CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Application No. 62/984,433, filed March 3, 2020 and titled CONTRIOL SYSTEM FOR A LIGHT SOURCE, and which is incorporated herein in its entirety by reference.

TECHNICAL FIELD

[0002] This disclosure relates to a control system for a light source, for example, a deep ultraviolet light source.

BACKGROUND

[0003] Photolithography is the process by which semiconductor circuitry is patterned on a substrate such as a silicon wafer. An optical source generates deep ultraviolet (DUV) light used to expose a photoresist on the wafer. DUV light may include wavelengths from, for example, about 100 nanometers (nm) to about 400 nm. Often, the optical source is a laser source (for example, an excimer laser) and the DUV light is a pulsed laser beam. The DUV light from the optical source interacts with a projection optical system, which projects the beam through a mask onto the photoresist on the silicon wafer. In this way, a layer of chip design is patterned onto the photoresist. The photoresist and wafer are subsequently etched and cleaned, and then the photolithography process repeats.

SUMMARY

[0004] In one aspect, a light source includes: a light-generation apparatus configured to be in an active state during a first time period, an idle state during a second time period, and the active state during a third time period; and a control system. The first time period occurs before the second time period, and the second time period occurs before the third time period. An excitation signal is applied to the light-generation apparatus in the active state and is not applied to the light-generation apparatus in the idle state. The control system is configured to estimate a property of the excitation signal for application to the light-generation apparatus during the third time period based on the duration of the second time period and a value of the property during the first time period.

[0005] Implementations may include one or more of the following features.

[0006] The light-generation apparatus may include: a discharge chamber configured to hold a gaseous gain medium; and a plurality of electrodes in the discharge chamber. The excitation signal may include a voltage signal applied to at least one of the plurality of electrodes, and the property of the excitation signal may include a magnitude of the voltage signal. The voltage signal may include a time-varying voltage signal. The control system may include a memory module configured to store at least one value representing the magnitude of the voltage signal applied to the electrodes in the first time period. The value of the property during the first time period may include a minimum voltage applied to the electrodes during the first time period. The control system may be configured to estimate the property of the excitation signal for application to the light-generation apparatus during the third time period based on the duration of the second time period, the minimum voltage applied to the electrodes during the first time period, and an adaptive parameter associated with the first time period. The gaseous gain medium may include a gain medium configured to emit deep ultraviolet (DUV) light in response to the voltage signal being applied to at least one of the electrodes. The gaseous gain medium may include argon fluoride (ArF), krypton fluoride (KrF), or xenon chloride (XeCl).

[0007] The control system also may be configured to determine an error metric based on the estimated property of the excitation signal and an actual value of the property of the excitation signal applied to the light-generation apparatus during the third time period. The control system may be configured to update a value of an adaptive parameter based on the error metric. The control system may be configured to update a value for each of a plurality of adaptive parameters, and each of the plurality of adaptive parameters may be associated with a different duration of the second time period. [0008] The control system may be configured to determine whether to initiate a warm-up procedure based on the estimated property of the excitation signal. If the warm-up procedure is initiated, the control system may be configured to determine a warm-up procedure metric related to a duration of the warm-up procedure. The warm-up procedure metric may be a number of times to excite the light- generation apparatus during the warm-up procedure.

[0009] The light-generation apparatus may include a master oscillator and a power amplifier.

[0010] The light-generation apparatus may include a single discharge chamber.

[0011] The light-generation apparatus may include a plurality of discharge chambers, and each of the discharge chambers may be configured to emit a pulsed light beam toward a beam combiner.

[0012] In another aspect, a controller for a light source includes a control system. The control system is configured to: access information related to a duration of an idle period of the light source; access information related to a value of a property of an excitation signal applied to the light source during a time period that occurred prior to the idle period; and estimate an updated value of the property of the excitation signal based on the duration of the idle period and the value of the property of the excitation signal during the time period that occurred prior to the idle period.

[0013] Implementations may include one or more of the following features.

[0014] The control system may be configured to apply the excitation signal with the updated value of the property to the light source after the idle period. The control system may be configured to determine an error metric based on the estimated updated value of the property and an actual value of the property of the excitation signal applied to the light-generation apparatus after the idle period.

The control system may be configured to update a value of an adaptive parameter based on the error metric. The control system may be configured to update a value for each of a plurality of adaptive parameters, each of the plurality of adaptive parameters associated with a different duration of the second time period.

[0015] The control system also may be configured to determine whether to initiate a warm-up procedure for the light source based on the estimated updated value of the property.

[0016] The control system may be configured to access the information related to the duration of an idle period of a light source and the information related to the value of a property of an excitation signal during the time period that occurred prior to the idle period from a computer-readable memory module.

[0017] The control system also may include: a computer-readable memory module; and one or more electronic processors coupled to the computer-readable memory module.

[0018] In another aspect, a method includes: accessing information related to a duration of an idle period of a light source; accessing information related to a value of a property of an excitation signal applied to the light source during a time period that occurred prior to the idle period; and estimating an updated value of the property of the excitation signal based on the duration of the idle period and the value of the property of the excitation signal during the time period that occurred prior to the idle period.

[0019] Implementations of any of the techniques described above may include a DUV light source, a system, a method, a process, a device, or an apparatus. The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims.

DRAWING DESCRIPTION

[0020] FIGS. 1A-1C are block diagrams of a light source at three different times.

[0021] FIGS. 2A-2C are block diagrams of another light source at three different times.

[0022] FIG. 3 is a flow chart of a process for estimating a value of a property of an excitation signal. [0023] FIG. 4 is a flow chart of a process for determining whether to initiate a warm-up procedure. [0024] FIG. 5A is a plot of idle time as a function of time.

[0025] FIG. 5B is a plot of a voltage metric as a function of time.

[0026] FIG. 5C is a plot of voltage applied to electrodes of a first DUV light source as a function of time.

[0027] FIG. 5D is a plot of voltage applied to electrodes of a second DUV light source as a function of time.

[0028] FIG. 5E is a plot of an error metric as a function of idle time for the second DUV light source of FIG. 5D.

[0029] FIG. 5F is a plot of the error metric as a function of idle time for the second DUV light source of FIG. 5D.

[0030] FIG. 6 is a block diagram of a photolithography system.

[0031] FIG. 7A is a block diagram of an optical lithography system.

[0032] FIG. 7B is a block diagram of a projection optical system for the optical lithography system of FIG. 7 A.

DETAILED DESCRIPTION

[0033] Each of FIGS. 1 A-1C is a block diagram of a light source 100 at a different time. FIG. 1A shows the light source 100 at a time tl. FIG. IB shows the light source 100 at a time t2. FIG. 1C shows the light source 100 at a time t3. The time tl occurs during a first time period, the time t2 occurs during a second time period, and the time t3 occurs during a third time period. The first time period occurs before the second time period, and the second time period occurs before the third time period. Three time periods are shown for illustration purposes. However, the light source 100 may operate over more than three time periods.

[0034] The light source 100 includes a light-generation apparatus 110 and a control system 150, which estimates a property of an excitation signal 109. The excitation signal 109 may be generated by the control system 150 or by a separate apparatus (such as a voltage source or a current source) that is controlled by the control system 150. The excitation signal 109 is any type of signal that is sufficient to cause the light-generation apparatus 110 to generate a light beam 105. For example, the excitation signal 109 may be a signal that is applied to an excitation mechanism (such as the excitation mechanism 211 of FIGS. 2A-2C, the electrodes 611 A and 61 lb of FIG. 6, or the electrodes 711-la, 711- lb of FIG. 7) in the light-generation apparatus 110. The light beam 105 may be, for example, a pulsed or continuous wave laser beam. The light-generation apparatus 110 may be a deep ultraviolet (DUV) optical system that emits a pulsed light beam in the DUV range (for example, wavelengths from about 100 nanometers (nm) to about 400 nm). In some implementations, the light- generation apparatus 110 emits a burst of pulses during each active period. A burst of pulses includes hundreds or thousands of pulses of light.

[0035] The excitation signal 109 is applied to the light-generation apparatus 110 or a component of the light-generation apparatus 110 when the light-generation apparatus 110 is in an active state. The light-generation apparatus 110 produces the light beam 105 during the active state. The light- generation apparatus 110 also has an inactive or idle state. While in the inactive or idle state, the excitation signal 109 is not applied to the light-generation apparatus 110 or its components, and the light-generation apparatus 110 does not produce the light beam 105. During the idle or inactive state, the light-generation apparatus 110 may be, for example, powered off or turned off, or powered on and not producing any light. In the example of FIGS. 1A-1C, the light-generation apparatus 110 is in the active state in the first and third time periods and the idle state in the second time period. The temporal duration of the second time period is also referred to as the idle time, and the second time period is also referred to as the idle period.

[0036] As discussed in greater detail below, the control system 150 estimates a property of the excitation signal 109 to apply to the light-generation apparatus 110 during the third time period based on the duration of the idle period and a value of the property of the excitation signal 109 that was applied to the light-generation apparatus 110 during a prior active time period (for example, the first time period). The property may be, for example, an amplitude of a voltage and/or current signal provided to an excitation mechanism in the light-generation apparatus 110.

[0037] By determining the property of the excitation signal 109 using the idle time and a value of the property during the first time period, the control system 150 improves the performance of the light source 100. For example, some prior techniques determine the excitation signal based only on the idle time. These prior techniques, for example, use a pre-determined excitation signal if the idle time is greater than a pre-determined threshold and or cause the light-generation apparatus 110 to enter into a warm-up mode if the idle time is greater than the pre-determined idle time threshold.

[0038] On the other hand, the control system 150 implements a technique that considers a prior value of the property of the excitation signal 109 to estimate an updated value of the excitation signal 109. The approach employed by the control system 150 results in a more accurate determination of the property of the excitation signal 109 to be applied in the third time period and improves the use of the warm-up procedure. For example, the control system 150 reduces or eliminates unnecessary performance of the warm-up procedure while also helping to ensure that the warm-up procedure is invoked appropriately.

[0039] Moreover, the control system 150 also may determine an adaptive parameter that accounts for changes in one or more characteristics of the light-generation apparatus 110 over time. For example, the energy efficiency of the light-generation apparatus 110 may change over time. The energy efficiency is a relationship between the amount of energy that is provided to the light-generation apparatus 110 to produce light having a certain amount of energy. For example, in implementations in which the excitation signal 109 is a voltage signal that is applied to electrodes in the light- generation apparatus 110, as the energy efficiency of the light-generation apparatus 110 decreases, a greater amount of voltage is needed to produce the light beam 105. The energy efficiency of the light- generation apparatus 110 also may decrease during the idle time. As discussed in greater detail below, the adaptive parameter may estimate and track changes in the energy efficiency of the light- generation apparatus 110. By accounting for characteristics of the light-generation 110 that change over time, the control system 150 improves the accuracy of the estimate of the property of the excitation signal 109. [0040] Referring to FIGS. 2A-2C, block diagrams of a light source 200 are shown. The light source 200 is an implementation of the light source 100. Each of FIGS. 2A-2C shows the light source 200 at a different time. The light source 200 is shown in the active state in FIGS. 2 A and 2C and in the idle state in FIG. 2B. The light source 200 includes a light-generation apparatus 210 and a control system 250. The light-generation apparatus 210 includes an excitation mechanism 211 and a gain medium 212

[0041] The light-generation apparatus 210 produces a light beam 205 in the active state. The excitation signal 209 is applied to the light-generation apparatus 210 and excites the excitation mechanism 211 when the light-generation apparatus 210 is in the active state (FIGS. 2 A and 2C). The light-generation apparatus 210 also has an inactive or idle state (FIG. 2B). When the light-generation apparatus 210 is in the idle state, the excitation signal 209 is not applied to the light-generation apparatus and does not excite the excitation mechanism 211. In the example of FIGS. 2A-2C, the light source 200 is in the active state during a first time period (which includes the time tl) and a third time period (which includes the time t3). The light source 200 is in the idle state during a second time period (which includes the time t2). The duration of the second time period is also referred to as the idle time. Three time periods are shown for illustration purposes. However, the light source 200 may operate over more than three time periods.

[0042] The excitation mechanism 211 excites the gain medium 212 in response to the excitation signal 209. The gain medium 212 is any medium suitable for producing a light beam at the wavelength, energy, and bandwidth required for the application. For example, the gain medium 212 may be a gas, a crystal, a glass, a semiconductor, or a liquid.

[0043] The excitation mechanism 211 is any mechanism capable of exciting the gain medium 212. For example, the excitation mechanism 211 may be a plurality of electrodes that excite a gaseous gain medium. The excitation signal 209 may be, for example, an electrical signal (such as a voltage signal) or a command signal that causes an additional element (such as a voltage or current source) to generate an electrical signal that is provided to the excitation mechanism 211. The excitation signal 209 may be a time- varying direct current (DC) electrical signal or an alternating current (AC) electrical signal, such as a sine wave voltage signal or a square wave voltage signal. In these implementations, the property of the excitation signal 209 may be the maximum amplitude of the time-varying signal, the average amplitude of the time-varying signal, the minimum amplitude of the time-varying signal, the frequency of the time-varying signal, the duty cycle of the time-varying signal, and/or or any other property related to the time-varying signal.

[0044] The control system 250 estimates the property of the excitation signal 209. The property may be, for example, an amplitude, frequency, and or duty cycle of a voltage and or current signal provided to the excitation mechanism 211 in the light-generation apparatus 210. The control system 250 estimates the property of the excitation signal 209 based on a prior or earlier idle time and a prior or earlier value of the property of the excitation signal 209. To estimate the property of the excitation signal 209, the control system 250 may implement a process such as the process 300 discussed with respect to FIG. 3. The control system 250 also may implement other processes, such as the process 400 discussed with respect to FIG. 4, as stand-alone processes or with the process 300. Moreover, the control system 250 may be used with any type of optical source. For example, the control system 250 may be used with the photolithography system 600 (FIG. 6) or the optical lithography system 700 (FIG. 7).

[0045] The control system 250 includes an electronic processing module 251, a computer-readable memory module 252, and an I/O interface 253. The electronic processing module 251 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 processing module 251 may include any type of electronic processor. The electronic processor or processors of the electronic processing module 251 execute instructions and access data stored on the memory module 252. The electronic processor or processors are also capable of writing data to the memory module 252.

[0046] The memory module 252 may be volatile memory, such as RAM, or non-volatile memory. In some implementations, and the memory module 252 includes non-volatile and volatile portions or components. The memory module 252 may store data and information that is used in the operation of the control system 250. For example, the memory module 252 may store information related to the idle period and information related to the value of the property of the excitation signal 209 applied to the light-generation apparatus 210 during one or more time periods that occurred in the past and prior to the most recent idle time. The memory module 252 may store one or more values associated with the excitation signal 209 applied during the active period that occurred immediately prior to the most recent idle period. For example, the excitation signal 209 may be a voltage signal or a signal that specifies voltages to be produced by a voltage source. In this example, the memory module 252 may store the average, minimum, and maximum values of the voltage signal during the most recent active period. The memory module 252 also may store information received from the light source 200 and/or the light-generation apparatus 210.

[0047] The I/O interface 253 is any kind of interface that allows the control system 250 to exchange data and signals with an operator, the light-generation apparatus 210, and or an automated process running on another electronic device. For example, in implementations in which rules or instructions stored on the memory module 252 may be edited, the edits may be made through the I/O interface 253. In another example, the I/O interface 253 receives data from the light-generation apparatus 210 and/or from hardware and/or software subsystems of the light-generation apparatus 210. For example, the light-generation apparatus 210 may provide the control system 250 with the idle time and other information about the light-generation apparatus 210 through the I/O interface 253. The I/O interface 253 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 253 also may allow communication without physical contact through, for example, an IEEE 802.11, Bluetooth, or a near- field communication (NFC) connection.

[0048] The control system 250 is coupled to the light-generation apparatus 210 through a data connection 254. The data connection 254 may be a physical cable or other physical data conduit (such as a cable that supports transmission of data based IEEE 802.3), a wireless data connection (such as a data connection that provides data via IEEE 802.11 or Bluetooth), or a combination of wired and wireless data connections. The data that is provided over the data connection may be set through any type of protocol or format. The data connection 254 is connected to the light-generation apparatus 210 at respective communication interfaces (not shown). The communication interfaces may be any kind of interface capable of sending and receiving data. For example, the data interfaces may be an Ethernet interface, a serial port, a parallel port, or a USB connection. In some implementations, the data interfaces allow data communication through a wireless data connection.

For example, the data interfaces may be an IEEE 811.11 transceiver, Bluetooth, or an NFC connection. The control system 250 may be connected to systems and/or components within the light-generation apparatus 210. For example, the control system 250 may be connected to the excitation mechanism 211.

[0049] In the example of FIGS. 2A-2C, the control system 250 is shown as being separate from the light-generation apparatus 210 and connected via the data connection 254. However, in some implementations, the control system 250 is implemented as part of the light-generation apparatus 210 such that the light-generation apparatus 210 and the control system 250 are part of a single, integrated package (for example, enclosed within the same housing). In these implementations, the data connection 254 may be a data path that allows communication between software modules with one of the software modules implementing aspects of the control system 250 and the other of the software modules implementing other functionality for the light-generation apparatus 210.

[0050] FIG. 3 is a flow chart of a process 300. The process 300 is an example of a process for estimating a value of a property of the excitation signal 209. The process 300 may be performed by a control system that is associated with a light-generation apparatus. For example, the process 300 may be performed by the control system 150 (FIG. 1) or the control system 250 (FIGS. 2A-2C). In the discussion below, the process 300 is discussed with respect to the control system 250 and the light- generation apparatus 210. For example, and referring to FIGS. 2A-2C, the process 300 may be implemented as a collection of instructions (for example, a computer program or computer software) that are stored on the memory module 252 and performed by one or more electronic processors in the electronic processing module 251.

[0051] Information relating to a duration of an idle period of the light-generation apparatus 210 is accessed (310). The duration of the idle period (also referred to as the idle time) is the length of the continuous time period during which the light-generation apparatus 210 is in the idle or inactive state. The idle time relates to an idle period that occurred in the past and may be the duration of the most recent idle period. For example, the idle time may be the duration of the second time period that includes the time t2 shown in FIG. 2B.

[0052] The idle time may be stored on the memory module 252. In these implementations, the control system 250 accesses the idle time value from the memory module 252. The idle time value is not necessarily accessed from the memory module 252. For example, in some implementations, the idle time is provided by an operator through the I/O interface 253. Moreover, the information relating to the idle time may be a numerical value that represents the idle time, or the information may take other forms. For example, the information relating to the idle time may include a time at which the idle period began and a time at which the idle time ended. In these implementations, the control system 250 is configured to determine the idle time based on the accessed information.

[0053] Information relating to a value of a property of the excitation signal 209 applied to the light- generation apparatus 210 during an active period that occurred prior to the idle period is accessed (320). For example, the information may be the maximum voltage applied to the excitation mechanism 211 during the most recent active period. The information may include more than one value of the property during the prior active period. For example, the information may include a maximum and minimum voltage applied to the excitation mechanism 211 during the prior active period. In another example, the information may include a time series that represents the voltage applied to the excitation mechanism 211 at regular intervals during the active period. The information relating to the value of the property of the excitation signal 209 may be accessed from the memory module 252 or the information may be accessed through the I/O interface 253.

[0054] An updated value of the property of the excitation signal 209 is estimated based on the duration of the idle period and the value of the property of the excitation signal 209 during the time period that occurred prior to the idle period (330). The discussion below relates to an example in which the excitation signal 209 is a time-varying voltage signal that is applied to the excitation mechanism 211. The property of the excitation signal 209 that is estimated is the maximum amplitude of voltage (V ) to be applied to the excitation mechanism 211 after the idle period ends. The excitation signal 209 produces a burst of pulses that includes many individual pulses (for example, hundreds or thousands). Each pulse is created by a corresponding pulse in the time- varying voltage excitation signal 209. In the discussion below, the maximum voltage (V _max ) is the greatest magnitude of voltage applied to the excitation mechanism 211 to form a pulse of light during an active period and the minimum voltage (V _mm ) applied to the excitation mechanism 211 to form a pulse of light during an active period. The maximum voltage ( V _max ) usually occurs relatively early in a particular burst, while the light-generation apparatus 210 is experiencing transient effects related to beginning to produce the light beam 205 after an idle period. The minimum voltage (Vi _m ) usually occurs later in the burst, after the transient effects have ended and the light-generation apparatus 210 is in the steady state.

[0055] The value of the voltage signal of the excitation signal 209 to be applied to the excitation mechanism 211 after the idle time ends may be estimated as shown in Equation (1): nax(i) = Vmint ^~ 1) + a(i - 1)DG(ί) Equation (1), where / is an integer value that indexes the active periods of the light-generation apparatus 210,

V _max (i) is the estimated value of the maximum voltage of the excitation signal 209 for use in the /th active period, V _mm (i-1 ) is the value of the minimum voltage of the excitation signal 209 during the (i- 1 )th active period, a( i-1 ) is the value of an adaptive parameter a associated with the ( i-1 )th active period, and AT(i) is the idle time of the idle period that immediately preceded the /th active period.

The /th active period is the current active period, and the (i-1 )th active period is the active period immediately preceding the current active period. The idle period having the idle time AT(i) is between the /th active time period and the ( i-1 )th active period.

[0056] For example, the current or /th active period may be the third time period that includes t3 as shown in FIG. 2C, and the prior active time period may be the first time period that includes tl as shown in FIG. 2A. Continuing this example, the idle time is the second time period that includes t2 as shown in FIG. 2B. Thus, in this example, V _mm (i-1 ) is the minimum voltage applied to the excitation mechanism 211 during the first time period (and is based on the information accessed at (320)), AT(i) is the duration of the second time period or the idle time (and is based on the information accessed at (310)), and the estimated property of the excitation signal is the maximum voltage to be applied to the excitation mechanism 211 during the third time period.

[0057] The discussion above relates to an example in which the excitation signal 209 is a time- varying voltage signal that is applied to the excitation mechanism 211, and the metric of the excitation signal 209 that is estimated is the value of the maximum voltage ( V _max ). However, other metrics may be estimated. For example, in some implementations, a constant voltage is applied to the excitation mechanism 211, and the output energy of the light beam 205 is estimated after the idle period, based on knowledge of the idle time and the output energy of the light beam 205 before the idle time. In other words, the approach discussed above may be used to predict generated optical energy in the light beam 205 after the idle period.

[0058] The adaptive parameter a(i-l ) is the value of the adaptive parameter associated with the first time period. The value of the adaptive parameter associated with the first time period may be stored on the memory module 252 or provided to the control system 250 via the I/O interface 253. The process 300 may end, return to (310), or continue to (340).

[0059] In some implementations, the adaptive parameter a is updated for each active period. In these implementations, an error metric is determined (340). The error metric is based on the estimated property of the excitation signal 209 (as estimated in 330) to be applied to the excitation mechanism 211 during an active period and an actual value of the property of the excitation signal 209 applied during that active period. The error metric may be determined as shown in Equation (2): e _V (0 = nax (0 - nax (0 Equation (2), where i is an integer value that indexes the active periods of the light-generation apparatus 210, e _v (i) is the error metric associated with the ith active period, V _maX ( i ) is the actual value of the applied maximum voltage of the excitation signal 209 applied to the light-generation apparatus 210 during the ith active period, V _max (i) is the estimated maximum voltage of the excitation signal 209 for the ith active period.

[0060] The value of the adaptive parameter may be updated (350). The adaptive parameter is any parameter that represents a characteristic of the light-generation apparatus 210 that changes over time. For example, the adaptive parameter may be an estimate of the energy efficiency of the light- generation apparatus 210. The energy efficiency relates input energy (voltage provided to the excitation mechanism 211) to output energy (optical energy in the light beam 205). The relationship may be approximately linear. The slope of the linear relationship with respect to or relative to the idle time may be used as the adaptive parameter.

[0061] The value of the adaptive parameter (a) may be updated as shown in Equation (3): a(i) = a(i — 1) + hb _n (i) Equation (3), where i is an integer value that indexes the active periods of the light-generation apparatus 210, h is a step sized or weighting factor, and e _v ( i ) is the error metric of the ith active period. In the example of Equation (3), i is the current active period (for example, the third time period that includes the time t3 in FIG. 2C), and i-1 is the immediately preceding active period (for example, the first time period that includes the time tl in FIG. 2A).

[0062] The step size or weighting factor h remains constant over time unless it is intentionally changed by the operator of the light source 200. The step size or weighting factor h determines how much the error value e _v (i) affects the adaptive parameter a. A relatively large value of the step size or weighting factor h results in a larger change in the adaptive parameter a as compared to a relatively small value. The step size or weighting factor h may be set by the manufacturer when the light- generation apparatus 210 is assembled and stored on the memory module 252 and/or an operator may update the step size or weighting factor via the I/O interface 253. [0063] The updated value of the adaptive parameter a may be stored on the memory module 252 in association with the ith active period such that the control system 250 may access the value of the adaptive parameter for later use.

[0064] In some implementations, more than one instance of the adaptive parameter is used, with each instance being associated with a particular idle time or range of idle times. For example, two (2), five (5), seven (7), or more instances of the adaptive parameter a may be initialized and then updated based on Equations (

Equation (4), Equation (5), cc _j (i) = a _j (i - 1) + h _]bn (ΐ) Equation (6), where j is one instance and a, is the adaptive parameter corresponding to the /th instance, and DT is the idle time range associated with the f ^h instance. The idle time ranges are not necessarily the same. For example, in one implementation, five (j=5) instances of the adaptive parameter «,· are initialized, with one instance of the adaptive parameter «,· for each of the following idle time ranges DΊ(ί): 0 to 60 seconds (s), 61 to 120 s, 121 to 600 s, 601 to 3600 s, and greater than 3600 s. Thus, if the idle time is 60 seconds or less, a(l) is used as «,·. If the idle time is 3600 seconds or greater, a(5) is used as «,·. [0065] Using more than one instance of the adaptive parameter «,· improves the overall accuracy of the process 300. For example, the energy efficiency of the light-generation apparatus 210 generally decreases as the idle time increases. Although the relationship between the energy efficiency and the idle time is often linear for relatively short idle times (for example, idle times less than 10 minutes), for relatively long idle times, the energy efficiency decreases relative to the idle time in a manner that is not necessarily linear. To account for this, a plurality of instances of the adaptive parameter a _· , each associated with a different range of idle times, may be used. This approach may result in a more efficient process and ensure accurate results for longer idle times because the ranges of idle times may be selected such that the energy efficiency is linear or approximately linear over each of the ranges such that Equation (3) may be used to update the various adjustable parameters.

[0066] In some implementations, the control system 250 intentionally does not update the adaptive parameter under certain conditions even if the control system 250 updates the adaptive parameter under other conditions. For example, the light-generation apparatus 210 may have one or more calibration modes and/or maintenance modes in which the light-generation apparatus 210 is in an active state but performs under conditions that are not reflective of typical use conditions. If the adaptive parameter is updated during the calibration and/or maintenance modes, the values of the adaptive parameter may become inaccurate and may affect the accuracy of calculations of the property of excitation signal after the light-generation apparatus 210 exits the maintenance and or calibration mode. Thus, the control system 250 may be configured to skip portions of the process 300, for example, (340) and (350), when the light-generation apparatus 210 is in a maintenance and or calibration mode. The control system 250 may receive an indication of entry into and exit from the maintenance and/or calibration mode via the I/O interface 253 from the light-generation apparatus 210 or from an operator.

[0067] Referring to FIG. 4, a flow chart of a process 400 is shown. The process 400 may be performed by a control system that is associated with a light-generation apparatus. For example, the process 400 may be performed by the control system 150 (FIG. 1) or the control system 250 (FIG. 2). In the discussion below, the process 400 is discussed with respect to the control system 250 and the light-generation apparatus 210. The process 400 may be implemented as a collection of instructions (for example, a computer program or computer software) that are stored on the memory module 252 and performed by one or more electronic processors in the electronic processing module 251.

[0068] The process 400 is an example of a process for determining whether to initiate a warm-up procedure. When the light-generation apparatus 210 begins producing the light beam 205 immediately after an idle period, one or more properties (for example, wavelength, bandwidth, energy, and or temporal pulse duration) of the light beam 205 may not meet a specification associated with an application that uses the light beam 205. In such a situation, the light-generation apparatus 210 may be considered to be in a cold start condition. In a cold start condition, the light-generation apparatus 210 produces the light beam 205, but the light beam 205 is inadequate for the application. The warm-up procedure is applied to the light-generation apparatus 210 to remedy the cold start condition. During the warm-up procedure, the excitation signal 209 is provided to the excitation mechanism 211, but the light beam 205 is not provided to a downstream tool or system (or is not used by the downstream tool or system) until the light beam 205 satisfies the performance specification.

For example, the light beam 205 may be blocked or diverted while the warm-up procedure is performed.

[0069] Some prior techniques initiate the warm-up procedure solely based on the idle time. For example, these prior techniques may initiate the warm-up procedure if the idle time immediately prior to the active period exceeds a threshold. However, because idle time alone does not always provide an accurate indication of whether or not the warm-up procedure should be initiated, such an approach may result in the warm-up procedure being invoked unnecessarily for some relatively long idle times. Moreover, such an approach may result in the warm-up procedure erroneously not being performed for some relatively short idle times. FIGS. 5A and 5B show an example of how relying solely on idle time may lead to an incorrect determination of whether or not to initiate the warm-up procedure. On other hand, and as discussed below, the control system 250 implements the process 400, which uses the estimated value of the property of the excitation signal 209 to determine whether or not to initiate the warm-up procedure.

[0070] An estimated property of the excitation signal 209 is analyzed to determine whether to initiate the warm-up procedure (410). The estimated property of the excitation signal 209 may be determined at (330) of process 300 and passed to the process 400 (for example, through a function call). In some implementations, the process 400 is performed independently of the process 300. In these implementations, the estimated property of the excitation signal 209 may be provided by an operator of the light source 200 through the I/O interface 253 or read from the memory module 252.

[0071] The estimated value of the property of the excitation signal 209 may be analyzed by comparing the estimated value to a threshold. For example, the estimated value may be an estimated maximum voltage value (V _max ). In this example, higher values of the estimated maximum voltage indicate a relatively low efficiency and that the warm-up procedure should be performed. On the other hand, a relatively low estimated maximum voltage indicates a relatively high efficiency without the need for a warm up procedure.

[0072] At (420), the process 400 determines whether or not to initiate the warm-up procedure based on the analysis performed at (410). If the warm-up procedure is not performed, the process 400 ends or returns to (330) of FIG. 3. If the warm-up procedure is performed, a warm-up procedure metric is determined (430). The warm-up procedure metric may be, for example, one or more characteristics of the excitation signal 209 that is to be applied to the excitation mechanism 211 during the warm-up procedure. For example, the metric may indicate a temporal duration of the excitation signal 209 and/or a number of voltage pulses to apply to the excitation mechanism 211 during the warm-up procedure. In some implementations, the number of voltage pulses may be calculated based on the estimated maximum voltage value (V _max ) (for example, as estimated by Equation 1) and a voltage that is desired after the warm-up procedure. The number of voltage pulses may be estimated by comparing the voltage that is desired after the warm-up procedure to an actual voltage that is achieved after the warm-up procedure. Specifically, a voltage error between the voltage that is desired after the warm-up procedure and the actual voltage that is achieved after the warm-up procedure may be updated adaptively to estimate the number of voltage pulses to be applied to the excitation mechanism 211 during the next warm-up procedure. In some implementations, the warm-up procedure metric is a pre -determined value that is stored on the memory module 252.

[0073] The excitation signal 209 with the determined properties is applied to the excitation mechanism 211 (440) to perform the warm-up. After the warm-up procedure is completed, the process 400 ends or returns to the process 300.

[0074] FIGS. 5A and 5B illustrate that relying on the idle time alone is insufficient to accurately detect a cold start condition and to accurately determine whether to initiate the warm-up procedure. FIG. 5A is a plot of idle time in seconds as a function of time. FIG. 5B is a plot of a voltage metric as a function of time. In the example of FIG. 5B, the voltage metric is the voltage that is applied to the electrodes immediately after the idle period. FIGS. 5A and 5B have the same X-axis.

[0075] The light source has a first active period ta_l. The light source is in a first idle period ti_l after the first active period. The light source is in a second active period ta_2 after the first idle period. The light source is in a second idle period ti_2 after the second active period. During the first active period, the duty cycle of the light beam produced by the light source is relatively low, as shown by the open circle symbols. During the second active period, the duty cycle of the light beam is higher, as shown by the solid circle symbols, which are closer together in time. This indicates that the excitation mechanism is being excited more rapidly in the second active than in the first active period. The first idle time (tl on FIGS. 5A and 5B) is less than the second idle time (t2 on FIGS. 5A and 5B). However, the first voltage metric (AVI) is greater than the second voltage metric (AV2), and a process such as the process 400 would determine that a warm-up procedure would be beneficial after the first idle time but not the second idle time. However, an approach that only considered the idle time and that compared the idle times to a threshold having a value between the first idle time and the second idle time, the opposite result would occur. Thus, the traditional approach that only considers idle time would initiate an unnecessary warm-up procedure after the second idle time and would not initiate a beneficial warm-up procedure after the first idle time. Accordingly, a process such as the process 400, which considers an estimate of the value of a property of the excitation signal (such as the amount of voltage to apply immediately after the idle period) allows the warm-up procedure to be more used in a more effective manner.

[0076] FIGS. 5C and 5D show examples of actual measurements for a process such as the process 300. FIG. 5C is a plot of voltage applied to electrodes of a first DUV light source as a function of time. FIG. 5D is a plot of voltage applied to electrodes of a second DUV light source as a function of time. In each of FIG. 5C and 5D, the actual voltage applied to the electrodes is represented by the line with the open circle symbols that is labeled as 596. Each point of data in the series labeled as 596 is the maximum burst average voltage over a plurality of consecutive bursts. The value of the voltage predicted at element (330) of the process 300 using a single adaptive parameter a is represented by the line with the open square symbols that is labeled as 594. The value of the voltage predicted at element (330) of the process 300 using a plurality of instances of the adaptive parameter a is represented by the line with the x symbols that is labeled as 595. In both implementations, the process 300 estimates the value of the property of the excitation signal with a reasonable accuracy, and the implementation with the plurality of instances of the adaptive parameter a results in improved accuracy in some situations.

[0077] FIGS. 5E and 5F show the error metric determined at (340) of the process 300 of FIG. 3 as a function of idle time in seconds. The data show in FIGS. 5E and 5F were simulated using the second DUV light source discussed with respect to FIG. 5D. In FIGS. 5E and 5F, the open circles represent error metrics for an implementation of the process 300 in which a single adaptive parameter a was used, and the x symbols represent error metrics for an implementation in which a plurality of instances of the adaptive parameter a was used. As shown in FIG. 5E, for idle times of about 18 seconds or less, the single adaptive parameter and multiple adaptive parameter approaches predict the value of the property of the excitation signal with similar accuracy (within about 2%). As shown in FIG. 5F, for idle times greater than about 50 seconds, the multiple adaptive parameter approach archives better accuracy.

[0078] The examples of FIGS. 3 and 4 are discussed with respect to the light-generation apparatus 210. However, the control system 250 may be used with other light sources. For example, the control system 250 may be used with a DUV laser that includes a single discharge chamber that encloses a gaseous gain medium and electrodes configured to excite the gain medium. In these examples, the control system 250 estimates the voltage to apply to the electrodes for active periods that occur immediately after an idle period. In another example, the control system 250 may be used with a DUV light source that includes more than one discharge chamber, and each discharge chamber encloses a gaseous gain medium and electrodes configured to excite the medium. In these examples, the control system 250 estimates the voltage to apply to the electrodes in one, more than one, or all of the discharge chambers for active periods that occur immediately after an idle period. FIGS. 6, 7A, and 7B show examples of DUV light sources that include more than one discharge chamber and may be used with the control system 150 or the control system 250.

[0079] Referring to FIG. 6, a block diagram of a photolithography system 600 is shown. An optical source 610 produces a pulsed light beam 605, which is provided to a lithography exposure apparatus 669. The optical source 610 may be, for example, an excimer optical source that outputs the pulsed light beam 605 (which may be a laser beam). As the pulsed light beam 605 enters the lithography exposure apparatus 669, it is directed through a projection optical system 675 and projected onto a wafer 670 to form one or more microelectronic features on a photoresist on the wafer 670. The photolithography system 600 also includes the control system 250, which, in the example of FIG. 6, is connected to components of the optical source 610 and the lithography exposure apparatus 669. In this example, the control system 250 may receive data related to the pulsed light beam 605 or other information from the lithography exposure apparatus 669 and/or may send commands to the lithography exposure apparatus 669. In other examples, the control system 250 is connected only to the optical source 610.

[0080] In the example shown in FIG. 6, the optical source 610 is a two-stage laser system that includes a master oscillator (MO) 631 that provides a seed light beam 624 to a power amplifier (PA) 630. The MO 631 and the PA 630 may be considered to be subsystems of the optical source 610 or systems that are part of the optical source 610. The power amplifier 630 receives the seed light beam 624 from the master oscillator 631 and amplifies the seed light beam 624 to generate the light beam 605 for use in the lithography exposure apparatus 669. For example, the master oscillator 631 may emit a pulsed seed light beam, with seed pulse energies of approximately 1 milliJoule (mJ) per pulse, and these seed pulses may be amplified by the power amplifier 630 to about 10 to 15 mJ.

[0081] The master oscillator 631 includes a discharge chamber 614 having two elongated electrodes 611 A, a gain medium 612 that is a gas mixture, and a fan for circulating gas between the electrodes 611 A. A resonator is formed between a line narrowing module 616 on one side of the discharge chamber 614 and an output coupler 618 on a second side of the discharge chamber 614. The line narrowing module 616 may include a diffractive optic such as a grating that finely tunes the spectral output of the discharge chamber 614.

[0082] The master oscillator 631 also includes a line center analysis module 620 that receives an output light beam from the output coupler 618 and a beam coupling optical system 622 that modifies the size or shape of the output light beam as needed to form the seed light beam 624. The line center analysis module 620 is a measurement system that may be used to measure or monitor the wavelength of the seed light beam 624. The line center analysis module 620 may be placed at other locations in the optical source 610, or it may be placed at the output of the optical source 610.

[0083] The gas mixture used in the discharge chamber 614 may be any gas suitable for producing a light beam at the wavelength and bandwidth required for the application. For an excimer source, the gas mixture may contain a noble gas (rare gas) such as, for example, argon or krypton, a halogen, such as, for example, fluorine or chlorine and traces of xenon apart from helium and/or neon as buffer gas. Specific examples of the gas mixture include argon fluoride (ArF), which emits light at a wavelength of about 193 nm, krypton fluoride (KrF), which emits light at a wavelength of about 248 nm, or xenon chloride (XeCl), which emits light at a wavelength of about 351 nm. The excimer gain medium (the gas mixture) is pumped with short (for example, nanosecond) current pulses in a high- voltage electric discharge by application of a voltage 609 to the elongated electrodes 611A.

[0084] The power amplifier 630 includes a beam coupling optical system 632 that receives the seed light beam 624 from the master oscillator 631 and directs the light beam through a discharge chamber 640, and to a beam turning optical element 648, which modifies or changes the direction of the seed light beam 624 so that it is sent back into the discharge chamber 640. The discharge chamber 640 includes a pair of elongated electrodes 61 IB, a gain medium 612 that is a gas mixture, and a fan for circulating the gas mixture between the electrodes 61 IB.

[0085] The output light beam 605 is directed through a bandwidth analysis module 662, where various parameters (such as the bandwidth or the wavelength) of the beam 605 may be measured.

The output light beam 605 may also be directed through a beam preparation system 663. The beam preparation system 663 may include, for example, a pulse stretcher, where each of the pulses of the output light beam 605 is stretched in time, for example, in an optical delay unit, to adjust for performance properties of the light beam that impinges the lithography exposure apparatus 669. The beam preparation system 663 also may include other components that are able to act upon the beam 605 such as, for example, reflective and or refractive optical elements (such as, for example, lenses and mirrors), filters, and optical apertures (including automated shutters). [0086] The light beam 605 is a pulsed light beam and may include one or more bursts of pulses that are separated from each other in time. Each burst may include one or more pulses of light. In some implementations, a burst includes hundreds of pulses, for example, 100-400 pulses.

[0087] As discussed above, when the gain medium 612 is pumped by applying voltage 609 to the electrodes 611 A, the gain medium 612 emits light. When voltage 609 is applied to the electrodes 611 A in pulses, the light emitted from the medium 612 is also pulsed. Thus, the repetition rate of the pulsed light beam 605 is determined by the rate at which voltage 609 is applied to the electrodes 611 A, with each application of voltage 609 producing a pulse of light. The pulse of light propagates through the gain medium 612 and exits the chamber 614 through the output coupler 618. Thus, a train of pulses is created by periodic, repeated application of voltage 609 to the electrodes 611 A. The repetition rate of the pulses may range between about 500 Hz and 6,000 Hz. In some implementations, the repetition rate is be greater than 6,000 Hz, and may be, for example, 12,000 Hz or greater

[0088] The signals from the control system 250 may also be used to control the electrodes 611 A,

61 IB within the master oscillator 631 and the power amplifier 630, respectively, for controlling the respective pulse energies of the master oscillator 631 and the power amplifier 630, and thus, the energy of the light beam 605. There may be a delay between the signal provided to the electrodes 611 A and the signal provided to the electrodes 61 IB. The amount of delay may influence properties of the beam 605, such as the amount of coherence in the pulsed light beam 605. The pulsed light beam 605 may have an average output power in the range of tens of watts, for example, from about 50 W to about 130 W. The irradiance (that is, the average power per unit area) of the light beam 605 at the output may range from 60 W/cm ² to 80 W/cm ² .

[0089] Referring to FIG. 7A, a block diagram of an optical lithography system 700 is shown. The optical lithography system 700 includes an optical source system 710, which produces an exposure beam 705 that is provided to a scanner apparatus 780. The scanner apparatus 780 exposes a wafer 770 with the exposure beam 705. In the example shown, the control system 250 is connected to the optical source system 710 and the scanner apparatus 780. In other examples, the control system 250 is connected only to the optical source system 710.

[0090] The scanner apparatus 780 exposes a wafer 770 with a shaped exposure beam 705’ . The shaped exposure beam 705’ is formed by passing the exposure beam 705 through a projection optical system 781.

[0091] The optical source system 710 includes optical oscillators 740-1 to 740-N, where N is an integer number that is greater than one. Each optical oscillator 740-1 to 740-N generates a respective light beam 704-1 to 704-N. The details of the optical oscillator 740-1 are discussed below. The other N-l optical oscillators in the optical source system 710 include the same or similar features. [0092] The optical oscillator 740-1 includes a discharge chamber 715-1, which encloses a cathode

711-la and an anode 711-lb. The discharge chamber 715-1 also contains a gaseous gain medium

712-1. A potential difference between the cathode 711 - 1 a and the anode 711 - 1 b forms an electric field in the gaseous gain medium 712-1. The potential difference may be generated by controlling a voltage source 797 coupled to the control system 250 to apply a voltage 709 to the cathode 711-la and/or the anode 711-lb. The electric field provides energy to the gain medium 712-1 sufficient to cause a population inversion and to enable generation of a pulse of light via stimulated emission. Repeated creation of such a potential difference forms a train of pulses of light to make the light beam 704-1. The repetition rate of the pulsed light beam 704-1 is determined by the rate at which voltage 709 is applied to the electrodes 711-la, 711-lb. The duration of the pulses in the pulsed light beam 704-1 is determined by the duration of the application of the voltage 709 to the electrodes 711-1 a and 711-lb. The repetition rate of the pulses may range, for example, between about 500 Hz and 6,000 Hz. In some implementations, the repetition rate may be greater than 6,000 Hz, and may be, for example, 12,000 Hz or greater. Each pulse emitted from the optical oscillator 740-1 may have a pulse energy of, for example, approximately 1 milliJoule (mJ).

[0093] The gaseous gain medium 712-1 may be any gas suitable for producing a light beam at the wavelength, energy, and bandwidth required for the application. For an excimer source, the gaseous gain medium 712-1 may contain a noble gas (rare gas) such as, for example, argon or krypton, a halogen, such as, for example, fluorine or chlorine and traces of xenon apart from a buffer gas, such as helium. Specific examples of the gaseous gain medium 712-1 include argon fluoride (ArF), which emits light at a wavelength of about 193 nm, krypton fluoride (KrF), which emits light at a wavelength of about 248 nm, or xenon chloride (XeCl), which emits light at a wavelength of about 351 nm. The gain medium 712-1 is pumped with short (for example, nanosecond) current pulses in a high-voltage electric discharge by application of the voltage 709 to the electrodes 711-la, 711-lb. [0094] A resonator is formed between a line narrowing module 716-1 on one side of the discharge chamber 715-1 and an output coupler 718-1 on a second side of the discharge chamber 715-1. The line narrowing module 716-1 may include a diffractive optic such as, for example, a grating and/or a prism, that finely tunes the spectral output of the discharge chamber 715-1. In some implementations, the line narrowing module 716-1 includes a plurality of diffractive optical elements. For example, the line narrowing module 716-1 may include four prisms, some of which are configured to control a center wavelength of the light beam 704-1 and others of which are configured to control a spectral bandwidth of the light beam 704-1.

[0095] The optical oscillator 740-1 also includes a line center analysis module 720-1 that receives an output light beam from the output coupler 718-1. The line center analysis module 720-1 is a measurement system that may be used to measure or monitor the wavelength of the light beam 704-1. The line center analysis module 720- 1 may provide data to the control system 250, and the control system 250 may determine metrics related to the light beam 704-1 based on the data from the line center analysis module 720-1. For example, the control system 250 may determine a beam quality metric or a spectral bandwidth based on the data measured by the line center analysis module 720-1. [0096] The optical source system 710 also includes gas supply system 790 that is fluidly coupled to an interior of the discharge chamber 715-1 via a fluid conduit 789. The fluid conduit 789 is any conduit that is capable of transporting a gas or other fluid with no or minimal loss of the fluid. For example, the fluid conduit 789 may be a pipe that is made of or coated with a material that does not react with the fluid or fluids transported in the conduit 789. The gas supply system 790 includes a chamber 791 that contains and/or is configured to receive a supply of the gas or gasses used in the gain medium 712-1. The gas supply system 790 also includes devices (such as pumps, valves, and/or fluid switches) that enable the gas supply system 790 to remove gas from or inject gas into the discharge chamber 715-1. The gas supply system 790 is coupled to the control system 250. The gas supply system 790 may be controlled by the control system 250 to perform, for example, a refill procedure.

[0097] The other N-l optical oscillators are similar to the optical oscillator 740-1 and have similar or the same components and subsystems. For example, each of the optical oscillators 740-1 to 740-N includes electrodes similar to the electrodes 711-la, 711-lb, a line narrowing module similar to the line narrowing module 716-1, and an output coupler similar to the output coupler 718-1. The optical oscillators 740-1 to 740-N may be tuned or configured such that all of the light beams 704-1 to 704-N have the same properties or the optical oscillators 740-1 to 740-N may be tuned or configured such that at least some optical oscillators have at least some properties that are different from other optical oscillators. For example, all of the light beams 704-1 to 704-N may have the same center wavelength, or the center wavelength of each light beam 704-1 to 704-N may be different. The center wavelength produced by a particular one of the optical oscillators 740-1 to 740-N may be set using the respective line narrowing module.

[0098] Moreover, the voltage source 797 may be electrically connected to the electrodes in each optical oscillator 740-1 to 740-N, or the voltage source 797 may be implemented as a voltage system that includes N individual voltage sources, each of which is electrically connected to the electrodes of one of the optical oscillators 740-1 to 740-N.

[0099] The optical source system 710 also includes a beam control apparatus 787 and a beam combiner 788. The beam control apparatus 787 is between the gaseous gain media of the optical oscillators 740-1 to 740-N and the beam combiner 788. The beam control apparatus 787 determines which of the light beams 704-1 to 704-N are incident on the beam combiner 788. The beam combiner 788 forms the exposure beam 705 from the light beam or light beams that are incident on the beam combiner 788. In the example shown, the beam control apparatus 787 is represented as a single element. However, the beam control apparatus 787 may be implemented as a collection of individual beam control apparatuses. For example, the beam control apparatus 787 may include a collection of shutters, with one shutter being associated with each optical oscillator 740-1 to 740-N.

[0100] The optical source system 710 may include other components and systems. For example, the optical source system 710 may include a beam preparation system 763 that includes a bandwidth analysis module that measures various properties (such as the bandwidth or the wavelength) of a light beam. The beam preparation system 763 also may include a pulse stretcher (not shown) that stretches each pulse that interacts with the pulse stretcher in time. The beam preparation system 763 also may include other components that are able to act upon light such as, for example, reflective and/or refractive optical elements (such as, for example, lenses and mirrors), and/or filters. In the example shown, the beam preparation system 763 is positioned in the path of the exposure beam 705.

However, the beam preparation system 763 may be placed at other locations within the optical lithography system 700. Moreover, other implementations are possible. For example, the optical source system 710 may include N instances of the beam preparation system 763, each of which is placed to interact with one of the light beams 704-1 to 704-N. In another example, the optical source system 810 may include optical elements (such as mirrors) that steer the light beams 704-1 to 704-N toward the beam combiner 788.

[0101] The scanner apparatus 780 may be a liquid immersion system or a dry system. The scanner apparatus 780 includes a projection optical system 781 through which the exposure beam 705 passes prior to reaching the wafer 770, and a sensor system or metrology system 799. The wafer 770 is held or received on a wafer holder 783. Referring also to FIG. 7B, the projection optical system 781 includes a slit 784, a mask 785, and a projection objective, which includes a lens system 786. The lens system 786 includes one or more optical elements. The exposure beam 705 enters the scanner apparatus 780 and impinges on the slit 784, and at least some of the beam 705 passes through the slit 784 to form the shaped exposure beam 705’ . In the example of FIGS. 7A and 7B, the slit 784 is rectangular and shapes the exposure beam 705 into an elongated rectangular shaped light beam, which is the shaped exposure beam 705’ . The mask 785 includes a pattern that determines which portions of the shaped light beam are transmitted by the mask 785 and which are blocked by the mask 785. Microelectronic features are formed on the wafer 770 by exposing a layer of radiation-sensitive photoresist material on the wafer 770 with the exposure beam 705’ . The design of the pattern on the mask is determined by the specific microelectronic circuit features that are desired.

[0102] The metrology system 799 includes a sensor 771. The sensor 771 may be configured to measure a property of the shaped exposure beam 705’ such as, for example, bandwidth, energy, pulse duration, and or wavelength. The sensor 771 may be, for example, a camera or other device that is able to capture an image of the shaped exposure beam 705’ at the wafer 770, or an energy detector that is able to capture data that describes the amount of optical energy at the wafer 770 in the x-y plane. [0103] Other aspects of the invention are set out in the following numbered clauses.

1. A light source comprising: a light-generation apparatus configured to be in an active state during a first time period, an idle state during a second time period, and the active state during a third time period, the first time period occurring before the second time period and the second time period occurring before the third time period, and wherein an excitation signal is applied to the light-generation apparatus in the active state and is not applied to the light-generation apparatus in the idle state; and a control system configured to estimate a property of the excitation signal for application to the light- generation apparatus during the third time period based on the duration of the second time period and a value of the property during the first time period.

2. The light source of clause 1, wherein the light-generation apparatus comprises: a discharge chamber configured to hold a gaseous gain medium; and a plurality of electrodes in the discharge chamber, and wherein the excitation signal comprises a voltage signal applied to at least one of the plurality of electrodes, and the property of the excitation signal comprises a magnitude of the voltage signal.

3. The light source of clause 2, wherein the voltage signal comprises a time-varying voltage signal.

4. The light source of clause 2, wherein the control system comprises a memory module configured to store at least one value representing the magnitude of the voltage signal applied to the electrodes in the first time period.

5. The light source of clause 2, wherein the value of the property during the first time period comprises a minimum voltage applied to the electrodes during the first time period.

6. The light source of clause 5, wherein the control system is configured to estimate the property of the excitation signal for application to the light-generation apparatus during the third time period based on the duration of the second time period, the minimum voltage applied to the electrodes during the first time period, and an adaptive parameter associated with the first time period.

7. The light source of clause 2, wherein the gaseous gain medium comprises a gain medium configured to emit deep ultraviolet (DUV) light in response to the voltage signal being applied to at least one of the electrodes.

8. The light source of clause 7, wherein the gaseous gain medium comprises argon fluoride (ArF), krypton fluoride (KrF), or xenon chloride (XeCl).

9. The light source of clause 1, wherein the control system is further configured to determine an error metric based on the estimated property of the excitation signal and an actual value of the property of the excitation signal applied to the light-generation apparatus during the third time period.

10. The light source of clause 9, wherein the control system is further configured to update a value of an adaptive parameter based on the error metric. 11. The light source of clause 10, wherein the control system is configured to update a value for each of a plurality of adaptive parameters, and each of the plurality of adaptive parameters is associated with a different duration of the second time period.

12. The light source of clause 1, wherein the control system is further configured to determine whether to initiate a warm-up procedure based on the estimated property of the excitation signal.

13. The light source of clause 12, wherein if the warm-up procedure is initiated, the control system is further configured to determine a warm-up procedure metric related to a duration of the warm-up procedure.

14. The light source of clause 13, wherein the warm-up procedure metric is a number of times to excite the light-generation apparatus during the warm-up procedure.

15. The light source of clause 1, wherein the light-generation apparatus comprises a master oscillator and a power amplifier.

16. The light source of clause 1, wherein the light-generation apparatus comprises a single discharge chamber.

17. The light source of clause 1, wherein the light-generation apparatus comprises a plurality of discharge chambers, and each of the discharge chambers is configured to emit a pulsed light beam toward a beam combiner.

18. A controller for a light source, the controller comprising a control system, wherein the control system is configured to: access information related to a duration of an idle period of the light source; access information related to a value of a property of an excitation signal applied to the light source during a time period that occurred prior to the idle period; and estimate an updated value of the property of the excitation signal based on the duration of the idle period and the value of the property of the excitation signal during the time period that occurred prior to the idle period.

19. The controller of clause 18, wherein the control system is further configured to apply the excitation signal with the updated value of the property to the light source after the idle period.

20. The controller of clause 19, wherein the control system is further configured to determine an error metric based on the estimated updated value of the property and an actual value of the property of the excitation signal applied to the light-generation apparatus after the idle period.

21. The controller of clause 20, wherein the control system is further configured to update a value of an adaptive parameter based on the error metric.

22. The controller of clause 21, wherein the control system is configured to update a value for each of a plurality of adaptive parameters, and each of the plurality of adaptive parameters is associated with a different duration of the second time period. 23. The controller of clause 18, wherein the control system is further configured to determine whether to initiate a warm-up procedure for the light source based on the estimated updated value of the property.

24. The controller of clause 18, wherein the control system is configured to access the information related to the duration of an idle period of a light source and the information related to the value of a property of an excitation signal during the time period that occurred prior to the idle period from a computer-readable memory module.

25. The controller of clause 18, wherein the control system comprises: a computer-readable memory module; and one or more electronic processors coupled to the computer-readable memory module.

26. A method comprising: accessing information related to a duration of an idle period of a light source; accessing information related to a value of a property of an excitation signal applied to the light source during a time period that occurred prior to the idle period; and estimating an updated value of the property of the excitation signal based on the duration of the idle period and the value of the property of the excitation signal during the time period that occurred prior to the idle period.

[0104] Other implementations are within the scope of the claims.