Patent Description:
This disclosure relates to monitoring light emissions. The light emissions are emissions of light that occur in a vacuum chamber of an extreme ultraviolet (EUV) light source.

Extreme ultraviolet ("EUV") light, for example, electromagnetic radiation having wavelengths of <NUM> nanometers (nm) or less (also sometimes referred to as soft x-rays), and including light at a wavelength of, for example, <NUM> or less, between <NUM> and <NUM>, or between <NUM> and <NUM>, 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.

In one general aspect, a system includes a vacuum chamber including an interior region, the interior region is configured to receive a target and a light beam, the target includes target material that emits extreme ultraviolet (EUV) light when in a plasma state; a detection system configured to image the interior region, the detection system configured to detect light emission from atoms, ions, or molecules in the interior region and to produce a representation of a spatial distribution of the light emission in the interior region; and a control system coupled to the detection system, the control system configured to: analyze the representation of the spatial distribution of the light emission to determine a spatial distribution of the light emission from atoms, ions, or molecules in the interior region; and determine whether to adjust at least one property of the light beam and/or at least one property of the vacuum chamber based on the spatial distribution of the light emission.

Implementations may include one or more of the following features. The light emission may include fluorescence. The fluorescence may include laser-induced fluorescence. The control system being configured to analyze the representation also may include the control system being configured to compare the spatial distributions of fluorescence in the interior region at least at two the different times to estimate a velocity of ions in the interior region, and to compare the estimated velocity to a velocity specification, and the control system may be configured to determine whether to adjust a pressure of the gas based on the comparison of the estimated velocity to the velocity specification.

The system also may include one or more spectral filters configured to be positioned relative to the detection system, the spectral filters being configured to only allow some wavelengths to reach the detection system. Each of the one or more spectral filters may be configured to transmit light having a wavelength in one of a plurality of emission lines of the target material. In some implementations, at least one of the one or more spectral filters is configured to transmit a wavelength in a visible light range. The vacuum chamber may be further configured to contain a gas in the interior region, and the spectral filter may be configured to transmit light having a wavelength at an emission line of the gas.

The control system may be configured to receive a plurality of representations of the interior region, each of the plurality of representations may be associated with a different time, and the control system being configured to analyze the representation of the interior region may include the control system being configured to analyze each of the plurality of representations to determine the spatial distribution of the light emission in the interior region at each of the different times. The light emission in the interior region may result from an energy event in the interior region, and the different times are all times that occur after the energy event. The energy event may include an interaction between the light beam and the target, and the light emission may be an emission from: (a) the target material, (b) a plasma formed from the interaction between the light beam and the target material, and/or (c) debris formed from the interaction of the light beam and the target.

The control system may be configured to receive an extended exposure representation of the interior region, the extended exposure representation of the interior region including an average of the spatial distribution of the emission in the interior region over a temporal period. The vacuum chamber may be further configured to contain a gas in the interior region, the energy event may be an interaction that adds energy to the gas, and the light emission may be an emission from the gas. The interaction that adds energy to the gas may include (a) an interaction between the light beam and the gas, (b) an interaction between the gas and a plasma formed from an interaction between the light beam and the target, and/or (c) an interaction between ions and the gas.

The control system being configured to analyze the representation to determine a spatial distribution of the light emission in the interior region may include the control system being configured to estimate a shape and/or a spatial distribution of intensity of the light emission.

In some implementations, the system also includes a first spectral filter configured to transmit light having a wavelength in a first wavelength band; and a second spectral filter configured to transmit light having a wavelength in a second wavelength band, and the control system being configured to analyze the representation may include: the control system being configured to estimate an amount of light emission in the first wavelength band and to estimate an amount of light emission in the second wavelength band, and the control system may be further configured to estimate an ionization fraction of the target material based on comparing the estimated amount of light emission at the first wavelength band and the estimated amount of light emission at the second wavelength band. The control system may determine whether to adjust at least one property of the light beam based on the estimated ionization fraction. The control system may determine whether to adjust a pointing direction of the light beam based on the estimated ionization fraction.

The light beam may include a main pulse light beam having an energy sufficient to convert at least some of the target material to a plasma that emits EUV light.

The light beam may include a pre-pulse light beam.

The representation of the spatial distribution may include a representation of a two-dimensional representation.

The light beam may include a pulsed light beam, and the control system being configured to adjust at least one property of the light beam may include the control system being configured to adjust at least one property of a later-occurring pulse of the pulsed light beam.

In another general aspect, an EUV light source includes a vacuum chamber configured to: contain a gas in an interior region and to receive a target and a light beam, the target including target material that emits extreme ultraviolet (EUV) light in a plasma state; a monitor including at least one sensor, the at least one sensor being configured to detect emissions from the gas in the interior region and to produce an indication of the detected emissions; and a control system coupled to the monitor, the control system configured to: analyze the indication of detected emissions; and determine whether to adjust at least one property of the light beam and/or at least one property of the vacuum chamber based on the analysis.

Implementations may include one or more of the following features. The monitor may include a detection system configured to image a portion of the interior region and to produce a representation of a spatial distribution of detected emissions in the portion. The control system may be configured to receive a plurality of representations over a period of time, each representation indicating a spatial distribution of detected emissions in the portion at a different time in the period of time, the control system being configured to determine whether to adjust at least one property of the light beam and/or at least one property of the vacuum chamber based on two or more of the plurality of representations.

The gas may include hydrogen, and the detected emission may include an H-alpha (H-α) and/or an H-beta (H-β) emission from the hydrogen.

The EUV light source also may include a first spectral filter configured to transmit a first band of wavelengths; and a second spectral filter configured to transmit a second band of wavelengths, where, in operational use, the first spectral filter and the second spectral filter may be between the portion and the detection system; and the control system being configured to analyze the detected emissions may include the control system being configured to compare a representation of emissions that are transmitted by the first spectral filter to a representation of emissions that are transmitted by the second spectral filter; and the determination of whether to adjust at least one property of the light beam and/or at least one property of the vacuum chamber may be based on the comparison.

The EUV light source also may include a pressure controller coupled to interior of the vacuum chamber, the pressure controller being configured to change a pressure of the gas in the interior of the vacuum chamber, and the control system may be coupled to the pressure controller.

In another general aspect, a method of controlling an EUV light source includes providing a target to a target region in a vacuum chamber, the vacuum chamber containing a gas in an interior region; causing an interaction between a light beam and the target in the target region; detecting light emission from atoms, ions, and/or molecules in the interior region of the vacuum chamber, the light emission being a response to an energy event in the vacuum chamber, the energy event including an event that adds energy to the target and/or the gas; analyzing the detected light emission to determine a spatial distribution of light emission in the interior region;
and determining whether to adjust a property of the light beam and/or the gas based on the analysis.

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.

It is emphasized that, according to common practice, the various features of the drawings are not necessarily to scale. On the contrary, the dimensions of the various features may be arbitrarily expanded or reduced for clarity. Like numerals denote like features throughout the specification and drawings.

Techniques for controlling an extreme ultraviolet (EUV) lithography system and/or an EUV source based on analysis of emissions of light that occur within a vacuum chamber of the EUV source are disclosed.

Referring to <FIG>, a block diagram of an extreme ultraviolet (EUV) light source <NUM> is shown. The EUV light source <NUM> includes a sensor system <NUM> and a control system <NUM>. The sensor system <NUM> monitors emissions of light that occur inside a vacuum chamber <NUM> and provides information about the emissions to the control system <NUM>. The emissions are analyzed by the control system <NUM>, which is configured to make adjustments to one or more components of the EUV light source <NUM> based on the analysis of the emissions. The emissions may be emissions from a plasma <NUM>, a gas <NUM>, target material in a target 121p, and/or debris <NUM>. Monitoring emissions in the vacuum chamber <NUM> allows determination and control of a variety of parameters of the EUV source <NUM> that affect the performance of the EUV source. For example, information from the sensor system <NUM> may be used to determine a portion or fraction of a target material (or fuel) that is ionized by a plasma-generation event and/or to determine an amount of energy deposited into a gas <NUM> that is in the vacuum chamber <NUM>. Knowledge of such parameters allows the control system <NUM> to improve the performance of the EUV light source <NUM>.

The monitored emissions are light emitted from one or more substances in the vacuum chamber <NUM>. The substances include or are atoms, molecules, and/or ions. The emissions may be any kind of emission that involves light emerging from the substance. For example, the emissions may be optical emissions that occur as a result of atoms being excited by a high-temperature source. In another example, the emissions may be fluorescence from an atom, molecule, or ion. Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation.

Moreover, the emissions may be laser-induced fluorescence. Laser-induced fluorescence is a process by which an atom, ion, or molecule absorbs laser light and an electron of the substance is excited to a higher energy level. After the excitation, the electron decays to a lower energy level and the atom, ion, or molecule emits light. This emitted light is laser-induced fluorescence. The laser-induced fluorescence may be generated by irradiating the substance with an optical beam <NUM> (which may be a laser) and/or by irradiating the substance with a laser beam <NUM> generated by a probe laser <NUM>. Any laser that is suitable to excite the substances in the manner of interest may be used as the probe laser <NUM>. For example, the probe laser <NUM> may be a laser (such as an optical parametric oscillator or other type of tunable laser) that is capable of being tuned to produce one of several different wavelengths.

The wavelength of a particular emission is determined by the properties of the substance and the amount of energy that is used to excite the substance. Moreover, a particular substance may produce emissions of more than one wavelength. For example, hydrogen gas emits light at a wavelength of <NUM> nanometers (nm) when a hydrogen electron transitions from the third lowest energy level to the second lowest energy level. This emission is called H-alpha (H-α) emission. However, hydrogen gas also emits light of other wavelengths. For example, hydrogen gas emits light at a wavelength of <NUM> when a hydrogen electron transitions from the fourth lowest energy level to the second lowest energy level. This emission is called H-beta (H-β) emission. Hydrogen gas also has other emission lines. The wavelength emitted by the hydrogen gas depends on the amount of excitation energy, which determines the energy level to which an electron is excited from the ground state. Similarly, other substances that may be present in the vacuum chamber <NUM> emit light of particular wavelengths depending on their respective physical properties and the energy with which the substance is excited.

By analyzing these emissions, the control system <NUM> is able to monitor conditions within the vacuum chamber <NUM> and adjust the environment in the vacuum chamber <NUM> accordingly. In particular, the control system <NUM> is configured to analyze and adjust one or more properties of a subsequent (later-occurring) pulse of an optical beam <NUM> and/or one or more properties of the vacuum chamber <NUM> based on the monitored emissions. The properties of a subsequent pulse of the optical beam <NUM> that may be adjusted include, for example, a size (for example, beam waist at a plasma formation location <NUM>), an average and/or maximum energy, a temporal duration, and/or a position relative to the plasma formation location <NUM>. The properties of the vacuum chamber <NUM> that may be adjusted include, for example, a pressure of a gas <NUM>, a temperature of the gas <NUM>, a flow rate of the gas <NUM>, a flow direction of the gas <NUM>, the size of the target 121p, and/or a spacing of targets in the stream of targets <NUM>.

Various components of the EUV source <NUM> are discussed prior to discussing the control system <NUM> in more detail.

The EUV source <NUM> also includes a target supply system <NUM> that emits a stream <NUM> of targets. The target supply system <NUM> includes a target formation apparatus <NUM>, which defines an orifice <NUM> that is fluidly coupled to a reservoir <NUM>. In operational use, the target material is in a flowable state (for example, the target material is molten and at a temperature that is above its melting point) and the reservoir <NUM> is pressurized to a pressure P. The pressure P is greater than the pressure in the vacuum chamber <NUM>. Thus, in operational use, target material flows through the orifice <NUM> and into the vacuum chamber <NUM> to form the stream of targets <NUM>. In the example of <FIG>, the stream of targets <NUM> travels from the orifice <NUM> to the plasma formation location <NUM> generally in the x direction, with the target 121p (which is one of the targets in the stream <NUM>) being at the plasma formation location <NUM> at the time depicted in <FIG>.

The targets in the stream of targets <NUM> may be droplets of target material. The target material may be 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 a target mixture that includes a target substance and impurities such as non-target particles. The target substance is the substance that, when in a plasma state, has an emission line in the EUV range. The target substance may be, for example, a droplet of liquid or molten metal, a portion of a liquid stream, solid particles or clusters, solid particles contained within liquid droplets, a foam of target material, or solid particles contained within a portion of a liquid stream. The target substance can be, 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 target substance can be the element tin, which can be used as pure tin (Sn); as a tin compound, for example, SnBr<NUM>, SnBr<NUM>, SnH<NUM>; as a tin alloy, for example, tin-gallium alloys, tin-indium alloys, tin-indium-gallium alloys, or any combination of these alloys. Moreover, in the situation in which there are no impurities, the target material includes only the target substance.

During operation of the EUV source <NUM>, the plasma <NUM> is formed by interacting the optical beam <NUM> with the target 121p at the plasma formation location <NUM>. Plasma includes fine or small particles collectively called plasma particles. The plasma particles may be, for example, vaporized, atomized, and/or ionized particles of the fuel, and the monitored emissions may include emissions from any of these substances. The interaction of a light beam with target material, where the light beam has an energy sufficient to convert at least some of the target material to a plasma, is referred to as a plasma-generating event. Each plasma-generating event also generally produces debris (for example, fragments or pieces of target material that are not converted to the plasma <NUM>) and the monitored emissions may include emissions from the debris. Thus, during operation of the EUV source <NUM>, the plasma <NUM> and the debris <NUM> are present in the chamber <NUM> after a plasma-generating event.

The EUV source <NUM> also includes a light-generation module <NUM>, which generates the optical beam <NUM>. The light-generation module <NUM> may be, for example, a carbon dioxide (CO<NUM>) laser or a solid state laser. The light-generation module <NUM> may include various other components that are not shown in <FIG>, such as pre-amplifiers, power amplifiers, optical elements (such as mirrors) used to direct light, and beam combiners. In some implementations, the light-generation module <NUM> includes more than one optical source and may include more than one laser and may include different types of lasers. <FIG> shows an example of a light generation module <NUM> that includes more than one optical source.

The optical beam <NUM> may be a train of pulses each of which is separated from the nearest pulse in time. <FIG> shows an example of a temporal profile (optical power as a function of time) of a pulse <NUM> within the train. The pulse <NUM> is an example of one of the pulses that may be part of the optical beam <NUM>. The pulse <NUM> has a peak power <NUM> and a finite temporal duration <NUM>. In the example of <FIG>, the pulse duration <NUM> is the time during which the pulse <NUM> has a non-zero power. The time for the pulse <NUM> to increase from zero to the peak power <NUM> is the rise time of the pulse. In other implementations, the pulse duration <NUM> and/or the rise time may be based on other metrics. For example, the pulse duration <NUM> may be less than the time during which the pulse <NUM> has a non-zero power, such as the full-width at half maximum (FWHM) of the pulse <NUM>. Similarly, the rise time may be measured between two values other than zero optical power and the peak optical power <NUM>.

In the example shown, the power of the pulse <NUM> increases from zero power to the peak power <NUM> monotonically and decreases from the peak power <NUM> to zero monotonically. Other temporal profiles are possible. For example, the power of a pulse may increase from zero to the peak power non-monotonically. A pulse may have more than one peak energy point. Moreover, the pulses in the train of pulses that make up the optical beam <NUM> may have different temporal profiles.

The optical beam <NUM> is directed to the vacuum chamber <NUM> on an optical path <NUM> by a beam delivery system <NUM> that includes one or more optical components <NUM>. The optical components <NUM> may include any components that are able to interact with the optical beam <NUM>. The components <NUM> also may include devices that are able to form and/or shape the pulse <NUM>. For example, the optical components <NUM> may include passive optical devices such as mirrors, lenses, and/or prisms, and any associated mechanical mounting devices and/or electronic drivers. These components may steer and/or focus the optical beam <NUM>. Additionally, the optical components <NUM> may include components that modify one or more properties of the optical beam <NUM>. For example, the optical components <NUM> may include active optical devices, such as acousto-optic modulators and/or electro-optic modulators, capable of changing the temporal profile of the optical beam <NUM> to form the pulse <NUM>.

The pulse <NUM> leaves the beam delivery system <NUM> and enters the vacuum chamber <NUM>. The pulse <NUM> passes through an aperture <NUM> of an optical element <NUM> to reach the plasma formation location <NUM>. An interaction between the pulse <NUM> and the target material in the target 121p produces the plasma <NUM> that emits light <NUM>. The light <NUM> includes light with wavelengths that correspond to the emission lines of the target material in the target 121p.

The light <NUM> includes EUV light <NUM> and out-of-band light. Out-of-band light is light at wavelengths not in the EUV light range. For example, the target material may include tin. In these implementations, the light <NUM> includes the EUV light <NUM> and also includes out-of-band light such as deep ultraviolet (DUV), visible, near infrared (NIR), mid-wavelength infrared (MWIR), and/or long-wavelength infrared (LWIR) light. The EUV light <NUM> may include light having a wavelength of, for example, <NUM> nanometers (nm), <NUM>-<NUM>, <NUM>-<NUM>, or less than <NUM>. The DUV light may include light having wavelengths between about <NUM>-<NUM>, the visible light can include light having wavelengths between about <NUM>-<NUM>, the NIR light may include light having wavelengths between about <NUM>-<NUM>, the MWIR light may have a wavelength between about <NUM>-<NUM>, and the LWIR light may have a wavelength between about <NUM>-<NUM>.

The optical element <NUM> has a reflective surface <NUM> that is positioned to receive at least some of the light <NUM>. The reflective surface <NUM> has a coating that reflects the EUV light <NUM> but does not reflect out-of-band components of the light <NUM> or reflects only a nominal amount of the out-of-band components of the light <NUM>. In this way, the reflective surface <NUM> directs only the EUV light <NUM> to the lithography apparatus <NUM>.

The EUV source <NUM> also includes a gas management system <NUM> that supplies the gas <NUM> to the vacuum chamber <NUM>. The gas <NUM> may be, for example, hydrogen or oxygen. The gas management system <NUM> may include pumps, valves, and other components used in the management of gas. The gas management system <NUM> is configured to control various properties of the gas <NUM> that is supplied to the vacuum chamber such as, for example, temperature, pressure, and/or flow rate. For example, the gas management system <NUM> may supply the gas <NUM> at a flow rate that is sufficient to move debris (such as the debris <NUM>) in a controlled fashion and/or control the temperature and/or pressure of the gas <NUM> to influence aspects of plasma production.

The EUV light source <NUM> also includes the sensor system <NUM>, which provides a signal <NUM> that includes data related to monitored emissions to the control system <NUM>. As noted above, the monitored emissions may include emissions from the plasma <NUM>, emissions from the gas <NUM>, and/or emissions from the debris <NUM>. The sensor system <NUM> includes a sensor module <NUM> that includes one or more sensors <NUM>. The sensor <NUM> is any detector or sensor capable of detecting or sensing light having the wavelengths of emissions of interest. Thus, in the example of <FIG>, the sensor <NUM> may be a sensor capable of detecting emissions from the plasma <NUM>, a sensor that is capable of detecting one or more wavelengths that may be emitted from the gas <NUM>, and/or a sensor that is capable of sensing wavelengths of light emitted from the debris <NUM>.

In some implementations, the sensor <NUM> is capable of producing data that includes spatial information about the emissions. For example, the sensor <NUM> may be a two-dimensional array of sensors, with each sensor being configured to sense light emitted from a particular portion of the vacuum chamber <NUM>. Each sensor is fixed and has a known location relative to the portion of the vacuum chamber <NUM> the sensor monitors, thus, the relative location of the sensed emissions may also be determined. In these implementations, the spatial information shows how the emissions are distributed in the vacuum chamber <NUM>. The data from the sensor <NUM> may be used to form a two-dimensional spatial representation (such as an image) of the vacuum chamber <NUM> (or a portion of the vacuum chamber <NUM>), with the image showing the relative locations of the monitored emissions within the vacuum chamber <NUM>.

Moreover, the sensor <NUM> may be capable of producing many two-dimensional spatial representations of the monitored emissions in the vacuum chamber <NUM> over a period of time. For example, the sensor <NUM> may be a video sensor that captures frames (images) that are collected at a frame rate determined by the video sensor. In these implementations, each frame is a representation of the emissions in the vacuum chamber <NUM> at a different time. In another example, the sensor is a camera with an exposure mechanism that allows the sensor to sense emissions over a finite period of time. In these implementations, the data produced by the sensor <NUM> represents the time-average of emissions in the vacuum chamber <NUM>. The sensor module <NUM> may include more than one sensor. In these implementations, the sensors <NUM> are positioned at different locations relative to a particular region of the vacuum chamber <NUM> such that the data produced by the sensors <NUM> may be used to generate a three-dimensional spatial representation of the monitored emissions.

Furthermore, the sensor system <NUM> also may include a spectral filter module <NUM>. The spectral filter module <NUM> includes one or more spectral filters <NUM>. The spectral filters <NUM> allow control of which specific wavelength or wavelengths are sensed by the sensors <NUM>. In this way, particular emissions may be separated from the total emissions in the vacuum chamber such that only emissions of interest are monitored. When included in the sensor system <NUM>, the spectral filters <NUM> are positioned on an optical path between the sensor <NUM> and a monitored portion of the interior of the vacuum chamber <NUM>.

The spectral filter <NUM> is any filter that is capable of allowing only some wavelengths, or a particular wavelength, to reach the sensor <NUM> while substantially preventing any other wavelength from reaching the sensor <NUM>. The spectral filter <NUM> may be, for example, a spectral filter that only allows visible light to reach the sensor <NUM> or a spectral filter that only allows particular wavelengths within the visible spectrum to reach the sensor <NUM>. The spectral filter <NUM> may separate wavelengths based on transmission, reflection, and/or absorption. For example, the spectral filter <NUM> may be a multi-layer dielectric stack that transmits wavelengths within a band of wavelengths while reflecting or absorbing all other wavelengths. In another example, the spectral filter <NUM> may be a dichroic mirror or a grating that reflect different wavelengths in different directions.

The spectral filter module <NUM> may include more than one spectral filter <NUM>. For example, in some implementations, the sensor module <NUM> includes more than one sensor <NUM>, and the spectral filter module <NUM> includes a spectral filter <NUM> for each of the sensors.

The EUV light source <NUM> also includes the control system <NUM>, which uses information from the sensor system <NUM> to analyze the emissions in the vacuum chamber <NUM>. The control system <NUM> also provides command signals <NUM>, which are generated based on information about the emissions in the vacuum chamber <NUM>, to the light-generation module <NUM>, the target supply system <NUM>, the gas management system <NUM>, and/or the beam delivery system <NUM>.

The control system <NUM> includes an analysis module <NUM>. The analysis module <NUM> analyzes the information from the sensor system <NUM> and determines whether to make an adjustment to the optical beam <NUM> and/or the vacuum chamber <NUM> based on the analysis. The operation of the control system <NUM> and the analysis module <NUM> is discussed further with respect to <FIG>. In the example of <FIG>, the analysis module <NUM> of the control system <NUM> is implemented using an electronic processor <NUM>, an electronic storage <NUM>, and an I/O interface <NUM>. The electronic processor <NUM> includes 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 (RAM), or both. The electronic processor <NUM> may be any type of electronic processor. The electronic processor <NUM> executes the instructions that make up the analysis module <NUM>.

The electronic storage <NUM> may be volatile memory, such as RAM, or non-volatile memory. In some implementations, and the electronic storage <NUM> includes non-volatile and volatile portions or components. The electronic storage <NUM> may store data and information that is used in the operation of the control system <NUM>. For example, the electronic storage <NUM> may store the instructions (for example, in the form of a computer program) that implement the analysis module <NUM>. The analysis module <NUM> receives information from the sensor system <NUM> and also may receive information from the light-generation module <NUM>, the gas management system <NUM>, the supply system <NUM>, and/or the beam delivery system <NUM>.

The electronic storage <NUM> also may store instructions, perhaps as a computer program, that, when executed, cause the electronic processor <NUM> to communicate with components in the light-generation module <NUM>, the gas management system <NUM>, the beam delivery system <NUM>, the supply system <NUM>, and/or the sensor system <NUM>. For example, the instructions may be instructions that cause the electronic processor <NUM> to provide the command signal <NUM> generated by the analysis module <NUM> to the light-generation module <NUM>, the gas management system <NUM>, the supply system <NUM>, and/or the beam delivery system <NUM>.

The command signal <NUM> is a signal that causes a component in the light-generation module <NUM> and/or the beam delivery system <NUM> to act in a manner that adjusts the optical beam <NUM> or a signal that causes the gas management system <NUM> to adjust a property of the gas <NUM>. For example, the command signal <NUM> may be a signal that includes information sufficient to cause a valve and/or pump in the gas management system <NUM> to start operating, stop operating, or to continue operating but in a different manner. In another example, the command signal <NUM> is a signal that is capable of adjusting properties of the target supply system <NUM> that change the rate at which targets arrive at the plasma formation location <NUM>. In this example, the command signal <NUM> may be a signal that includes information sufficient to cause the target formation apparatus <NUM> to vibrate at a different rate such that the size and/or rate of targets arriving at the plasma formation location <NUM> changes. In yet another example, the command signal <NUM> is a signal that operates on the light-generating module <NUM> and/or the beam delivery system <NUM> to change a property of the beam <NUM>. For example, the command signal <NUM> may be a signal sufficient to cause a mirror in the beam delivery system <NUM> to move or a signal sufficient to adjust the operation of an electro-optic modulator in the beam delivery system <NUM>.

The I/O interface <NUM> is any kind of interface that allows the control system <NUM> to exchange data and signals with an operator, the light-generation module <NUM>, one or more components of the light-generation module <NUM>, the lithography apparatus <NUM>, and/or an automated process running on another electronic device. For example, in some implementations, the analysis module <NUM> may be programmed by an end-user to include analysis specific to the end-user. In these implementations, the analysis module may be programmed through the I/O interface <NUM>. The I/O interface <NUM> may include one or more of a visual display, a keyboard, and a communications interface, such as a parallel port, a Universal Serial Bus (USB) connection, and/or any type of network interface, such as, for example, Ethernet. The I/O interface <NUM> also may allow communication without physical contact through, for example, an IEEE <NUM>, Bluetooth, or a near-field communication (NFC) connection.

Referring to <FIG>, a block diagram of an EUV light source <NUM> is shown. The EUV light source <NUM> is another example of an implementation of an EUV light source. The EUV light source <NUM> is the same as the EUV light source <NUM> (<FIG>), except the EUV light source <NUM> uses a light-generation module <NUM> that includes a first optical source 208_1, which emits a first optical beam 206_1, and a second optical source 208_2, which emits a second optical beam 206_2. A pulse 204_1 is a pulse of the first optical beam 206_1, and a pulse 204_2 is a pulse of the second optical beam 206_2. The pulse 204_2 may be referred to as a "pre-pulse" optical beam, and the pulse 204_1 may be referred to as a "main pulse" optical beam.

The EUV light source <NUM> includes the optical element <NUM>, but for simplicity only the aperture <NUM> of the optical element <NUM> is shown in <FIG>. The pulse 204_2 propagates along a beam path 207_2, passes through the aperture <NUM> of the optical element <NUM>, and is delivered to an initial target region <NUM> via a beam delivery system 211_2. The initial target region <NUM> receives an initial target 221p from the supply system <NUM>. The initial target region <NUM> is displaced in the -x direction relative to the plasma formation location <NUM>.

The pulse 204_2 interacts with the target 221p at the initial target region <NUM> to condition the target 221p and form the modified target <NUM>. The conditioning may enhance the ability of the target 221p to absorb the pulse 204_1. For example, although the EUV light-emitting plasma <NUM> is not generally produced at the initial target region <NUM>, the interaction between the pulse 204_2 and the target 221p may change the shape, volume, and/or size of the distribution of the target material in the initial target 221p and/or may reduce the density gradient of the target material along the direction of propagation of the main pulse 204_1. Moreover, the interaction between the pulse 204_2 and the initial target 221p may produce a pre-plasma or a plasma that does not necessarily emit EUV light. The modified target <NUM> may be, for example, a disk-shaped distribution of target material that has a larger extent in the x-y plane than the target 221p and a smaller extent along the z axis than the target 221p. The modified target <NUM> drifts to the plasma formation location <NUM> and is irradiated by the pulse 204_1 to form the plasma <NUM>.

In the implementation of <FIG>, the control system <NUM> is coupled to the optical source 208_2 and the beam delivery system 211_2 such that the control system <NUM> may be used to control the properties of the second optical beam 206_2 (or a subsequent or later-occurring pulse of the optical beam 206_2). For example, the control system <NUM> may adjust the pulse energy of a later-occurring pulse of the optical beam 206_2, the location of a later-occurring pulse of the optical beam 206_2 relative to the expected location of the target 221p, and/or the duration of a later-occurring pulse of the optical beam 206_2. In this way, the control system <NUM> may be used to control the parameters of the conditioning of the initial target 221p. The control system <NUM> is also coupled to the optical source 208_1 and the beam delivery system 211_1 and may be used to control the properties of the optical beam 206_1 (or a pulse of the optical beam 206_1). Furthermore, the control system <NUM> is coupled to the gas management system <NUM> and is capable of adjusting one or more properties of the gas <NUM>.

The optical sources 208_1 and 208_2 may be, for example, two lasers. For example, the optical sources 208_1, 208_2 may be two carbon dioxide (CO<NUM>) lasers. In other implementations, the optical sources 208_1, 208_2 may be different types of lasers. For example, the optical source 208_2 may be a solid state laser, and the optical source 208_1 may be a CO<NUM> laser. In the example of <FIG>, the first and second optical beams 206_1, 206_2 are pulsed. The first and second optical beams 206_1, 206_2 may have different wavelengths. For example, in implementations in which the optical sources 208_1, 208_2 include two CO<NUM> lasers, the wavelength of the first optical beam 206_1 may be about <NUM> micrometers (µm) and the wavelength of the second optical beam 206_2 may be between <NUM> and <NUM>. The wavelength of the second optical beam 206_2 may be about <NUM>. In these implementations, the optical beams 206_1, 206_2 are generated from different lines of the CO<NUM> laser, resulting in the optical beams 206_1, 206_2 having different wavelengths even though both beams are generated from the same type of source.

The pulses 204_1 and 204_2 have different energies and may have different durations. For example, the pre-pulse 204_2 may have a duration of at least <NUM> ns, for example, the pre-pulse may have a duration of <NUM>-<NUM> ns and a wavelength of <NUM> or <NUM>. In one example, the pre-pulse of radiation is a laser pulse that has energy of <NUM>-<NUM> mJ, a pulse duration of <NUM>-<NUM> nanoseconds (ns), and a wavelength of <NUM>-<NUM> micrometers (µm). In some examples, the pre-pulse may have a duration of less than <NUM> ns. For example, the pre-pulse may have a duration of <NUM> picoseconds (ps) or less, <NUM> ps or less, between <NUM>-<NUM> ps, or between <NUM>-<NUM> ps.

Each of the beam delivery systems 211_1 and 211_2 is similar to the beam delivery system <NUM> (<FIG>). In the example of <FIG>, the first optical beam 206_1 and the second optical beam 206_2 interact with separate beam delivery systems and travel on separate optical paths. However, in other implementations, the first optical beam 206_1 and the second optical beam 206_2 may share all or part of the same optical path and also may share the same beam delivery system.

Referring to <FIG>, a flowchart of a process <NUM> is shown. The process <NUM> is an example of a process that may be performed by the control system <NUM>.

Light emitted from a substance in the vacuum chamber <NUM> is detected (<NUM>). The emitted light is detected by the sensor <NUM>. The substance may be an atom, ion, and/or molecule. The substance may be part of the gas <NUM>, the plasma <NUM>, and/or the debris <NUM>. The light emitted from the substance may be fluorescence or laser-induced fluorescence. The emitted light is detected by the sensor <NUM>. The sensor <NUM> produces data that indicates the characteristics of the emitted light. For example, the data may indicate the intensity of the emitted light. In some implementations, the data indicates the relative location of the emitted light in the vacuum chamber <NUM>. In these implementations, the data may be used to form a two-dimensional representation, such as an image. Moreover, in some implementations, the sensor module <NUM> includes more than one sensor <NUM>. The more than one sensor <NUM> may be positioned relative to a particular portion of the vacuum chamber <NUM>. In these implementations, the data from the sensors <NUM> may be used together to form a stereoscopic representation that represents the spatial distribution of the light emissions in the vacuum chamber <NUM> in three dimensions.

In some implementations, the sensor <NUM> collects data over a relatively short period (for example <NUM> microseconds (µs) or shorter, such as a period of <NUM> nanoseconds (ns) or less) such that the detected emissions are associated with a single plasma-generating event. These implementations allow one or more components of the EUV light source <NUM> or <NUM> to be changed on a pulse-to-pulse basis. Moreover, monitoring over relatively short periods allows generation of fast time-resolved images, such as shown in <FIG> and <FIG>. In other implementations, the sensor <NUM> collects data over a longer period such that the detected emissions are associated with more than one plasma-generating event.

An indication of the detected emissions is analyzed (<NUM>). The indication is data received via the signal <NUM> from the sensor system. The signal <NUM> includes information that describes the emissions, such as the intensity of the detected emissions. In some implementations, the signal <NUM> includes location information about the detected emissions. For example, the signal <NUM> may include a read out of a two-dimensional array of the sensors <NUM>, with the intensity of emissions detected by each sensor in the array being included in the signal <NUM>. Based on such information, the control system <NUM> determines the relative location of the detected emissions.

As discussed above, in some implementations, the sensor system <NUM> includes the spectral filter module <NUM> and one or more spectral filters <NUM>. In these implementations, the spectral filters <NUM> determine which wavelengths reach the sensor or sensors <NUM>. For example, the spectral filters <NUM> may include a filter that is designed to only allow wavelengths associated with H-α emissions to reach the sensor <NUM>. In these implementations, the signal <NUM> may include data that indicates that a particular signal <NUM> includes information related to detected H-α emissions.

Furthermore, the signal <NUM> may include data that relates to the conditions under which the emissions were generated. For example, the signal <NUM> may include information about the sensor, such as exposure time. In another example, the signal <NUM> may include information about the environment in the vacuum chamber. Examples of such environmental information include the temperature, pressure, and/or flow rate of the gas <NUM> and information about the optical beam <NUM>, such as pulse duration, pulse energy, and/or pulse wavelength.

The analysis module <NUM> of the control system <NUM> is capable of performing a variety of analyses on the indication of the detected emissions. The various analyses may be stored on the electronic storage <NUM> as, for example, computer programs that are executable by the electronic processor <NUM>. Any type of analysis on the detected emissions may be performed. Specific examples of data and corresponding analysis of that data are discussed with respect to <FIG> and <FIG>. Analysis other than the ones discussed in these examples may be performed by the analysis module <NUM>.

Referring also to <FIG>, an example in which the emissions are laser-induced fluorescence of neutral atomic tin is shown. In this example, tin is used as the target material, and the neutral atomic tin may be tin debris and/or tin that is not converted into the plasma <NUM>. In the example of <FIG>, the sensor <NUM> is a camera that images the plasma formation location <NUM> and produces a two-dimensional image of the vacuum chamber <NUM>.

In the example of <FIG>, the sensor <NUM> is an intensified charge coupled device (ICCD) with an exposure time of about <NUM> nanoseconds (ns), the spectral filter <NUM> was placed between the sensor <NUM> and the plasma formation location <NUM>, and the laser-induced fluorescence is formed by exciting neutral tin atoms with the laser beam <NUM> from the probe laser <NUM>. In this example, the probe laser <NUM> is a tunable laser, and the laser beam <NUM> is a pulsed light beam with pulses that had a duration of a few nanoseconds (for example, <NUM> ns or less). Additionally, the probe laser <NUM> was tuned such that the laser beam <NUM> had a wavelength of <NUM>, which excites neutral atomic tin that is in the ground state. Some fraction or percentage of the neutral tin atoms decay via an electronic transition that emits light (laser-induced fluorescence) at <NUM>. The spectral filter <NUM> in this example is a band-pass filter centered at <NUM>. Additionally, <FIG> relate to a system that uses a pre-pulse and a main pulse. Thus, these figures are discussed with respect to <FIG>.

<FIG> is an image 400A of the laser-induced fluorescence from neutral tin in the vacuum chamber <NUM> at <NUM> nanoseconds (ns) after the pre-pulse (the pulse 204_2 of <FIG>) interacts with the initial target 221p (<FIG>). <FIG> is an image 400B of the laser-induced fluorescence from neutral tin in the vacuum chamber <NUM> at <NUM> ns after the pre-pulse interacts with the initial target 221p. <FIG> is an image 400C of the laser-induced fluorescence from neutral tin in the vacuum chamber <NUM> at <NUM> ns after the main pulse (the pulse 204_1 of <FIG>) interacts with the modified target <NUM> (<FIG>). <FIG> is an image 400D of the laser-induced fluorescence from neutral tin in the vacuum chamber <NUM> at <NUM> ns after the main pulse interacts with the modified target <NUM>. Each pixel of each image 400A-400D represents an amount of laser-induced fluorescence in a particular region of the vacuum chamber <NUM>. The interaction between the main pulse and the modified target <NUM> is a plasma-generating event.

The analysis module <NUM> determines the amount of tin ionized by the interactions by analyzing the images 400A-400D to determine the intensity of certain spectral lines and the relative intensity of these lines. For example, the intensity of emissions from neutral tin may be compared to the intensity of singly or doubly ionized tin to determine the fraction of atoms of target material that are ionized after the plasma-generating event. The intensity of emissions is proportional to the number of tin atoms, neutral tin atoms in the example of <FIG>. Thus, if the intensity of emissions from neutral tin atoms decreases at the same time that intensity from ion species increases, then this is evidence of changing ionization fraction.

Other features of the images 400A-400D may be analyzed. For example, the spatial distribution of the intensity may be analyzed to estimate the distance traveled by the neutral tin atoms and/or the velocity of the neutral tin atoms. For example, as seen in <FIG>, the distance traveled from the origin (the location of the interaction between the main pulse and the modified target <NUM>) and the elapsed time (<NUM> ns after the interaction in this example) gives the velocity of those neutral tin atoms.

Furthermore, the orientation angle of the fluorescence is indicative of the angle or orientation of the modified target <NUM> relative to the direction of propagation of the main pulse and the orientation angle of the fluorescence changes as the orientation of the modified target <NUM> changes. Thus, the orientation of the modified target <NUM> also may be determined from images such the images 400A-400D.

The analysis module <NUM> also determines other information about the emissions from the images 400A-400D. For example, the analysis module <NUM> may apply a morphological operator to identify a ring structure <NUM> in the image 400C. The ring structure <NUM> expands in space as time since the plasma-generating event passes. The analysis module <NUM> also identifies the ring structure <NUM> in the image 400D. By comparing the spatial characteristics of the ring structure <NUM> in the image 400C to the ring structure <NUM> in the image 400D, the velocity of the tin atoms may be estimated. For example, the radius and/or the diameter of the ring structure <NUM> in the images 401C and 401D may be compared and used, with knowledge of the amount of time between the images 400C and 400D, to estimate the velocity of the tin atoms. Moreover, in some implementations, velocity of the tin atoms is determined from a single image. For example, velocity of the tin atoms may be determined from a single image when the time from the interaction of the main pulse and the modified target <NUM> is known for that single image. When the velocity of the tin atoms is determined from two or more images, changes in velocity of the tin atoms also may be determined.

Furthermore, the morphological operator may be used to determine an orientation of the ring structure <NUM>. The orientation of the ring structure provides an indication of the orientation of the modified target <NUM>. For example, the major and minor axes of the ring structure <NUM> may be estimated after identifying the ring structure <NUM>, and the orientation of the ring structure <NUM> may be estimated from the axes.

The images 400A-400D are provided as examples of data that the sensor system <NUM> may provide to the control system <NUM>. Other types of laser-induced fluorescence may be monitored. For example, images showing laser-induced fluorescence of ions of target material formed during a plasma-generation event may be generated and provided to the control system <NUM>. In another example, the emissions from the gas <NUM> are analyzed. The gas <NUM> may emit light due to for example, heat in the vacuum chamber <NUM> from the pulse <NUM> (or the pulse 204_1 and/or the pulse 204_2 of <FIG>) and/or ions moving in the gas <NUM>, by the formation of the plasma <NUM>, or by direct excitation by the probe laser <NUM>. <FIG> shows an example related to analyzing the emissions from the gas <NUM> to determine the amount of energy deposited into the gas <NUM> as a result of a plasma-generation event.

In the example of <FIG>, the gas <NUM> was hydrogen gas and the sensor <NUM> was a camera that produced two-dimensional images of the plasma formation location <NUM>. The pulse of light converts at least some of the target material to plasma that emits EUV light. In the example of <FIG>, the pulse energy was <NUM> milliJoules (mJ), the wavelength of the pulse was <NUM>, and the duration of the pulse was <NUM> ns. The target was a tin droplet that had a radius of about <NUM>. In this implementation, the spectral filter <NUM> was a band-pass filter with a narrow spectral band centered on the H-α emission wavelength and was placed between the plasma formation location <NUM> and the sensor <NUM>. Thus, H-α emissions to reached the sensor <NUM> but light of other wavelengths was substantially prevented from reaching the sensor <NUM>.

Four two-dimensional images 500A-500D (of many more images taken) are shown in <FIG>. Each of the images 500A-500D was obtained at a different time. Thus, the images 500A-500D are images of the relative intensity or amount of H-α emissions at the plasma formation location <NUM> at four different times.

The analysis module <NUM> is configured to analyze images such as 500A-500D to determine spatial characteristics of a shockwave or blast-wave <NUM>. The blast-wave <NUM> is formed in the gas <NUM> by the plasma-generating event. The spatial characteristics may include, for example, the radius, diameter, orientation of the semi-major axis, the orientation of the major axis, the orientation of the minor axis, and/or circumference of the blast-wave <NUM>. The analysis module <NUM> locates the blast-wave <NUM> in one or more of the images collected by the camera by applying morphological operators and imaging processing techniques to the images. For example, the general shape of the blast-wave <NUM> is known to be a circle, and the analysis module <NUM> may apply a morphological filter that detects circular objects within images to locate the blast-wave <NUM> in an image from the camera. In another example, the analysis module <NUM> may apply an edge detector that relies on the difference in intensity between emissions at the edge of the shockwave <NUM> and the background.

Once the spatial characteristics of the blast-wave <NUM> have been estimated, the analysis module <NUM> applies the Taylor-Sedov equation to estimate an amount of energy (E) deposited into the gas <NUM>. The Taylor-Sedov equation is: <MAT> where E is the energy deposited into the gas <NUM>, r is the radius of the blast-wave, ρo is the density of the gas <NUM>, and t-to is the time since the plasma-generating event. The radius (r) at a particular time (t) is estimated from an image of the plasma formation location <NUM> captured at the time (t). The analysis module <NUM> estimates the amount of energy deposited into the gas <NUM> using Equation <NUM> and the estimate of the radius of the blast-wave <NUM> at a particular time (t). The analysis module <NUM> also may determine other information from the images 500A-500D. For example, <FIG> also includes a plot of relative total H-α emission as a function of time since the plasma-generation event. To generate the plot <NUM>, the value of each pixel in an image collected by the camera at a particular time was summed and normalized. The results were plotted as a function of time. The images 500A-500D correspond to four of the points included on the plot <NUM>.

The data shown in <FIG> and <FIG> are examples of the types of data that the sensor system <NUM> may provide to the control system <NUM> via the signal <NUM>. However, the sensor system <NUM> may be configured to collect any other data about the emissions in the vacuum chamber <NUM>, and the analysis module <NUM> also may be configured to analyze such data. For example, in some implementations, the plasma formation location <NUM> is monitored by more than one sensor <NUM>, each of which has a spectral filter <NUM> corresponding to a particular emission line of the target material or the gas <NUM>. In these implementations, each sensor <NUM> provides data that specifies the spatial distribution of one of the emission lines of the substance at the plasma generation location <NUM> under the same operating conditions. The measured emissions from each sensor is compared to the measured emissions measured by the other sensors to determine properties of the environment in the vacuum chamber <NUM>. For example, in the case of comparing different possible emissions from the target material, such a comparison results in an estimate of the portion of the target material that was ionized to form the plasma <NUM>.

Furthermore, the analysis module <NUM> may be configured to compare the spatial distribution of a certain type of emission at two different times after a plasma event. For example, in an implementation in which tin is used as the target material, the sensor <NUM> may be used with a filter <NUM> that only allows an emissions from ionized tin to reach the sensor <NUM>. By comparing images of the emission of the ionized tin taken at different times, the analysis module <NUM> is able to estimate the velocity and/or direction of motion of the tin ions.

Accordingly, the analysis module <NUM> analyzes information and data from the sensor system <NUM>.

In addition to analyzing the data provided from the sensor system <NUM>, the control system <NUM> also determines whether to make adjustments to the EUV light source <NUM> or <NUM> based on the analysis (<NUM>). The adjustment to the EUV light source <NUM> or <NUM> may be an adjustment to any component of the EUV light source <NUM> or <NUM> and may include an adjustment to more than one component of the EUV light source <NUM> or <NUM>. Whether an adjustment is made and the nature of the adjustment (if any) depends on the results of the analysis discussed with respect to (<NUM>).

The EUV light source <NUM> or <NUM> may be associated with various performance specifications, and the analysis of the emissions may be used to determine whether the EUV light source <NUM> or <NUM> is operating within one or more performance specifications. Conversion efficiency (CE) is an example of a performance specification. The conversion efficiency is the ratio of the energy supplied to the EUV light source <NUM> or <NUM> that is converted into the EUV light. The CE depends on the ionization fraction (the portion of target material that is converted to ions). As discussed above, the analysis of the emissions may be used to estimate ionization fraction. To increase the ionization fraction, the duration and/or energy of the pulses in the optical beam <NUM> may be increased. Thus, if the CE is below the specified CE, the control system <NUM> may issue a command signal <NUM> to the light-generation module <NUM> to change the duration and/or intensity of the pulses in the optical beam <NUM>.

In another example, the control system <NUM> may issue the command signal <NUM> to the light generation-module <NUM> (<FIG>) such that properties of the pre-pulse 204_2 are changed. As discussed above, the pre-pulse 204_2 conditions the target by changing the shape and/or density of the target such that the modified target <NUM> (<FIG>) is more favorable to plasma production. The light-generation module <NUM> may be adjusted such that the intensity and/or duration of the pre-pulse 204_2 are such that a later-produced modified target <NUM> has a lower density and/or a different shape. Moreover, in some implementations, the control system <NUM> issues the command signal to the beam steering system 211_1 such that the position of the pre-pulse 204_2 relative to the initial target location <NUM> is changed. Furthermore, the size of the targets in the stream <NUM> may be adjusted to reduce the ionization fraction. In these implementations, the command signal <NUM> is provided to the target supply system <NUM> to, for example, change the frequency of vibration of the target formation apparatus <NUM> such that the size of the targets in the stream <NUM> is reduced.

In another example, the analysis of the emissions produces an estimated ion velocity that is greater than a desired ion velocity. In this example, the control system <NUM> issues the command signal <NUM> to the gas management system <NUM>. The gas management system <NUM> causes the pressure of the gas <NUM> to increase such that ions created in subsequent plasma-generating events have a lower velocity. In yet another example, the analysis of the emissions shows a relatively high amount of tin atoms at a time relatively soon after the plasma-generating event. A relatively high amount of tin atoms shortly after the plasma-generating event is an indication of excess debris in the vacuum chamber <NUM>. The control system <NUM> may issue the command signal <NUM> to the gas management system <NUM> to increase the flow rate of the gas <NUM> and/or change the direction of the flow of the gas <NUM> to move the debris away from the optical element <NUM>.

In yet another example, analysis of the emissions is used to produce an estimate of the amount of energy deposited into the gas <NUM>. The estimated amount of energy is compared to a threshold and/or a specification (for example, a range of acceptable amounts of energy), and, if the estimated amount of energy is above the threshold and/or does not meet the threshold, the control system <NUM> may issue a command to the light generation module <NUM> to reduce the power of the pre-pulse 204_2. Reducing the power or the pre-pulse 204_2 generally reduces the amount of ions and/or pre-pulse plasma produced during the interaction between the pre-pulse 204_2 and the initial target 221p, and thereby reduces the energy deposited into the gas <NUM>.

In some implementations, the control system <NUM> issues command signals <NUM> to more than one component or system of the EUV light source <NUM> or <NUM>. For example, to increase the ionization fraction, the control system <NUM> may issue the command signal <NUM> to the light-generation module <NUM> or <NUM>, the target supply system <NUM>, and the gas management system <NUM>. Moreover, under some conditions, all performance specifications are met and/or the EUV light source <NUM> is operating acceptably, and no adjustments are made.

After determining whether to adjust the EUV light source <NUM> or <NUM>, the control system <NUM> determines whether to continue monitoring the vacuum chamber <NUM> (<NUM>). If the monitoring continues, the process <NUM> returns to (<NUM>). If the monitoring does not continue, the process <NUM> ends. Moreover, in some implementations, the process <NUM> runs continuously during operation of the EUV light source <NUM> or <NUM> such that the control system continuously monitors the EUV light source <NUM> or <NUM>. In these implementations, the control system <NUM> does not determine whether to continue monitoring the vacuum chamber <NUM> and instead monitors the vacuum chamber <NUM> continuously and without interruption during operation of the EUV light source <NUM> or <NUM>.

<FIG> is a block diagram of a lithographic apparatus <NUM> that includes a source collector module SO. The lithographic apparatus <NUM> includes:.

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

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 may 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 may be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. The pattern imparted to the radiation beam may correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix.

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.

In the example of <FIG>, the apparatus is of a reflective type (for example, employing a reflective mask). The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more patterning device tables). In such "multiple stage" machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.

Referring to <FIG>, the illuminator IL receives an extreme ultraviolet radiation beam from the source collector module SO. Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has at least one element, for example, xenon, lithium or tin, with one or more emission lines in the EUV range. In one such method, often termed laser produced plasma ("LPP") the required plasma is produced by irradiating a fuel, such as a droplet, stream or cluster of material having the required line-emitting element, with a laser beam. The source collector module SO may be part of an EUV radiation system including a laser, not shown in <FIG>, for providing the laser beam exciting the fuel. The resulting plasma emits output radiation, for example, EUV radiation, which is collected using a radiation collector, disposed in the source collector module. The laser and the source collector module may be separate entities, for example when a carbon dioxide (CO<NUM>) laser is used to provide the laser beam for fuel excitation.

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

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 PS1 can 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 M1, M2 and substrate alignment marks P1, P2.

The depicted apparatus may be used in at least one of the following modes:.

Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

<FIG> shows an implementation of the lithographic apparatus <NUM> in 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 structure <NUM> of the source collector module SO. The systems IL and PS are likewise contained within vacuum environments of their own. An EUV radiation emitting plasma <NUM> may be formed by a laser produced LPP plasma source. The function of source collector module SO is to deliver EUV radiation beam <NUM> from the plasma <NUM> such 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 aperture <NUM> in the enclosing structure <NUM>. The virtual source point IF is an image of the radiation emitting plasma <NUM>.

From the aperture <NUM> at the intermediate focus IF, the radiation traverses the illumination system IL, which in this example includes a facetted field mirror device <NUM> and a facetted pupil mirror device <NUM>. These devices form a so-called "fly's eye" illuminator, which is arranged to provide a desired angular distribution of the radiation beam <NUM>, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA (as shown by reference <NUM>). Upon reflection of the beam <NUM> at the patterning device MA, held by the support structure (mask table) MT, a patterned beam <NUM> is formed and the patterned beam <NUM> is imaged by the projection system PS via reflective elements <NUM>, <NUM> onto 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 structure <NUM>. 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 in <FIG>.

Considering source collector module SO in more detail, a laser energy source including a laser <NUM> is arranged to deposit laser energy <NUM> into 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 plasma <NUM> is a highly ionized plasma with electron temperatures of several <NUM>'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 collector <NUM> and focused on the aperture <NUM>. The plasma <NUM> and the aperture <NUM> are located at first and second focal points of collector CO, respectively.

Although the collector <NUM> shown in <FIG> is 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 generator <NUM> is arranged within the enclosure <NUM>, arranged to fire a high frequency stream <NUM> of droplets towards the desired location of plasma <NUM>. The droplet generator <NUM> may be the target formation apparatus <NUM> and/or includes an adhesive such as the adhesive <NUM>. In operation, laser energy <NUM> is delivered in a synchronism with the operation of droplet generator <NUM>, to deliver impulses of radiation to turn each fuel droplet into a plasma <NUM>. The frequency of delivery of droplets may be several kilohertz, for example <NUM>. In practice, laser energy <NUM> is 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 energy <NUM> is delivered to the cloud at the desired location, to generate the plasma <NUM>. A trap <NUM> is provided on the opposite side of the enclosing structure <NUM>, to capture fuel that is not, for whatever reason, turned into plasma.

The droplet generator <NUM> comprises a reservoir <NUM> which contains the fuel liquid (for example, molten tin) and a filter <NUM> and a nozzle <NUM>. The nozzle <NUM> is configured to eject droplets of the fuel liquid towards the plasma <NUM> formation location. The droplets of fuel liquid may be ejected from the nozzle <NUM> by a combination of pressure within the reservoir <NUM> and 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 beams <NUM>, <NUM>, <NUM>. At each part of the apparatus, a local reference frame of X, Y and Z axes may be defined. In the example of <FIG>, 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 stream <NUM>, while the Y axis is orthogonal to that, pointing out of the page as indicated in <FIG>. 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 diagram <FIG>, 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 apparatus <NUM> as 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 collector <NUM> and 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 apparatus <NUM>.

Referring to <FIG>, an implementation of an LPP EUV light source <NUM> is shown. The light source <NUM> may be used as the source collector module SO in the lithographic apparatus <NUM>. Furthermore, the light-generation module <NUM> of <FIG> may be part of the drive laser <NUM>. The drive laser <NUM> may be used as the laser <NUM> (<FIG>).

The LPP EUV light source <NUM> is formed by irradiating a target mixture <NUM> at a plasma formation location <NUM> with an amplified light beam <NUM> that travels along a beam path toward the target mixture <NUM>. The target material discussed with respect to <FIG>, <FIG>, and <FIG>, and the targets in the stream <NUM> of <FIG> and <FIG> may be or include the target mixture <NUM>. The plasma formation location <NUM> is within an interior <NUM> of a vacuum chamber <NUM>. When the amplified light beam <NUM> strikes the target mixture <NUM>, a target material within the target mixture <NUM> is 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 mixture <NUM>. 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 source <NUM> also includes the supply system <NUM> that delivers, controls, and directs the target mixture <NUM> in 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 mixture <NUM> includes 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, SnBr<NUM>, SnBr<NUM>, SnH<NUM>; as a tin alloy, for example, tin-gallium alloys, tin-indium alloys, tin-indium-gallium alloys, or any combination of these alloys. The target mixture <NUM> may also include impurities such as non-target particles. Thus, in the situation in which there are no impurities, the target mixture <NUM> is made up of only the target material. The target mixture <NUM> is delivered by the supply system <NUM> into the interior <NUM> of the chamber <NUM> and to the plasma formation location <NUM>.

The light source <NUM> includes a drive laser system <NUM> that produces the amplified light beam <NUM> due to a population inversion within the gain medium or mediums of the laser system <NUM>. The light source <NUM> includes a beam delivery system between the laser system <NUM> and the plasma formation location <NUM>, the beam delivery system including a beam transport system <NUM> and a focus assembly <NUM>. The beam transport system <NUM> receives the amplified light beam <NUM> from the laser system <NUM>, and steers and modifies the amplified light beam <NUM> as needed and outputs the amplified light beam <NUM> to the focus assembly <NUM>. The focus assembly <NUM> receives the amplified light beam <NUM> and focuses the beam <NUM> to the plasma formation location <NUM>.

In some implementations, the laser system <NUM> may 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 system <NUM> produces an amplified light beam <NUM> due to the population inversion in the gain media of the laser amplifiers even if there is no laser cavity. Moreover, the laser system <NUM> may produce an amplified light beam <NUM> that is a coherent laser beam if there is a laser cavity to provide enough feedback to the laser system <NUM>. The term "amplified light beam" encompasses one or more of: light from the laser system <NUM> that is merely amplified but not necessarily a coherent laser oscillation and light from the laser system <NUM> that is amplified and is also a coherent laser oscillation.

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

The light source <NUM> includes a collector mirror <NUM> having an aperture <NUM> to allow the amplified light beam <NUM> to pass through and reach the plasma formation location <NUM>. The collector mirror <NUM> may be, for example, an ellipsoidal mirror that has a primary focus at the plasma formation location <NUM> and a secondary focus at an intermediate location <NUM> (also called an intermediate focus) where the EUV light may be output from the light source <NUM> and may be input to, for example, an integrated circuit lithography tool (not shown). The light source <NUM> may also include an open-ended, hollow conical shroud <NUM> (for example, a gas cone) that tapers toward the plasma formation location <NUM> from the collector mirror <NUM> to reduce the amount of plasma-generated debris that enters the focus assembly <NUM> and/or the beam transport system <NUM> while allowing the amplified light beam <NUM> to reach the plasma formation location <NUM>. For this purpose, a gas flow may be provided in the shroud that is directed toward the plasma formation location <NUM>.

The light source <NUM> may also include a master controller <NUM> that is connected to a droplet position detection feedback system <NUM>, a laser control system <NUM>, and a beam control system <NUM>. The light source <NUM> may include one or more target or droplet imagers <NUM> that provide an output indicative of the position of a droplet, for example, relative to the plasma formation location <NUM> and provide this output to the droplet position detection feedback system <NUM>, 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 system <NUM> thus provides the droplet position error as an input to the master controller <NUM>. The master controller <NUM> may therefore provide a laser position, direction, and timing correction signal, for example, to the laser control system <NUM> that may be used, for example, to control the laser timing circuit and/or to the beam control system <NUM> to control an amplified light beam position and shaping of the beam transport system <NUM> to change the location and/or focal power of the beam focal spot within the chamber <NUM>.

The supply system <NUM> includes a target material delivery control system <NUM> that is operable, in response to a signal from the master controller <NUM>, for example, to modify the release point of the droplets as released by a target material supply apparatus <NUM> to correct for errors in the droplets arriving at the desired plasma formation location <NUM>. The target material supply apparatus <NUM> includes a target formation apparatus that employs an adhesive such as the adhesive <NUM>.

Additionally, the light source <NUM> may include light source detectors <NUM> and <NUM> that 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 detector <NUM> generates a feedback signal for use by the master controller <NUM>. 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 source <NUM> may also include a guide laser <NUM> that may be used to align various sections of the light source <NUM> or to assist in steering the amplified light beam <NUM> to the plasma formation location <NUM>. In connection with the guide laser <NUM>, the light source <NUM> includes a metrology system <NUM> that is placed within the focus assembly <NUM> to sample a portion of light from the guide laser <NUM> and the amplified light beam <NUM>. In other implementations, the metrology system <NUM> is placed within the beam transport system <NUM>. The metrology system <NUM> may 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 beam <NUM>. A beam analysis system is formed from the metrology system <NUM> and the master controller <NUM> since the master controller <NUM> analyzes the sampled light from the guide laser <NUM> and uses this information to adjust components within the focus assembly <NUM> through the beam control system <NUM>.

Thus, in summary, the light source <NUM> produces an amplified light beam <NUM> that is directed along the beam path to irradiate the target mixture <NUM> at the plasma formation location <NUM> to convert the target material within the mixture <NUM> into plasma that emits light in the EUV range. The amplified light beam <NUM> operates 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 system <NUM>. Additionally, the amplified light beam <NUM> may be a laser beam when the target material provides enough feedback back into the laser system <NUM> to produce coherent laser light or if the drive laser system <NUM> includes suitable optical feedback to form a laser cavity.

The preceding merely illustrates the principles of embodiments of the disclosure. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are included within its scope. Furthermore, all examples and conditional language recited herein are principally intended expressly to be only for pedagogical purposes and to aid the reader in understanding the principles of the invention and the concepts contributed by the inventors to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents and equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure.

This description of the exemplary embodiments is intended to be read in connection with the figures of the accompanying drawing, which are to be considered part of the entire written description. In the description, relative terms such as "lower," "upper," "horizontal," "vertical," "above," "below," "up," "down," "top" and "bottom" as well as derivatives thereof (e.g., "horizontally," "downwardly," "upwardly," etc.) should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description and do not require that the apparatus be constructed or operated in a particular orientation. Terms concerning attachments, coupling and the like, such as "connected" and "interconnected," refer to a relationship wherein structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both movable or rigid attachments or relationships, unless expressly described otherwise.

Claim 1:
A system comprising: a vacuum chamber (<NUM>)
comprising an interior region, wherein the interior region is configured to receive a target (<NUM>) and a light beam (<NUM>), the target comprises target material, and the target material emits extreme ultraviolet, EUV, light (<NUM>) when in a plasma state;
characterised by:
a detection system (<NUM>) configured to image the interior region, the detection system configured to detect light emission from atoms, ions, or molecules in the interior region and to produce a representation of a spatial distribution of the light emission in the interior region; and
a control system (<NUM>) coupled to the detection system, the control system configured to:
analyze the representation of the spatial distribution of the light emission to determine a spatial distribution of the light emission from atoms, ions, or molecules in the interior region; and
determine whether to adjust at least one property of the light beam and/or at least one property of the vacuum chamber based on the spatial distribution of the light emission.