Light source, EUV lithography system, and method for generating EUV radiation

A light source for extreme ultraviolet (EUV) radiation is provided. The light source includes a target droplet generator, a laser generator, a measuring device, and a controller. The target droplet generator is configured to provide a plurality of target droplets to a source vessel. The laser generator is configured to provide a plurality of first laser pulses according to a control signal to irradiate the target droplets in the source vessel, so as to generate plasma as the EUV radiation. The measuring device is configured to measure process parameters including temperature of the source vessel, droplet positions of the target droplets, and beam sizes and focal points of the first laser pulses. The controller is configured to provide the control signal according to at least two of the process parameters.

BACKGROUND

Extreme ultraviolet lithography (EUVL) has been developed to perform high-resolution photolithography in semiconductor manufacturing. The EUVL employs scanners using light in the extreme ultraviolet (EUV) region, having a wavelength of about 1-100 nm. Some EUV scanners provide 4×reduction projection printing, similar to some optical scanners, except that the EUV scanners use reflective rather than refractive optics, i.e., mirrors instead of lenses. One type of EUV light source is laser-produced plasma (LPP). LPP technology produces EUV light by focusing a high-power laser beam onto small tin droplet targets to form highly ionized plasma that emits EUV radiation with a peak maximum emission at 13.5 nm. The EUV light is then collected by a LPP collector and reflected by optics towards a lithography target, e.g., a wafer. The collector is subjected to damage and degradation due to the impact of particles, ions, radiation, and most seriously, tin deposition.

DETAILED DESCRIPTION

The advanced lithography process, method, and materials described in the current disclosure can be used in many applications, including fin-type field effect transistors (FinFETs). For example, the fins may be patterned to produce a relatively close spacing between features, for which the above disclosure is well suited. In addition, spacers used in forming fins of FinFETs can be processed according to the above disclosure.

FIG. 1shows a schematic and diagrammatic view of a lithography system10, in accordance with some embodiments of the disclosure. The lithography system10is operable to perform lithography exposing processes with respective radiation source and exposure mode.

The lithography system10includes a light source12, an illuminator14, a mask stage16, a projection optics module (or projection optics box (POB))20, a substrate stage24, and a gas supply module26. In some embodiments, the light source12and the gas supply module26are implemented in an extreme ultraviolet (EUV) radiation source apparatus11. Furthermore, the illuminator14, the mask stage16, the projection optics module20and the substrate stage24are implemented in an EUV scanner13. The elements of the lithography system10can be added to or omitted, and the invention should not be limited by the embodiment.

The light source12is configured to generate radiation having a wavelength ranging between about 1 nm and about 100 nm. In some embodiments, the light source12is capable of generating an EUV radiation (or light) with a wavelength centered at about 13.5 nm. In such embodiments, the light source12is also referred to as an EUV light source. In some embodiments, the light source12can be utilized to perform any high-intensity photon emission from excited target material.

The illuminator14includes various refractive optic components, such as a single lens or a lens system having multiple lenses (zone plates) or alternatively reflective optics (for EUV lithography system), such as a single mirror or a mirror system having multiple mirrors in order to direct light from the light source12onto a mask stage16, particularly to a mask18secured on the mask stage16. In such embodiments, the light source12generates light in the EUV wavelength range, and reflective optics is employed.

The mask stage16is configured to secure the mask18. In some embodiments, the mask stage16includes an electrostatic chuck (e-chuck) to secure the mask18. This is because the gas molecules absorb EUV light and the lithography system for the EUV lithography patterning is maintained in a vacuum environment to avoid EUV intensity loss. In such embodiments, the terms mask, photomask, and reticle are used interchangeably.

In some embodiments, the mask18is a reflective mask. One exemplary structure of the mask18includes a substrate with a suitable material, such as a low thermal expansion material (LTEM) or fused quartz. In various examples, the LTEM includes TiO2 doped SiO2, or other suitable materials with low thermal expansion. The mask18includes reflective multilayer deposited on the substrate.

The reflective multilayer includes a plurality of film pairs, such as molybdenum-silicon (Mo/Si) film pairs (e.g., a layer of molybdenum above or below a layer of silicon in each film pair). Alternatively, the reflective multilayer may include molybdenum-beryllium (Mo/Be) film pairs, or other suitable materials that are configurable to highly reflect the EUV light. The mask18may further include a capping layer, such as ruthenium (Ru), disposed on the reflective multilayer for protection. The mask18further includes an absorption layer, such as a tantalum boron nitride (TaBN) layer, deposited over the reflective multilayer. The absorption layer is patterned to define a layer of an integrated circuit (IC). Alternatively, another reflective layer may be deposited over the reflective multilayer and is patterned to define a layer of an integrated circuit, thereby forming an EUV phase shift mask.

The projection optics module (or POB)20is configured to provide a patterned beam and project the patterned beam onto a semiconductor substrate22, so as to image the pattern of the mask18on to the semiconductor substrate22secured on a substrate stage24of the lithography system10. In some embodiments, the projection optics module20has refractive optics (such as for a UV lithography system) or alternatively reflective optics (such as for an EUV lithography system). The light directed from the mask18, carrying the image of the pattern defined on the mask, is collected by the projection optics module20. In some embodiments, the illuminator14and the projection optics module20are collectively referred to as an optical module of the lithography system10.

The semiconductor substrate22is a semiconductor wafer, and the semiconductor wafer may be made of silicon or other semiconductor materials. Alternatively or additionally, the semiconductor substrate22may include other elementary semiconductor materials such as germanium (Ge). In some embodiments, the semiconductor substrate22is made of a compound semiconductor such as silicon carbide (SiC), gallium arsenic (GaAs), indium arsenide (InAs), or indium phosphide (InP). In some embodiments, the semiconductor substrate22is made of an alloy semiconductor such as silicon germanium (SiGe), silicon germanium carbide (SiGeC), gallium arsenic phosphide (GaAsP), or gallium indium phosphide (GaInP). In some other embodiments, the semiconductor substrate22may be a silicon-on-insulator (SOI) or a germanium-on-insulator (GOI) substrate.

The semiconductor substrate22may have various device elements. Examples of device elements that are formed in the semiconductor substrate22include transistors (e.g., metal oxide semiconductor field effect transistors (MOSFET), complementary metal oxide semiconductor (CMOS) transistors, bipolar junction transistors (BJT), high-voltage transistors, high-frequency transistors, p-channel and/or n-channel field-effect transistors (PFETs/NFETs), etc.), diodes, and/or other applicable elements. Various processes are performed to form the device elements, such as deposition, etching, implantation, photolithography, annealing, and/or other suitable processes. In some embodiments, the semiconductor substrate22is coated with a resist layer sensitive to the EUV light. Various components including those described above are integrated together and are operable to perform lithography exposing processes.

The lithography system10may further include other modules or be integrated with (or be coupled with) other modules. For example, the gas supply module26is configured to provide hydrogen gas to the light source12, so as to decrease contamination of the light source12.

FIG. 2shows a light source12A of the lithography system10ofFIG. 1, in accordance with some embodiments of the disclosure. The light source12A employs a single pulse laser produced plasma (LPP) mechanism to generate plasma and further generate EUV light from the plasma.

In some embodiments, the light source12A includes a controller40, a target droplet generator30, a laser generator60A, a laser produced plasma (LPP) collector36, a measuring device50, a droplet catcher70, and a source vessel (or chamber)72. The above-mentioned elements of the light source12A may be held under vacuum. Furthermore, the source vessel72is maintained in a vacuum environment. It should be appreciated that the elements of the light source12A can be added to or omitted, and the invention should not be limited by the embodiments.

The controller40is coupled to the measuring device50, the target droplet generator30, and the laser generator60A. The controller40is configured to control the operations of the measuring device50, the target droplet generator30, and the laser generator60A. Furthermore, the controller40is configured to receive measuring information regarding condition variations (e.g., positions of the target droplets32and/or temperature TEMP) within the source vessel72from the measuring device50.

The target droplet generator30is configured to generate a plurality of target droplets32into the source vessel72. For example, the target droplets32are generated one at a time and a train of target droplets32move through the excitation zone34. In some embodiments, the target droplet generator30includes a gas supplier (not shown). The gas supplier is configured to supply a pumping gas to force target material out of the target droplet generator30and drive the flowing of the target droplets32. A flow velocity of the target droplets32from the target droplet generator30is controlled by the controller40. Furthermore, the flow velocity of the target droplets32from the target droplet generator30is a function of the pressure of the pumping gas in the target droplet generator30. For example, the target droplets32flow faster when the pressure of the pumping gas is increased, and the target droplets32flow slower when the pressure of the pumping gas is decreased.

In some embodiments, the target droplets32are tin (Sn) droplets. In some embodiments, the target droplets32have a diameter about 30 microns (μm). In some embodiments, the target droplets32are generated at a rate about 50 kilohertz (kHz) and are introduced into the excitation zone34in the light source12A at a speed about 70 meters per second (m/s). Other material can also be used for the target droplets32, for example, a tin containing liquid material such as eutectic alloy containing tin, lithium (Li), and xenon (Xe). The target droplets32may be in a solid or liquid phase.

The laser generator60A is configured to generate a pulse laser66according to a control signal CTRL from the controller40, to allow a conversion of the target droplets32into plasma. The laser generator60A includes a laser source62. In response to the control signal CTRL, the laser source62is configured to produce the pulse laser (or laser beam)66. The pulse laser66is used to irradiate the target droplets32, so as to generate increased emission of light. For example, the pulse laser66heats the target droplets32to a preset temperature. At the preset temperature, the material of the target droplets32shed their electrons and become a plasma that emits EUV light (or radiation)38. The EUV light38is collected by the collector36. The collector36further reflects and focuses the EUV light38for the lithography exposure processes. For example, the EUV light38collected by the collector36is irradiated to the illuminator14ofFIG. 1via an output port74of the source vessel72, so as to direct the EUV light38from the light source12onto the mask stage16, particularly to the mask18secured on the mask stage16.

In such embodiments, the laser source62is a carbon dioxide (CO2) laser source. In some embodiments, the laser source62is a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser source. In some embodiments, the pulse laser66has a specific spot size (e.g., about 100-300 μm). The pulse laser66is generated to have certain driving powers to fulfill wafer volume production, such as a throughput of 125 wafers per hour. For example, the pulse laser66is equipped with about 19 kW driving power. It should be appreciated that many variations and modifications can be made to embodiments of the disclosure.

The pulse laser66from the laser source62is directed through a window (or lens)64of the collector36, into the excitation zone34, so as to irradiate the target droplets32in a lighting position LP (e.g., a focal point) of the excitation zone34. The window64adopts a suitable material substantially transparent to the pulse laser66.

The collector36is designed with the proper coating material and shape, functioning as a mirror for EUV collection, reflection, and focusing. In some embodiments, the collector36is designed to have an ellipsoidal geometry. In some embodiments, the coating material of the collector36is similar to the reflective multilayer of the EUV mask18ofFIG. 1. In some embodiments, the coating material of the collector36includes a ML (such as a plurality of Mo/Si film pairs) and may further include a capping layer (such as Ru) coated on the ML to substantially reflect the EUV light. In some embodiments, the collector36may further include a grating structure designed to effectively scatter the laser beam directed onto the collector36. For example, a silicon nitride layer may be coated on the collector36and patterned to have a grating structure.

The droplet catcher70is arranged opposite to the target droplet generator30and in the direction of the movement of the target droplets32. The droplet catcher70is configured to catch any target droplets32that are not hit by the pulse laser66. For example, some target droplets32may be purposely missed by the pulse laser66. Furthermore, the high-temperature plasma may cool down and become vapor or small particles (collectively, debris)39. When the target droplets32are not properly and accurately irradiated by the pulse laser66at the lighting position LP of the excitation zone34, the debris39is increased. For example, if the target droplets32are unstable, the unstable target droplets32are converted into unstable plasma and unexpected debris39is present, thereby increasing the debris39. The debris39may be deposited onto the surface of the collector36, thereby causing contamination thereof. Over time, the reflectivity of the collector36degrades due to debris accumulation and other factors such as ion damages, oxidation, and blistering. Once the reflectivity is degraded to a certain degree (e.g., less than 50%), the collector36reaches the end of its usable lifespan and needs to be swapped out in a replacement operation. When swapping out the collector36during the replacement operation, the lithography system10is shut down, and no lithography exposing process can be performed. When the number of the replacement operations or operation time of the replacement operations is increased, manufacturing cycle of the semiconductor substrate22is increased, thereby increasing manufacturing costs.

The measuring device50includes a thermal sensor52and a droplet detector54in certain embodiments. The thermal sensor52is configured to measure temperature TEMP within the source vessel72, and to provide the temperature TEMP to the controller40. In some embodiments, the thermal sensor52is a thermograph arranged in the source vessel72.

The droplet detector54is configured to measure the droplet positions of the target droplets32, and to provide information regarding the droplet positions to the controller40. In some embodiments, the droplet detector54is a droplet imager including a camera (not shown) capable of capturing images of the target droplets32. According to the images of the target droplets32, the droplet imager provides information regarding the droplet positions of the target droplets32to the controller40. In some embodiments, the droplet detector54is a droplet imager including a light generator (not shown) and a light detector (not shown). The light generator is configured to direct a laser beam to the target droplets32. In some embodiments, the light detector, such as a photodetector array, avalanche photodiode or photomultiplier, is arranged opposite the light generator, and is configured to receive the laser beam passing through the target droplets32, so as to obtain the droplet positions of the target droplets32. In some embodiments, the light detector is arranged adjacent to the light generator, and is configured to receive the laser beam reflecting from the target droplets32, so as to obtain the droplet positions of the target droplets32. Furthermore, the droplet positions may be determined in one or more axes.

In some embodiments, the measuring device50further includes a laser meter56for measuring a beam size of the pulse laser66and providing the measured beam size to the controller40. Furthermore, the installation position of the laser meter56within the source vessel72is determined according to various applications. In some embodiments, the laser meter56is capable of measuring the lighting position LP and providing the measured lighting position LP to the controller40, so as to detect whether the lighting position LP is offset.

In the light source12A, according to the temperature TEMP of the source vessel72and the droplet positions of the target droplets32from the measuring device50, the controller40is capable of providing the control signal CTRL to the laser generator60A, so as to control whether to generate the pulse laser66. For example, when the temperature TEMP of the source vessel72or the droplet positions of the target droplets32are normal, the controller40determines that the target droplets32are stable, and the controller40is configured to provide the control signal CTRL to the laser generator60A, so as to provide the pulse laser66. Conversely, when the temperature TEMP of the source vessel72and/or the droplet position of the target droplet32are abnormal, the controller40determines that the target droplets32are unstable, and the controller40is configured to provide the control signal CTRL to the laser generator60A, so as to stop providing the pulse laser66until the temperature TEMP of the source vessel72or the droplet position of the target droplet32is normal. Therefore, converting the unstable target droplets32with the pulse laser66is avoided, thereby reducing contamination on the surface of the collector36and extending the usable lifespan of the collector36. A detailed description of the operation will be provided below.

In some embodiments, in addition to the temperature TEMP of the source vessel72and the droplet positions of the target droplets32from the measuring device50, the controller40is configured to provide the control signal CTRL further according to the beam size of the pulse laser66and the lighting position LP measured by the laser meter56. For example, when the beam size of the pulse laser66or the lighting position LP are normal, the controller40is configured to provide the control signal CTRL to the laser generator60A, so as to provide the pulse laser66. Conversely, when the beam size of the pulse laser66and/or the lighting position LP are abnormal, the controller40is configured to provide the control signal CTRL to the laser generator60A, so as to stop providing the pulse laser66until the beam size of the pulse laser66or the lighting position LP is normal. Therefore, converting the target droplets32with the unstable pulse laser66is avoided, thereby reducing contamination on the surface of the collector36and extending the usable lifespan of the collector36.

In some embodiments, the controller40further provides the control signal CTRL to the EUV scanner13ofFIG. 1, so as to notify the EUV scanner13whether to suspend a lithography exposing process. For example, when the temperature TEMP of the source vessel72or the droplet positions of the target droplets32are normal, the controller40determines that the target droplets32are stable and provides the control signal CTRL to the EUV scanner13, so as to perform the lithography exposing process. Conversely, when the temperature TEMP of the source vessel72and the droplet positions of the target droplets32are abnormal, the controller40determines that the target droplets32are unstable and provides the control signal CTRL to the EUV scanner13, so as to suspend the lithography exposing process.

FIG. 3shows a light source12B of the lithography system10ofFIG. 1, in accordance with some embodiments of the disclosure. The light source12B employs a dual-pulse laser LPP mechanism to generate plasma and further generate EUV light from the plasma.

Compared with the laser generator60A of the light source12A inFIG. 2, the laser generator60B of the light source12B inFIG. 3includes a first laser source62A and a second laser source62B. The first laser source62A is configured to produce a pre-pulse laser66A. The second laser source62B is configured to produce a main pulse laser66B. The pre-pulse laser66A is used to heat (or pre-heat) the target droplets32to expand the target droplets32and create a low-density target plume33, which is subsequently irradiated by the main pulse laser66B, generating increased emission of light.

In some embodiments, the first laser source62A is a carbon dioxide (CO2) laser source. In some embodiments, the first laser source62A is a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser source. In some embodiments, the second laser source62B is a (CO2) laser source.

In such embodiments, the pre-pulse laser66A has less intensity and a smaller spot size than the main pulse laser66B. In some embodiments, the pre-pulse laser66A has a spot size of about 100 μm or less, and the main pulse laser66B has a spot size about 200-300 μm (e.g., 225 μm). The pre-pulse laser66A and the main pulse laser66B are generated to have certain driving powers to fulfill wafer volume production, such as a throughput of 125 wafers per hour. For example, the pre-pulse laser66A is equipped with about 2 kilowatts (kW) driving power, and the main pulse laser66B is equipped with about 19 kW driving power. In some embodiments, the total driving power of the pre-pulse laser66A and the main pulse laser66B, is at least 20 kW, such as 27 kW. However, it should be appreciated that many variations and modifications can be made to embodiments of the disclosure.

The pre-pulse laser66A from the first laser source62A and the main pulse laser66B from the second laser source62B are directed through the windows (or lenses)64A and64B of the collector36, respectively, into the excitation zone34and irradiate the target droplets32at a first lighting position LP1and a second lighting position LP2. In other words, the first lighting position LP1and the second lighting position LP2are the focal points of the pre-pulse laser66A and main pulse laser66B, respectively. The windows64A and64B may adopt a suitable material substantially transparent to the pre-pulse laser66A and the main pulse laser66B. In some embodiments, the generation of the pre-pulse laser66A and main pulse laser66B are synchronized with the generation of the target droplets32. When the target droplets32move through the excitation zone34, the pre-pulse lasers66A heat the target droplets32at the first lighting position LP1and convert them into low-density target plumes33. A delay between the pre-pulse laser66A and the main pulse laser66B is controlled by the controller40, to allow the target plume33to form and to expand to an optimal size and geometry. When the main pulse laser66B heats the target plume33at the second lighting position LP2, a high-temperature plasma is generated. The plasma emits EUV radiation38, which is collected by the collector36. The collector36further reflects and focuses the EUV radiation38for the lithography exposing processes.

As described above, the droplet catcher70is arranged opposite the target droplet generator30. The droplet catcher is configured to catch excessive target droplets32. For example, some target droplets32may be purposely missed by both pre-pulse laser66A and the main pulse laser66B, or some target plume33may be purposely missed by only the main pulse laser66B. Furthermore, when the target droplets32are not properly and accurately irradiated by the pre-pulse laser66A, the debris39is increased. Similarly, when the target plume33is not properly and accurately irradiated by the main pulse laser66B, the debris39is increased. For example, if the target droplets32are unstable, the unstable target droplets32are converted into unstable plasma, and unexpected debris39is present, thereby increasing the debris39. The debris39may be deposited onto the surface of the collector36, thereby causing contamination thereof. Thus, the reflectivity of the collector36is degraded and needs to be swapped out.

In the light source12B, according to the temperature TEMP of the source vessel72and the droplet positions of the target droplets32measured by the measuring device50, the controller40is capable of providing the control signal CTRL to the laser generator60B, so as to control whether to generate the pre-pulse laser66A and/or the main pulse laser66B. For example, when the temperature TEMP of the source vessel72or the droplet position of the target droplet32are normal, the controller40determines that the target droplets32are stable, and the controller40provides the control signal CTRL to the laser generator60B, so as to provide the pre-pulse laser66A and the main pulse laser66B. Conversely, when both the temperature TEMP of the source vessel72and the droplet position of the target droplet32are abnormal, the controller40determines that the target droplets32are unstable, and the controller40provides the control signal CTRL to the laser generator60B, so as to stop providing the pre-pulse laser66A and the main pulse laser66B until the temperature TEMP of the source vessel72or the droplet position of the target droplet32is normal. Therefore, converting the unstable target droplets32with the pre-pulse laser66A and/or the main pulse laser66B is avoided, thereby reducing contamination on the surface of the collector36and extending usable lifespan of the collector36. The details of the operation will be described below.

In some embodiments, in addition to the temperature TEMP of the source vessel72and the droplet positions of the target droplets32from the measuring device50, the controller40is configured to provide the control signal CTRL further according to the beam sizes of the pre-pulse laser66A and main pulse laser66B and the first and second lighting positions LP1and LP2measured by the laser meter56. For example, when the beam size of the pre-pulse laser66A and/or the main pulse laser66B, or the lighting positions LPI and/or LP2are normal, the controller40provides the control signal CTRL to the laser generator60B, so as to provide the pre-pulse laser66A and the main pulse laser66B. Conversely, when the beam size of the pre-pulse laser66A and/or the main pulse laser66B, and the lighting positions LP1and/or LP2are abnormal, the controller40provides the control signal CTRL to the laser generator60B, so as to stop providing the pre-pulse laser66A and the main pulse laser66B until the beam size of the pre-pulse laser66A and/or the main pulse laser66B, or the lighting positions LP1and/or LP2is normal. Therefore, converting the target droplets32with the unstable pre-pulse laser66A and/or the main pulse laser66B is avoided, thereby reducing contamination on the surface of the collector36and extending usable lifespan of the collector36. The details of the operation will be described below.

In some embodiments, the controller40further provides the control signal CTRL to the EUV scanner13ofFIG. 1, so as to notify the EUV scanner13whether to suspend a lithography exposing process. For example, when determining that the target droplets32are stable, the controller40is configured to provide the control signal CTRL to the EUV scanner13, so as to perform the lithography exposing process. Conversely, when determining that the target droplets32are unstable, the controller40is configured to provide the control signal CTRL to the EUV scanner13, so as to suspend the lithography exposing process.

FIG. 4shows a simplified flowchart of a method for generating extreme ultraviolet (EUV) radiation, in accordance with some embodiments of the disclosure. Furthermore, the method ofFIG. 4is performed by the lithography system10ofFIG. 1for a lithography exposure process.

Before the method ofFIG. 4is performed, the EUV mask18is loaded to the lithography system10that is configured to perform an EUV lithography exposing process. The mask18is a patterning mask and includes an IC pattern to be transferred to a semiconductor substrate, such as the wafer22. Furthermore, the mask18is secured on the mask stage16and an alignment of the mask18is performed.

Similarly, the semiconductor substrate22is loaded to the lithography system10before the method ofFIG. 4is performed. The wafer22is coated with a resist layer. In such embodiments, the resist layer is sensitive to the EUV light (radiation) from the light source12of the lithography system10.

In operation410ofFIG. 4, the target droplet generator30is configured to provide (or generate) the target droplets32into the source vessel72with the proper material, proper size, proper rate, and proper movement speed and direction. In some embodiments, the rate and the proper movement speed and/or direction of the target droplets32are controlled by the controller40according to a predetermined setting. As described above, the target droplets32are generated one at a time and a train of target droplets32move through the excitation zone34of the source vessel72.

In operation420ofFIG. 4, the temperature TEMP within the source vessel72is measured by the measuring device50, and then the measuring device50is configured to provide the temperature TEMP to the controller40. In some embodiments, the temperature TEMP is transmitted through an analog signal or a digital signal with one or multiple bits from the measuring device50to the controller40.

When obtaining the temperature TEMP, the controller40compares the temperature TEMP with a temperature threshold value TH_temp to obtain data (or a signal) T_vane, and the data T_vane is used to determine whether the temperature TEMP of the source vessel72is greater than the temperature threshold value TH_temp. Referring toFIG. 5,FIG. 5shows an example illustrating the relationship between the temperature TEMP and the data T_vane, in accordance with some embodiments of the disclosure. InFIG. 5, when the temperature TEMP is less than or equal to the temperature threshold value TH_temp, the controller40obtains the data T_vane with a low logic level (“L”). Conversely, when the temperature TEMP is greater than the temperature threshold value TH_temp, the controller40obtains the data T_vane with a high logic level (“H”).

In some embodiments, the controller40compares the temperature TEMP with a temperature range to obtain the data T_vane, and the data T_vane is used to determine whether the temperature TEMP of the source vessel72is greater than an upper temperature threshold value or less than a lower temperature threshold value (e.g., outside the temperature range). For example, when the temperature TEMP is within the temperature range (i.e., less than the upper temperature threshold value and greater than the lower temperature threshold value), the controller40obtains the data T_vane with a low logic level (“L”). Conversely, when the temperature TEMP does not fall within the temperature range (i.e., greater than the upper temperature threshold value or less than the lower temperature threshold value), the controller40obtains the data T_vane with a high logic level (“H”).

Referring back to the flowchart ofFIG. 4, in operation420ofFIG. 4, droplet position of each target droplet32is also measured by the measuring device50and information regarding the droplet positions is provided to the controller40. In some embodiments, the information regarding each droplet position is transmitted through an analog signal or a digital signal with one or multiple bits.

When obtaining the droplet positions, the controller40may calculate a standard deviation σ of the droplet positions in some embodiments, e.g., the controller40is configured to perform an arithmetic calculation on the standard deviation a. In such embodiments, the standard deviation σ of the droplet positions is a measure that is used to quantify the amount of variation or dispersion of a set of the droplet positions. In other words, the standard deviation σ is a performance indicator for the interval between two consecutive target droplets32. In some embodiments, the standard deviation σ is provided by the measuring device50. In some embodiments, other calculations may be made to determine whether droplet pointing is acceptable.

In some embodiments, the standard deviation σ is obtained according to the following equation:

σ=1N⁢∑i=1N⁢(xi-μ)2,
where {x1, x2, . . . , xN} represent the droplet positions of first, second, . . . , Nthtarget droplets32, N is the number of the target droplets32, and μ represents the mean value of the droplet positions.

When obtaining the standard deviation a, the controller40compares the standard deviation σ with a standard deviation threshold value TH_SD to obtain data (or a signal) X-int_σ, and the data X-int_σ is used to indicate whether the standard deviation σ of the target droplets32is greater than the standard deviation threshold value TH_SD. Referring toFIG. 6,FIG. 6shows an example illustrating the relationship between the standard deviation σ and the data X-int_σ, in accordance with some embodiments of the disclosure. InFIG. 6, when the standard deviation σ is less than or equal to the standard deviation threshold value TH_SD, the controller40obtains the data X-int_σ with a low logic level (“L”). Conversely, when the standard deviation σ is greater than the standard deviation threshold value TH_SD, the controller40obtains the data X-int_σ with a high logic level (“H”).

In some embodiments, the controller40compares the standard deviation σ with a standard deviation range to obtain the data X-int_σ, and the data X-int_σ is used to indicate whether the standard deviation σ of the target droplets32is greater than an upper standard deviation threshold value or less than a lower standard deviation threshold value (e.g., outside the standard deviation range). For example, when the standard deviation σ is within the standard deviation range, the controller40obtains the data X-int_σ with a low logic level (“L”). Conversely, when the standard deviation σ does not fall within the standard deviation range (i.e., greater than the upper standard deviation threshold value or less than the lower standard deviation threshold value), the controller40obtains the data X-int_σ with a high logic level (“H”).

Due to the standard deviation σ being sensitive to the vessel conditions of the source vessel72, such as the temperature TEMP, when the source vessel72is heated, the variation of the temperature TEMP leads to unstable pressure flow, thereby impacting the droplet positions of the target droplets32. For example, referring toFIG. 7,FIG. 7shows a pressure of the source vessel72corresponding to the temperature TEMP ofFIG. 5and the standard deviation σ ofFIG. 6, in accordance with some embodiments of the disclosure. When an obvious variation (labeled as UPF) is present in the pressure of the source vessel72, the pressure variation will cause the unstable pressure flow to impact the target droplets32, thereby generating unstable target droplets32. InFIG. 7, the obvious variation (labeled as UPF) of the pressure of the source vessel72is caused by the obvious variation in the temperature TEMP ofFIG. 5.

Referring back to the flowchart ofFIG. 4, in operation430, the controller40determines whether the temperature TEMP exceeds the temperature threshold value TH_temp and whether the standard deviation σ exceeds the standard deviation threshold TH_SD. When determining that the temperature TEMP does not exceed the temperature threshold value TH_temp (e.g., TEMP≤TH_temp) or the standard deviation σ does not exceed the standard deviation threshold TH_SD (e.g., σ≤TH_SD), the controller40is configured to provide the control signal CTRL to the laser generator (e.g.,60A ofFIG. 2 or 60BofFIG. 3) for notifying that the target droplets32are stable. Thus, the laser generator continues to provide the pulse laser66.

As described above, the standard deviation σ is a process parameter (or an indicator) to quantify the amount of variation or dispersion of a set of the droplet positions. In some embodiments, other process parameters may be used to quantify the amount of variation or dispersion of the droplet positions, such as an average value (or a mean value) of the droplet positions or a first derivatives of the droplet positions by time. For example, the controller40determines whether the temperature TEMP exceeds the temperature threshold value TH_temp and whether the average value is within a predetermined range. When determining that the temperature TEMP does not exceed the temperature threshold value TH_temp (e.g., TEMP≤TH_temp) or the average value does not exceed the predetermined range, the controller40is configured to provide the control signal CTRL to the laser generator (e.g.,60A ofFIG. 2 or 60BofFIG. 3) for notifying that the target droplets32are stable. Thus, the laser generator continues to provide the pulse laser66.

In operation440, in response to the control signal CTRL, the laser generator is configured to provide the laser pulses (e.g.,66ofFIG. 2 or 66A and 66BofFIG. 3), so as to convert the target droplets32into plasma as the EUV radiation for the lithography exposing process. For example, if the control signal CTRL indicates that the target droplets32are stable, the laser source62is configured to produce the pulse laser66inFIG. 2. Similarly, the laser source62A is configured to produce the pre-pulse laser66A, and the laser source62B is configured to produce the main pulse laser66B inFIG. 3.

For the EUV scanner13ofFIG. 1, the controller40further provides the control signal CTRL to the EUV scanner13, so as to notify the EUV scanner13to continue the lithography exposing process. Furthermore, the EUV radiation generated by the light source12is illuminated on the mask18(by the illuminator14), and is further projected on the resist layer coated on the wafer22(by the projection optics module20), thereby forming a latent image on the resist layer.

When the laser pulses are generated by the laser generator in response to the control signal CTRL (in operation440), the operation420of the flowchart inFIG. 4is performed again, so as to obtain the current temperature TEMP and the current droplet positions. Thus, the temperature TEMP and the standard deviation σ are updated. Next, when the controller40determines that the temperature TEMP does not exceed the temperature threshold value TH_temp or determines that the standard deviation σ does not exceed the standard deviation threshold TH_SD (in operation430), the controller40continues to provide the control signal CTRL for generating the laser pulses (in operation440).

Conversely, when the controller40simultaneously determines that the temperature TEMP exceeds the temperature threshold value TH_temp and the standard deviation σ exceeds the standard deviation threshold TH_SD (in operation430), the controller40determines that the target droplets32are unstable, and provides the control signal CTRL to the laser generator (e.g.,60A ofFIG. 2 or 60BofFIG. 3), so as to stop providing the pulse lasers (e.g.,66ofFIG. 2 or 66A and 66BofFIG. 3) in operation450until the temperature TEMP does not exceed the temperature threshold value TH_temp or the standard deviation σ does not exceed the standard deviation threshold TH_SD. For example, if the control signal CTRL indicates that the target droplets32are unstable, the laser source62is configured to stop producing the pulse laser66inFIG. 2. In some embodiments, if the control signal CTRL indicates that the target droplets32are unstable, only the laser source62B is configured to stop producing the main pulse laser66B inFIG. 3. In some embodiments, if the control signal CTRL indicates that the target droplets32are unstable, both the laser sources62A and62B are configured to stop producing the pre-pulse laser66A and the main pulse laser66B inFIG. 3.

When the target droplets32are unstable, the controller40further provides the control signal CTRL to the EUV scanner13ofFIG. 1, so as to notify the EUV scanner13to suspend the lithography exposing process. Furthermore, when the laser pulses are not generated by the laser generator in response to the control signal CTRL, the operation420of the flowchart inFIG. 4is performed again, so as to obtain the current temperature TEMP and the current droplet positions. Thus, the temperature TEMP and the standard deviation σ are updated. Next, in operation430, the controller40determines whether the current temperature TEMP exceeds the temperature threshold value TH_temp and determines whether the current standard deviation σ exceeds the standard deviation threshold TH_SD, and then subsequent operation (e.g., operation440or450ofFIG. 4) is performed in response to the control signal CTRL corresponding to the determination in operation430.

FIG. 8shows an example illustrating the control signal CTRL corresponding to the determination in operation430according to the temperature TEMP ofFIG. 5and the standard deviation σ ofFIG. 6, in accordance with some embodiments of the disclosure. As described above, when the temperature TEMP exceeds the temperature threshold value TH_temp, the controller40obtains the data T_vane with a high logic level. Furthermore, when the standard deviation σ exceeds the standard deviation threshold value TH_SD, the controller40obtains the data X-int_σ with a high logic level. The controller40performs an AND logic operation on the data T_vane and the data X-int_σ to obtain data (or signal) UNSTABLE_PD. In such embodiments, the data UNSTABLE_PD is obtained according to the convolution of the data T_vane and the data X-int_σ, and the data UNSTABLE_PD is used to represent the status of the target droplets32in the source vessel72. For example, if both the data T_vane and the data X-int_σ are at a high logic level, the data UNSTABLE_PD is at a high logic level, which represents that the target droplets32are unstable. If the data T_vane or the data X-int_σ is at a low logic level, the data UNSTABLE_PD is at a low logic level, which represents that the target droplets32are stable.

In such embodiments, the controller40provides the control signal CTRL according to the data UNSTABLE_PD, and the data UNSTABLE_PD is obtained according to the process parameters regarding to the temperature TEMP and the standard deviation σ. In such embodiments, the control signal CTRL is complement to the data UNSTABLE_PD. Therefore, when the control signal CTRL is at a high logic level (i.e., the target droplets32are stable), the laser generator is configured to provide the laser pulses in response to the control signal CTRL with a high logic level. Conversely, when the control signal CTRL is at a low logic level (i.e., the target droplets32are unstable), the laser generator is configured to stop providing the laser pulses in response to the control signal CTRL with a low logic level.

In some embodiments, other process parameters regarding to the droplet positions of the target droplets32may be used to obtain the data UNSTABLE_PD. For example, assuming that the number of the target droplets32is N, the other process parameter may be the average value of the droplet positions of first, second, . . . , Nthtarget droplets32, or the first derivatives of the droplet positions of first, second, . . . , Nthtarget droplets32by time.

In some embodiments, the process parameters regarding to the beam sizes of the pulse laser66, the pre-pulse laser66A and/or the main pulse laser66B may be used to obtain the data UNSTABLE_PD. For example, the process parameter may be the average value or the standard deviation of beam sizes of the pulse lasers66(or the pre-pulse laser66A, the main pulse laser66B) corresponding to first, second, . . . , Nthtarget droplets32.

In some embodiments, the process parameters regarding to the focal points of the pulse laser66, the pre-pulse laser66A and/or the main pulse laser66B may be used to obtain the data UNSTABLE_PD. For example, the process parameter may be the average value, the standard deviation or the first derivatives of the focal points of the pulse laser66(or the pre-pulse laser66A, the main pulse laser66B) corresponding to first, second, . . . , Nthtarget droplets32. Furthermore, the process parameter may be the average value, the standard deviation or the first derivatives of the focal point separation between the pre-pulse laser66A and the main pulse laser66B corresponding to first, second, . . . , Nthtarget droplets32. In some embodiments, the focal point separation between the pre-pulse laser66A and the main pulse laser66B represent a difference between the first lighting position LP1and the second lighting position LP2ofFIG. 3.

In some embodiments, two or more process parameters previously described are used to obtained the data UNSTABLE_PD according to various processes or applications. For example, inFIG. 8, the data UNSTABLE_PD is obtained according to the process parameter regarding to the temperature TEMP ofFIG. 5and the process parameter regarding to the standard deviation σ ofFIG. 6.

In some embodiments, if two of more process parameters used to obtained the data UNSTABLE_PD are outside the corresponding ranges (e.g., the temperature range or the standard deviation range) or are greater than the corresponding threshold values (e.g., the temperature threshold value TH_temp or the standard deviation threshold value TH_SD), the controller40determines that the target droplets32or the laser pulses66/66A/66B are unstable, and no pulse lasers are generated to irradiate the target droplets32in the source vessel72. Conversely, if one of the process parameters used to obtained the data UNSTABLE_PD is within the corresponding ranges or are less than or equal to the corresponding threshold values, the controller40determines that the target droplets32or the laser pulses66/66A/66B are stable, and the controller40is configured to provide the control signal CTRL to the laser generator60A/60B, so as to provide the pulse laser66/66A/66B.

In some embodiments, the control signal CTRL is a gating signal for suspending the lithography exposing process. As described above, the controller40further provides the control signal CTRL to notify the EUV scanner13whether to suspend the lithography exposing process. For example, when the control signal CTRL is at a high logic level (i.e., the target droplets32are stable), the EUV scanner13is configured to perform the lithography exposing process in response to the control signal CTRL with a high logic level. Conversely, when the control signal CTRL is at a low logic level (i.e., the target droplets32are unstable), the EUV scanner13is configured to suspend the lithography exposing process in response to the control signal CTRL with a low logic level, thereby preventing the unexpected debris39generation. Therefore, the usable lifespan of the collector36is increased. For example, the usable lifespan is increased from 30 days to 180 days. It should be appreciated that the logic levels of the data T_vane, X-int_σ, and UNSTABLE_PD and the control signal CTRL can be changed, and the invention should not be limited by the embodiments.

Embodiments for light source, EUV lithography system, and method for generating EUV radiation are provided. A target droplet generator30is configured to provide a plurality of target droplets32to a source vessel72. A measuring device50is configured to measure the temperature TEMP of the source vessel72, the droplet positions of the target droplets32and the pulse lasers66,66A and66B. Furthermore, the process parameter regarding to a standard deviation σ on the droplet positions of the target droplets32is obtained. If the temperature TEMP of the source vessel72does not exceed a temperature threshold value TH_temp or is within a temperature range, the controller40determines that the target droplets are stable, and a plurality of pulse lasers (e.g.,66ofFIG. 2 or 66A and 66BofFIG. 3) are generated to irradiate the target droplets32in the source vessel72. If the standard deviation σ does not exceed a standard deviation threshold value TH_SD or is within a standard deviation range, the controller40determines that the target droplets are stable, and the pulse lasers are also generated to irradiate the target droplets32in the source vessel72. However, if the temperature TEMP of the source vessel exceeds the temperature threshold value TH_temp or is outside the temperature range and the standard deviation σ exceeds the standard deviation threshold value TH_SD or is outside the standard deviation range, the controller40determines that the target droplets32are unstable, and no pulse lasers are generated to irradiate the target droplets32in the source vessel72. Thus, no unexpected debris39is generated when the controller40determines that the target droplets32are unstable, thereby contamination on the surface of the collector36is decreased. Therefore, the lifespan of the collector36is improved and then extended further. Also, the number of times that the collector36needs to be swapped is decreased. Furthermore, dose control and exposure quality can be secured due to more stable droplet conditions in the lithography exposing processes. Therefore, tool availability, productivity and reliability are also improved in the lithography exposing processes.

In some embodiments, a light source for extreme ultraviolet (EUV) radiation is provided. The light source includes a target droplet generator, a laser generator, a measuring device, and a controller. The target droplet generator is configured to provide a plurality of target droplets to a source vessel. The laser generator is configured to provide a plurality of first laser pulses according to a control signal to irradiate the target droplets in the source vessel, so as to generate plasma as the EUV radiation. The measuring device is configured to measure process parameters including temperature of the source vessel, droplet positions of the target droplets, and beam sizes and focal points of the first laser pulses. The controller is configured to provide the control signal according to at least two of the process parameters.

In some embodiments, an extreme ultraviolet (EUV) lithography system is provided for performing a lithography exposing process. The EUV lithography system includes an EUV scanner and an light source. The light source is configured to provide EUV radiation to the EUV scanner in response to a control signal. The light source includes a collector, a target droplet generator, a laser generator and a controller. The collector is configured to collect the EUV radiation and direct the EUV radiation to the EUV scanner. The target droplet generator is configured to provide a plurality of target droplets to a source vessel. The laser generator is configured to provide a plurality of first laser pulses according to the control signal to irradiate the target droplets in the source vessel, so as to generate plasma as the EUV radiation. The controller is configured to provide the control signal to the laser generator to stop providing the first laser pulses, and to provide the control signal to the EUV scanner to suspend the lithography exposing process according to at least two process parameters including temperature of the source vessel, the droplet positions of the target droplets, and beam sizes and focal points of the first laser pulses.

In some embodiments, a method for generating extreme ultraviolet (EUV) radiation is provided. A plurality of target droplets are provided to a source vessel. A temperature of the source vessel is measured. A droplet position of each of the target droplets is measured, and a standard deviation or an average value of the droplet positions of the target droplets is obtained. It is determined whether the target droplets are stable according to the temperature of the source vessel and the standard deviation or the average value of the droplet positions of the target droplets. A plurality of first laser pulses are provided to irradiate the target droplets in the source vessel when the target droplets are stable, so as to generate plasma as the EUV radiation.