Patent Publication Number: US-2023152707-A1

Title: Forming multiple aerial images in a single lithography exposure pass

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 16/755,993, which is the national phase of International Application No. PCT/US2018/052949, filed Sep. 26, 2018 and titled FORMING MULTIPLE AERIAL IMAGES IN A SINGLE LITHOGRAPHY EXPOSURE PASS, which claims priority to U.S. Application No. 62/574,628, which was filed on Oct. 19, 2017. Each of these patent applications is incorporated herein in its entirety by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to forming multiple aerial images in a single lithography exposure pass. The techniques discussed below may be used, for example, to form a three-dimensional semiconductor component. 
     BACKGROUND 
     Photolithography is the process by which semiconductor circuitry is patterned on a substrate such as a silicon wafer. A photolithography optical source provides the deep ultraviolet (DUV) light used to expose a photoresist on the wafer. DUV light for photolithography is generated by excimer optical sources. Often, the optical source is a laser source and the pulsed light beam is a pulsed laser beam. The light beam is passed through a beam delivery unit, a reticle or a mask, and then projected onto a prepared silicon wafer. In this way, a chip design is patterned onto a photoresist that is then etched and cleaned, and then the process repeats. 
     SUMMARY 
     In one general aspect, a method of forming a three-dimensional semiconductor component using a photolithography system includes directing a pulsed light beam along a direction of propagation toward a mask, the pulsed light beam including a plurality of pulses of light; passing a set of the pulses of light in the light beam through the mask toward a wafer during a single exposure pass; generating, during the single exposure pass, at least a first aerial image and a second aerial image on the wafer based on pulses of light in the set of pulses that pass through the mask, the first aerial image being at a first plane on the wafer and the second aerial image being at a second plane on the wafer, the first plane and the second plane being spatially distinct from each other and separated from each other by a separation distance along the direction of propagation; and forming the three-dimensional semiconductor component based on an interaction between light in the first aerial image and a material in a first portion of the wafer and an interaction between light in the second aerial image and a material in a second portion of the wafer. At least one of the pulses in the set of pulses has a first primary wavelength and at least one of the other pulses in the set of pulses has a second primary wavelength that is different from the first primary wavelength, such that the separation distance is formed during the single exposure pass based on the difference between the first primary wavelength and the second primary wavelength. 
     Implementations may include one or more of the following features. At least one of the pulses in the set of pulses that passes through the mask during the single exposure pass may have more than one primary wavelength of light. 
     Each primary wavelength may be separated by a spectral separation of 200 femtometers (fm) to 500 picometers (pm) from the nearest other primary wavelength. 
     The separation distance between the first aerial image and the second aerial image may change during the single exposure pass. 
     The single exposure pass may be a first exposure pass, and the method also may include passing a second set of pulses of light in the light beam through the mask toward the wafer during a second exposure pass and after the first exposure pass is completed. The separation distance between the first aerial image and the second aerial image is different during the first exposure pass and the second exposure pass. 
     The separation distance between the first aerial image and the second aerial image may be set prior to the single exposure pass, and, in some implementations, the separation distance does not change during the single exposure pass. The separation distance between the first aerial image and the second aerial image may be set to accommodate one or more features of the photolithography system. 
     The set of pulses may include a first group of pulses of light and a second group of pulses of light, each pulse in the first group of pulses of light has the first primary wavelength, each pulse in the second group of pulses may have the second primary wavelength, and the method also may include: controlling a property of the first group of pulses to thereby control an amount of light in the first aerial image; and controlling a property of the second group of pulses to thereby control an amount of light in the second aerial image. The property of the first group may be a count of pulses in the first group, and the property of the second group may be a count of pulses in the second group. Controlling the count of pulses in the first group may include determining, before the single exposure pass begins, a first number of pulses to include in the first group of pulses, and controlling the second number of pulses may include determining, before the single exposure pass, a second number of pulses to include in the second group of pulses. The first group of pulses and the second group of pulses may include all of the pulses that pass through the mask in the single exposure pass. Determining the first number of pulses and the second number of pulses may include one or more of: (a) receiving input from an operator and (b) accessing a pre-defined setting associated with the photolithography system. The property of the first group of pulses may include an intensity of each pulse in the first group, and the property of the second group of pulses may include an intensity of each pulse in the second group. 
     The first plane on the wafer and the second plane on the wafer may be planes that are substantially perpendicular to the direction of propagation. 
     In some implementations, a first feature of the three-dimensional semiconductor is formed at the first plane, a second feature of the three-dimensional semiconductor is formed at the second plane, and the first and second features are displaced from each other by a sidewall that extends substantially parallel to the direction of propagation. 
     The three-dimensional semiconductor component may be a three-dimensional NAND flash memory component. 
     The first plane may correspond to a first focal plane and the second plane corresponds to a second focal plane, and the separation distance between the first plane and the second plane is based on a difference between one or more wavelengths in a pulse of light that passes through the mask or a difference between a wavelengths among discrete pulses in the set of pulses. 
     In another general aspect, a photolithography system includes a light source; a lithography scanner apparatus including: a mask positioned to interact with a pulsed light beam from the light source, and a wafer holder; and a control system coupled to the light source, the control system configured to cause the light source to emit the pulsed light beam toward the lithography scanner apparatus during a single exposure pass such that, during the single exposure pass, at least a first aerial image and a second aerial image are formed on a wafer received at the wafer holder based on pulses of light in a set of pulses of light that pass through the mask along a direction of propagation, the first aerial image being at a first plane on the wafer and the second aerial image being at a second plane on the wafer, the first plane and the second plane being spatially distinct from each other and separated from each other by a separation distance along the direction of propagation, and a three-dimensional semiconductor component is formed based on an interaction between light in the first aerial image and a material in a first portion of the wafer and an interaction between light in the second aerial image and a material in a second portion of the wafer. At least one of the pulses in the set of pulses has a first primary wavelength, at least one of the other pulses in the set of pulses has a second primary wavelength that is different from the first primary wavelength, and the separation distance between first aerial image and the second aerial image is based on the difference between the first primary wavelength and the second primary wavelength. 
     Implementations may include one or more of the following features. The control system may include a computer-readable storage medium, one or more electronic processors coupled to the computer-readable storage medium, and an input/output interface, and a recipe related to the photolithography system is stored on the computer-readable storage medium. The recipe may specify the separation distance. The recipe specifies the separation distance on a per-wafer or per-lot basis. The light source may include a krypton fluoride (KrF) gain medium or a argon fluoride (ArF) gain medium. 
     Implementations of any of the techniques described above and herein may include a process, an apparatus, a control system, instructions stored on a non-transient machine-readable computer medium, and/or a method. 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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1 A  is a block diagram of an example of an implementation of a photolithography system. 
         FIG.  1 B  is a block diagram of an example of an implementation of an optical system for the photolithography system of  FIG.  1 A . 
         FIG.  1 C  is a cross-sectional view of an example of a wafer exposed by the photolithography system of  FIG.  1 A . 
         FIG.  2 A  is a block diagram of another example of an implementation of a photolithography system. 
         FIG.  2 B  is a block diagram of an example of an implementation of a spectral feature selection module that may be used in a photolithography system. 
         FIG.  2 C  is a block diagram of an example of an implementation of a line narrowing module. 
         FIGS.  3 A- 3 C  are plots of data that relate to the production of pulses and/or bursts of pulses in an optical source, in which  FIG.  3 C  shows an amplitude of a trigger signal as a function of time. 
         FIG.  4    is a block diagram of another example of an implementation of a photolithography system. 
         FIG.  5    is a flow chart of an example of a process for forming a three-dimensional semiconductor component. 
         FIGS.  6 A and  6 B  each show an example of an optical spectrum of a single pulse of light. 
         FIG.  7    shows an example of an average optical spectrum for a single exposure pass. 
         FIGS.  8 A and  8 B  show side and top cross-sectional views, respectively, of an example of a wafer. 
         FIGS.  9 A and  9 B  show side and top cross-sectional views, respectively, of an example of a three-dimensional semiconductor component. 
         FIGS.  10 A and  10 B  show examples of simulated data. 
     
    
    
     DETAILED DESCRIPTION 
     Techniques for forming more than one aerial image, each at a different plane, in a single lithography pass, and forming a three-dimensional semiconductor component using the aerial images are discussed herein. 
     Referring to  FIG.  1 A , a photolithography system  100  includes an optical (or light) source  105  that provides a light beam  160  to a lithography exposure apparatus  169 , which processes a wafer  170  received by a wafer holder or stage  171 . The light beam  160  is a pulsed light beam that includes pulses of light separated from each other in time. The lithography exposure apparatus  169  includes a projection optical system  175  through which the light beam  160  passes prior to reaching the wafer  170 , and a metrology system  172 . The metrology system  172  may include, for example, a camera or other device that is able to capture an image of the wafer  170  and/or the light beam  160  at the wafer  170 , or an optical detector that is able to capture data that describes characteristics of the light beam  160 , such as intensity of the light beam  160  at the wafer  170  in the x-y plane. The lithography exposure apparatus  169  can be a liquid immersion system or a dry system. The photolithography system  100  also may include a control system  150  to control the light source  105  and/or the lithography exposure apparatus  169 . 
     Microelectronic features are formed on the wafer  170  by, for example, exposing a layer of radiation-sensitive photoresist material on the wafer  170  with the light beam  160 . Referring also to  FIG.  1 B , the projection optical system  175  includes a slit  176 , a mask  174 , and a projection objective, which includes a lens  177 . The light beam  160  enters the optical system  175  and impinges on the slit  176 , and at least some of the beam  160  passes through the slit  176 . In the example of  FIGS.  1 A and  1 B , the slit  176  is rectangular and shapes the light beam  160  into an elongated rectangular shaped light beam. A pattern is formed on the mask  174 , and the pattern determines which portions of the shaped light beam are transmitted by the mask  174  and which are blocked by the mask  174 . The design of the pattern is determined by the specific microelectronic circuit design that is to be formed on the wafer  170 . 
     The shaped light beam interacts with the mask  174 . The portions of the shaped light beam that are transmitted by the mask  174  pass through (and may be focused by) the projection lens  177  and expose the wafer  170 . The portions of the shaped light beam that are transmitted by the mask  174  form an aerial image in the x-y plane in the wafer  170 . The aerial image is the intensity pattern formed by the light that reaches the wafer  170  after interacting with the mask  174 . The aerial image is at the wafer  170  and extends generally in the x-y plane. 
     The system  100  is able to form a plurality of aerial images during a single exposure pass, with each of the aerial images being at a spatially distinct location along the z axis in the wafer  170 . Referring also to  FIG.  1 C , which shows a cross-sectional view of the wafer  170  in the y-z plane, the projection optical system  175  forms two aerial images  173   a ,  173   b  at different planes along the z axis in a single exposure pass. As discussed in greater detail below, each of the aerial images  173   a ,  173   b  is formed from light having a different primary wavelength. 
     The location of the aerial image along the z axis depends on the characteristics of the optical system  175  (including the projection lens  177  and the mask  174 ) and the wavelength of the light beam  160 . The focal position of the lens  177  depends on the wavelength of the light incident on the lens  177 . Thus, varying or otherwise controlling the wavelength of the light beam  160  allows the position of the aerial image to be controlled. By providing pulses having different primary wavelengths of light during a single exposure pass, a plurality (two or more) of aerial images, which are each at a different location along the z axis, may be formed in a single exposure pass without moving the optical system  175  (or any components of the optical system  175 ) and the wafer  170  relative to each other along the z axis. 
     In the example of  FIG.  1 A , light passing through the mask  174  is focused to a focal plane by the projection lens  177 . The focal plane of the projection lens  177  is between the projection lens  177  and the wafer stage  171 , with the position of the focal plane along the z axis depending on the properties of the optical system  175  and the wavelength of the light beam  160 . The aerial images  173   a ,  173   b  are formed from light having different wavelengths, thus the aerial images  173   a ,  173   b  are at different locations in the wafer  170 . The aerial images  173   a ,  173   b  are separated from each other along the z axis by a separation distance  179 . The separation distance  179  depends on the difference between the wavelength of the light that forms the aerial image  173   a  and the wavelength of the light that forms the aerial image  173   b.    
     The wafer stage  171  and the mask  174  (or other parts of the optical system  175 ) generally move relative to each other in the x, y, and z directions during scanning for routine performance corrections and operation, for example, the motion may be used to accomplish basic leveling, compensation of lens distortions, and for compensation of stage positioning error. This relative motion is referred to as incidental operational motion. However, in the system of  FIG.  1 A , the relative motion of the wafer stage  171  and the optical system  175  is not relied upon to form the separation distance  179 . Instead, the separation distance  179  is formed due to the ability to control the primary wavelengths in the pulses that pass through the mask  174  during the exposure pass. Thus, unlike some prior systems, the separation distance  179  is not created only by moving the optical system  175  and the wafer  170  relative to each other along the z direction. Moreover, the aerial images  173   a  and  173   b  are both present at the wafer  170  during the same exposure pass. In other words, the system  100  does not require that the aerial image  173   a  be formed in a first exposure pass and the aerial image  173   b  be formed in a second, subsequent exposure pass. 
     The light in the first aerial image  173   a  interacts with the wafer at a portion  178   a , and the light in the second aerial image  173   b  interacts with the wafer at a portion  178   b . These interactions may form electronic features or other physical characteristics, such as openings or holes, on the wafer  170 . Because the aerial images  173   a ,  173   b  are at different planes along the z axis, the aerial images  173   a ,  173   b  may be used to form three-dimensional features on the wafer  170 . For example, the aerial image  173   a  may be used to form a periphery region, and the aerial image  173   b  may be used to form a channel, trench, or recess that is at a different location along the z axis. As such, the techniques discussed herein may be used to form a three-dimensional semiconductor component, such as a three-dimensional NAND flash memory component. 
     Before discussing additional details related to forming multiple aerial images in a single exposure pass, example implementations of the light source  105  and the photolithography system  100  are discussed with respect to  FIGS.  2 A- 2 C,  3 A- 3 C, and  4   . 
     Referring to  FIG.  2 A , a block diagram of a photolithography system  200  is shown. The system  200  is an example of an implementation of the system  100  ( FIG.  1 A ). For example, in the photolithography system  200 , an optical source  205  is used as the optical source  105  ( FIG.  1 A ). The optical source  205  produces a pulsed light beam  260 , which is provided to the lithography exposure apparatus  169 . The optical source  205  may be, for example, an excimer optical source that outputs the pulsed light beam  260  (which may be a laser beam). As the pulsed light beam  260  enters the lithography exposure apparatus  169 , it is directed through the projection optical system  175  and projected onto the wafer  170 . In this way, one or more microelectronic features are patterned onto a photoresist on the wafer  170  that is then developed and cleaned prior to subsequent process steps, and the process repeats. The photolithography system  200  also includes the control system  250 , which, in the example of  FIG.  2 A , is connected to components of the optical source  205  as well as to the lithography exposure apparatus  169  to control various operations of the system  200 . The control system  250  is an example of an implementation of the control system  250  of  FIG.  1 A . 
     In the example shown in  FIG.  2 A , the optical source  205  is a two-stage laser system that includes a master oscillator (MO)  212  that provides a seed light beam  224  to a power amplifier (PA)  230 . The MO  212  and the PA  230  may be considered to be subsystems of the optical source  205  or systems that are part of the optical source  205 . The power amplifier  230  receives the seed light beam  224  from the master oscillator  212  and amplifies the seed light beam  224  to generate the light beam  260  for use in the lithography exposure apparatus  169 . For example, the master oscillator  212  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  230  to about 10 to 15 mJ. 
     The master oscillator  212  includes a discharge chamber  240  having two elongated electrodes  217 , a gain medium  219  that is a gas mixture, and a fan for circulating gas between the electrodes  217 . A resonator is formed between a line narrowing module  216  on one side of the discharge chamber  240  and an output coupler  218  on a second side of the discharge chamber  240 . The line narrowing module  216  may include a diffractive optic such as a grating that finely tunes the spectral output of the discharge chamber  240 .  FIGS.  2 B and  2 C  provide additional detail about the line narrowing module  216 . 
       FIG.  2 B  is a block diagram of an example of an implementation of a spectral feature selection module  258  that includes one or more instances of the line narrowing module  216 . The spectral feature selection module  258  couples to light that propagates in the optical source  205 . In some implementations (such as shown in  FIG.  2 B ), the spectral feature selection module  258  receives the light from the chamber  214  of the master oscillator  212  to enable the fine tuning of the spectral features such as wavelength and bandwidth within the master oscillator  212 . 
     The spectral feature selection module  258  may include a control module such as a spectral feature control module  254  that includes electronics in the form of any combination of firmware and software. The control module  254  is connected to one or more actuation systems such as spectral feature actuation systems  255 _ 1  to  255 _ n . Each of the actuation systems  255 _ 1  to  255 _ n  may include one or more actuators that are connected to respective optical features  256 _ 1  to  256 _ n  of an optical system  257 . The optical features  256 _ 1  to  256 _ n  are configured to adjust particular characteristics of the generated light beam  260  to thereby adjust the spectral feature of the light beam  260 . The control module  254  receives a control signal from the control system  250 , the control signal including specific commands to operate or control one or more of the actuation systems  255 _ 1  to  255 _ n . The actuation systems  255 _ 1  to  255 _ n  can be selected and designed to work together, that is, in tandem, or the actuation system  255 _ 1  to  255 _ n  may be configured to work individually. Moreover, each actuation system  255 _ 1  to  255 _ n  may be optimized to respond to a particular class of disturbances. 
     Each optical feature  256 _ 1  to  256 _ n  is optically coupled to the light beam  260  produced by the optical source  105 . The optical system  257  may be implemented as a line narrowing module  216 C such as that shown in  FIG.  2 C . The line narrowing module includes as the optical features  256 _ 1  to  256 _ n  dispersive optical elements such as a reflective gratings  291  and refractive optical elements such as prisms  292 ,  293 ,  294 ,  295 . One or more of the prisms  292 ,  293 ,  294 ,  295  may be rotatable. An example of this line narrowing module can be found in U.S. application Ser. No. 12/605,306, titled SYSTEM METHOD AN APPARATUS FOR SELECTING AND CONTROLLING LIGHT SOURCE BANDWIDTH and filed on Oct. 23, 2009 (the &#39;306 application). In the &#39;306 application, a line narrowing module is described that includes a beam expander (including the one or more prisms  292 ,  293 ,  294 ,  295 ) and the dispersive element such as the grating  291 . The respective actuation systems for the actuatable optical features such as the grating  291 , and one or more of the prisms  292 ,  293 ,  294 ,  295  are not shown in  FIG.  2 C . 
     Each of the actuators of the actuation systems  255 _ 1  to  255 _ n  is a mechanical device for moving or controlling the respective optical features  256 _ 1  to  256 _ n  of the optical system  257 . The actuators receive energy from the module  254 , and convert that energy into some kind of motion imparted to the optical features  256 _ 1  to  256 _ n  of the optical system  257 . For example, in the &#39;306 application, actuation systems are described such as force devices (to apply forces to regions of the grating) and rotation stages for rotating one or more of the prisms of the beam expander. The actuation systems  255 _ 1  to  255 _ n  may include, for example, motors such as stepper motors, valves, pressure-controlled devices, piezoelectric devices, linear motors, hydraulic actuators, and/or voice coils. 
     Returning to  FIG.  2 A , the master oscillator  212  also includes a line center analysis module  220  that receives an output light beam from the output coupler  218  and a beam coupling optical system  222  that modifies the size or shape of the output light beam as needed to form the seed light beam  224 . The line center analysis module  220  is a measurement system that may be used to measure or monitor the wavelength of the seed light beam  224 . The line center analysis module  220  may be placed at other locations in the optical source  205 , or it may be placed at the output of the optical source  205 . 
     The gas mixture used in the discharge chamber  240  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 to the elongated electrodes  217 . 
     The power amplifier  230  includes a beam coupling optical system  232  that receives the seed light beam  224  from the master oscillator  212  and directs the light beam through a discharge chamber  240 , and to a beam turning optical element  248 , which modifies or changes the direction of the seed light beam  224  so that it is sent back into the discharge chamber  240 . The discharge chamber  240  includes a pair of elongated electrodes  241 , a gain medium  219  that is a gas mixture, and a fan for circulating the gas mixture between the electrodes  241 . 
     The output light beam  260  is directed through a bandwidth analysis module  262 , where various parameters (such as the bandwidth or the wavelength) of the beam  260  may be measured. The output light beam  260  may also be directed through a beam preparation system  263 . The beam preparation system  263  may include, for example, a pulse stretcher, where each of the pulses of the output light beam  260  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  169 . The beam preparation system  263  also may include other components that are able to act upon the beam  260  such as, for example, reflective and/or refractive optical elements (such as, for example, lenses and mirrors), filters, and optical apertures (including automated shutters). 
     The photolithography system  200  also includes the control system  250 . In the implementation shown in  FIG.  2 A , the control system  250  is connected to various components of the optical source  205 . For example, the control system  250  may control when the optical source  205  emits a pulse of light or a burst of light pulses that includes one or more pulses of light by sending one or more signals to the optical source  205 . The control system  250  is also connected to the lithography exposure apparatus  169 . Thus, the control system  250  also may control the various aspects of the lithography exposure apparatus  169 . For example, the control system  250  may control the exposure of the wafer  170  and thus may be used to control how electronic features are printed on the wafer  170 . In some implementations, the control system  250  may control the scanning of the wafer  170  by controlling the motion of the slit  176  in the x-y plane ( FIG.  1 B ). Moreover, the control system  250  may exchange data with the metrology system  172  and/or the optical system  175 . 
     The lithography exposure apparatus  169  also may include, for example, temperature control devices (such as air conditioning devices and/or heating devices), and/or power supplies for the various electrical components. The control system  250  also may control these components. In some implementations, the control system  250  is implemented to include more than one sub-control system, with at least one sub-control system (a lithography controller) dedicated to controlling aspects of the lithography exposure apparatus  169 . In these implementations, the control system  250  may be used to control aspects of the lithography exposure apparatus  169  instead of, or in addition to, using the lithography controller. 
     The control system  250  includes an electronic processor  251 , an electronic storage  252 , and an I/O interface  253 . The electronic processor  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, or both. The electronic processor  251  may be any type of electronic processor. 
     The electronic storage  252  may be volatile memory, such as RAM, or non-volatile memory. In some implementations, and the electronic storage  252  includes non-volatile and volatile portions or components. The electronic storage  252  may store data and information that is used in the operation of the control system  250 , components of the control system  250 , and/or systems controlled by the control system  250 . The information may be stored in, for example, a look-up table or a database. For example, the electronic storage  252  may store data that indicates values of various properties of the beam  260  under different operating conditions and performance scenarios. 
     Moreover, the electronic storage  252  may store various recipes or process programs  259  that dictate parameters of the light beam  260  during use. For example, the electronic storage  252  may store a recipe that indicates the wavelength of each pulse in the light beam  260  for a particular exposure pass. The recipe may indicate different wavelengths for different exposure passes. The wavelength controlling techniques discussed below may be applied on a pulse-by-pulse basis. In other words, the wavelength content may be controlled for each individual pulse in an exposure pass to facilitate formation of the aerial images at the desired locations along the z axis. 
     The electronic storage  252  also may store instructions, perhaps as a computer program, that, when executed, cause the processor  251  to communicate with components in the control system  250 , the optical system  205 , and/or the lithography exposure apparatus  169 . 
     The I/O interface  253  is any kind of electronic interface that allows the control system  250  to receive and/or provide data and signals with an operator, the optical system  205 , the lithography exposure apparatus  169 , any component or system within the optical system  205  and/or the lithography exposure apparatus  169 , and/or an automated process running on another electronic device. For example, the I/O interface  253  may include one or more of a visual display, a keyboard, and a communications interface. 
     The light beam  260  (and the light beam  160 ) are pulsed light beams 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.  FIGS.  3 A- 3 C  provides an overview of the production of pulses and bursts in the optical source  205 .  FIG.  3 A  shows an amplitude of a wafer exposure signal  300  as a function of time,  FIG.  3 B  shows an amplitude of a gate signal  315  as a function of time, and  FIG.  3 C  shows an amplitude of a trigger signal as a function of time. 
     The control system  250  may be configured to send the wafer exposure signal  300  to the optical source  205  to control the optical source  205  to produce the light beam  260 . In the example shown in  FIG.  3 A , the wafer exposure signal  300  has a high value  305  (for example, 1) for a period of time  307  during which the optical source  205  produces bursts of pulses of light. The wafer exposure signal  300  otherwise has a low value  310  (for example, 0) when the wafer  170  is not being exposed. 
     Referring to  FIG.  3 B , the light beam  260  is a pulsed light beam, and the light beam  260  includes bursts of pulses. The control system  250  also controls the duration and frequency of the bursts of pulses by sending a gate signal  315  to the optical source  205 . The gate signal  315  has a high value  320  (for example, 1) during a burst of pulses and a low value  325  (for example, 0) during the time between successive bursts. In the example shown, the duration of time at which the gate signal  315  has the high value is also the duration of a burst  316 . The bursts are separated in time by an inter-burst time interval. During the inter-burst time interval, the lithography exposure apparatus  169  may position the next die on the wafer  170  for exposure. 
     Referring to  FIG.  3 C , the control system  250  also controls the repetition rate of the pulses within each burst with a trigger signal  330 . The trigger signal  330  includes triggers  340 , one of which is provided to the optical source  205  to cause the optical source  205  to produce a pulse of light. The control system  250  may send a trigger  340  to the source  205  each time a pulse is to be produced. Thus, the repetition rate of the pulses produced by the optical source  205  (the time between two successive pulses) may be set by the trigger signal  330 . 
     As discussed above, when the gain medium  219  is pumped by applying voltage to the electrodes  217 , the gain medium  219  emits light. When voltage is applied to the electrodes  217  in pulses, the light emitted from the medium  219  is also pulsed. Thus, the repetition rate of the pulsed light beam  260  is determined by the rate at which voltage is applied to the electrodes  217 , with each application of voltage producing a pulse of light. The pulse of light propagates through the gain medium  219  and exits the chamber  214  through the output coupler  218 . Thus, a train of pulses is created by periodic, repeated application of voltage to the electrodes  217 . The trigger signal  330 , for example, may be used to control the application of voltage to the electrodes  217  and the repetition rate of the pulses, which may range between about 500 and 6,000 Hz for most applications. In some implementations, the repetition rate may be greater than 6,000 Hz, and may be, for example, 12,000 Hz or greater 
     The signals from the control system  250  may also be used to control the electrodes  217 ,  241  within the master oscillator  212  and the power amplifier  230 , respectively, for controlling the respective pulse energies of the master oscillator  212  and the power amplifier  230 , and thus, the energy of the light beam  260 . There may be a delay between the signal provided to the electrodes  217  and the signal provided to the electrodes  241 . The amount of delay may influence properties of the beam  260 , such as the amount of coherence in the pulsed light beam  260 . 
     The pulsed light beam  260  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  260  at the output may range from 60 W/cm 2  to 80 W/cm 2 . 
     Referring also to  FIG.  4   , the wafer  170  is irradiated by the light beam  260 . The lithography exposure apparatus  169  includes the optical system  175  ( FIGS.  1 A and  1 B ). In the example of  FIG.  4   , the optical system  175  (not shown) includes an illuminator system  429 , which includes an objective arrangement  432 . The objective arrangement  432  includes the projection lens  177  ( FIG.  1 B ) and enables the image transfer to occur from the mask  174  to the photoresist on the wafer  170 . The illuminator system  429  adjusts the range of angles for the light beam  260  impinging on the mask  174 . The illuminator system  429  also may homogenize (make uniform) the intensity distribution of the light beam  260  in the x-y plane across the mask  174 . 
     In some implementations, an immersion medium may be supplied to cover the wafer  170 . The immersion medium may be a liquid (such as water) for liquid immersion lithography. In other implementations in which the lithography is a dry system, the immersion medium may be a gas such as dry nitrogen, dry air, or clean air. In other implementations, the wafer  170  may be exposed within a pressure-controlled environment (such as a vacuum or partial vacuum). 
     During an exposure pass, a plurality of N pulses of the light beam  260  illuminates the same area of the wafer  170 . N may be any integer number greater than one. The number of pulses N of the light beam  110  that illuminate the same area may be referred to as an exposure window or exposure pass  400 . The size of the window  400  may be controlled by the slit  176 . For example, the slit  176  may include a plurality of blades that are movable such that the blades form an aperture in one configuration and close the aperture in another configuration. By arranging the blades of the slit  176  to form an aperture of a particular size, the size of the window  400  also may be controlled. 
     The N pulses also determine an illumination dose for the exposure pass. The illumination dose is the amount of optical energy that is delivered to the wafer during the exposure pass. Thus, properties of the N pulses, such as the optical energy in each pulse, determine the illumination dose. Moreover, and as discussed in greater detail below, the N pulses also may be used to determine the amount of light in each of the aerial images  173   a ,  173   b . In particular, a recipe may specify that of the N pulses, a certain number of pulses have a first primary wavelength that forms the aerial image  173   a  and a certain number of pulses have a second primary wavelength that forms the aerial image  173   b.    
     Additionally, the slit  176  and/or the mask  174  may move in in a scanning direction in the x-y plane such that only a portion of the wafer  170  is exposed at a given time or during a particular exposure scan (or exposure pass). The size of the area on the wafer  170  exposed by the light beam  160  is determined by the distance between the blades in the non-scanning direction and by the length (distance) of the scan in the scanning direction. In some implementations, the value of N is in the tens, for example, from 10-100 pulses. In other implementations, the value of N is greater than 100 pulses, for example, from 100-500 pulses. An exposure field  479  of the wafer  170  is the physical area of the wafer  170  that is exposed in one scan of an exposure slit or window within the lithography exposure apparatus  169 . 
     The wafer stage  171 , the mask  174 , and the objective arrangement  432  are fixed to associated actuation systems to thereby form a scanning arrangement. In the scanning arrangement, one or more of the mask  174 , the objective arrangement  432 , and the wafer  170  (via the stage  171 ) may move relative to each other in the x-y plane. However, aside from incidental relative operational motion between the wafer stage  171 , the mask  174 , and the objective arrangement  432 , these elements are not moved relative to each other along the z axis during an exposure pass or an exposure pass. 
     Referring to  FIG.  5   , a flow chart of a process  500  is shown. The process  500  is an example of a process for forming a three-dimensional semiconductor component or a portion of such a component. The process  500  may be performed using the photolithography system  100  or  200 . The process  500  is discussed with respect to the system  200  shown in  FIG.  2 A . The process  500  is also discussed with respect to  FIGS.  6 A- 10 B . 
     The light beam  260  is directed toward the mask  174  ( 510 ). The light beam  260  is a pulsed light beam that includes a plurality of pulses, each of which are separated from each other in time such as shown in  FIG.  3 C .  FIGS.  6 A and  6 B  show examples of optical spectra of a single pulse that is part of the light beam  260 . Other pulses in the light beam  260  may have different optical spectra. 
     Referring to  FIG.  6 A , an optical spectrum  601 A of a pulse of light  600 A is shown. The pulse of light  600 A has non-zero intensity within a band of wavelengths. The band of wavelengths also may be referred to as the bandwidth of the pulse  600 A. 
     The information shown in  FIG.  6 A  is the instantaneous optical spectrum  601 A (or emission spectrum) of the pulse  600 A. The optical spectrum  601 A contains information about how the optical energy or power of a pulse of the light beam  260  is distributed over different wavelengths (or frequencies). The optical spectrum  601 A is depicted in the form of a diagram where the spectral intensity (not necessarily with an absolute calibration) is plotted as a function of the wavelength or optical frequency. The optical spectrum  601 A may be referred to as the spectral shape or intensity spectrum of a pulse of the light beam  260 . The pulse  600 A has a primary wavelength  602 A, which, in the example of  FIG.  6 A , is the peak intensity. Although the discussion of the pulses of the light beam  260  and the aerial images formed by the pulses of the light beam  260  refers to the primary wavelengths of the pulses, the pulses include wavelengths other than the primary wavelength and the pulses have a finite bandwidth that may be characterized by a metric. For example, the full width of the spectrum  601 A at a fraction (X) of the maximum peak intensity of the spectral shape (referred to as FWXM) may be used to characterize the light beam bandwidth. As another example, the width of the spectrum that contains a fraction (Y) of the integrated spectral intensity (referred to as EY) may be used to characterize the light beam bandwidth. 
     The pulse  600 A is shown as an example of a pulse that may be in the light beam  260 . When the pulse  600 A is used to expose a portion of the wafer  120 , the light in the pulse forms an aerial image. The location of the aerial image in the z direction ( FIGS.  1 C and  4   ) is determined by the value of the primary wavelength  602 A. The various pulses in the light beam  260  may have different primary wavelengths. For example, to generate two aerial images during a single exposure pass, some of the pulses of the light beam  260  have one primary wavelength (a first primary wavelength) and the other pulse of the light beam  260  have another primary wavelength (a second primary wavelength). The first and second primary wavelengths are different wavelengths. The wavelength difference between the first and second primary wavelengths may be referred to as the spectral separation. The spectral separation may be, for example, 200 femtometers (fm) to 5 picometers (pm). Although the wavelengths of the various pulses in the light beam  260  may be different, the shape of the optical spectra of the pulses may be the same. 
     The light source  205  may dither or switch the primary wavelength between the first and second primary wavelengths on a pulse-to-pulse basis such that every pulse has a different primary wavelength than a pulse that immediately precedes or follows the pulse in time. In these implementations, assuming that all of the pulses in the light beam  260  have the same intensity, distributing the first and second primary wavelengths in this manner results in two aerial images at different locations in the z direction with the same intensity. 
     In some implementations, a certain portion (for example, 33%) of the pulses have the first primary wavelength, and the remainder (67% in this example) have the second primary wavelength. In these implementations, assuming that all of the pulses in the light beam  260  have the same intensity, two aerial images are formed of different intensities. The aerial image formed by the pulses having the first primary wavelength has about half of the intensity of the aerial image formed by the pulses having the second primary wavelength. In this way, the dose provided to a particular location in the wafer  170  along the z axis may be controlled by controlling the portion of the N pulses that have each of the first and second primary wavelengths. 
     The portion of pulses that are to have a particular primary wavelength for an exposure pass may be specified in a recipe file  259  that is stored on the electronic storage  252 . The recipe  259  specifies the ratio of the various primary wavelengths for an exposure pass. The recipe  259  also may specify the ratio for other exposure passes, such that a different ratio may be used for other exposure passes and the aerial images may be adjusted or controlled on a field-by-field basis. 
     Referring to  FIG.  6 B , an optical spectrum  601 B of a pulse  600 B is shown. The pulse  600 B is another example of a pulse of the light beam  260 . The optical spectrum  601 B of the pulse  600 B has a different shape than the optical spectrum  601 A. In particular, the optical spectrum  601 B has two peaks that correspond to two primary wavelengths  602 B_ 1  and  602 B_ 2  of the pulse  600 B. The pulse  600 B is part of the light beam  260 . When the pulse  600 B is used to expose a portion of the wafer  120 , the light in the pulse forms two aerial images at different locations along the z axis on the wafer. The locations of the aerial images are determined by the wavelengths of the primary wavelengths  602 B_ 1  and  602 B_ 2 . 
     The pulses shown in  FIGS.  6 A and  6 B  may be formed by any hardware capable of forming such pulses. For example, a pulse train of pulses such as the pulse  600 A may be formed using a line narrowing module similar to the line narrowing module  216 C of  FIG.  2 C . The wavelength of the light diffracted by the grating  291  depends on the angle of the light that is incident on the grating. A mechanism to change the angle of incidence of light that interacts with the grating  291  may be used with such a line narrowing module to create a pulse train with N pulses for an exposure pass, where at least one of the N pulses has a primary wavelength that is different from the primary wavelength of another pulse of the N pulses. For example, one of the prisms  292 ,  293 ,  294 ,  295  may be rotated to change the angle of light that is incident on the grating  291  on a pulse-by-pulse basis. In some implementations, the line narrowing module includes a mirror that is in the path of the beam  260  and is movable to change the angle of light that is incident on the grating  291 . An example of such an implementation is discussed, for example, in U.S. Pat. No. 6,192,064, titled NARROW BAND LASER WITH FINE WAVELENGTH CONTROL, issued on Feb. 20, 2001. 
     A pulse such as the pulse  600 B ( FIG.  6 B ) also may be formed using a line narrowing module similar to the line narrowing module  216 C of  FIG.  2 C . For example, a stimulated optical element, such as an acousto-optic modulator, may be placed in the line narrowing module  216 C in the path of the beam  260 . The acousto-optic modulator deflects incident light at an angle that depends on the frequency of an acoustic wave used to excite the modulator. An acoustic modulator includes a material, such as glass or quartz, that allows acoustic waves to propagate, and a transducer coupled to the material. The transducer vibrates in response to an excitation signal and the vibrations create acoustic waves in the material. The acoustic waves form moving planes of expansion and compression that change the index of refraction of the material. As a result, the acoustic waves act as a diffraction grating such that incident light is diffracted and exits the material at several different angles simultaneously. Light from two or more of the orders may be allowed to reach the grating  291 , and the light in each of the various diffraction orders has a different angle of incidence on the grating  291 . In this way, a single pulse that includes two or more primary wavelengths may be formed. An example of a line narrowing module that includes an acousto-optic modulator is discussed, for example, in U.S. Pat. No. 7,154,928, titled LASER OUTPUT BEAM WAVEFRONT SPLITTER FOR BANDWIDTH SPECTRUM CONTROL, issued on Dec. 26, 2006. 
     A set of pulses of light are passed through the mask  174  toward the wafer  170  during a single exposure pass ( 520 ). As discussed above, N pulses of light may be provided to the wafer  170  during the exposure pass. The N pulses of light may be consecutive pulses of light in the beam  260 . The exposed portion of the wafer  170  sees an average of the optical spectrum of each of the N pulses over the exposure pass. Thus, if a portion of the N pulses have a first primary wavelength and the remaining N pulses have a second primary wavelength, the average optical spectrum at the wafer  170  will be an optical spectrum that includes a peak at the first primary wavelength and a peak at the second primary wavelength. Similarly, if all or some of the individual pulses of the N pulses have more than one primary wavelength, those primary wavelengths may form peaks in the average optical spectrum.  FIG.  7    shows an example of an average optical spectrum  701  at the wafer  170 . The averaged optical spectrum  701  includes a first primary wavelength  702 _ 1  and a second primary wavelength  702 _ 2 . In the example of  FIG.  7   , the first primary wavelength  702 _ 1  and the second primary wavelength  702 _ 2  are separated by a spectral separation  703  of about 500 fm however other combinations can also be considered. The spectral separation  703  is such that the first primary wavelength  702 _ 1  and the second primary wavelength  702 _ 2  are distinct, and the average optical spectrum  701  includes a spectral region  704  of little to no intensity between the wavelengths  702 _ 1  and  702 _ 2 . 
     Two or more aerial images, for example, the first based on the first primary wavelength and the second based on the second primary wavelength, are formed at the wafer  170  based on the average optical spectrum ( 530 ). Continuing the example of the averaged optical spectrum  701  and referring also to  FIG.  8 A , two aerial images  873   a  and  873   b  are formed in a single exposure pass based on the N pulses. The N pulses include pulses that have the first primary wavelength  702 _ 1  and pulses that have the second primary wavelength  702 _ 2 . The pulses that have the first primary wavelength  702 _ 1  form the first aerial image  873   a , and the pulses that have the second primary wavelength  702 _ 2  form the second aerial image  873   b . The aerial image  873   a  is formed at a first plane  878   a , and the aerial image  873   b  is formed at a second plane  878   b . The planes  878   a  and  878   b  are perpendicular to a direction of propagation of the light beam  260  at the wafer  170 . The planes  878   a  and  878   b  are separated along the z direction by a separation distance  879 . 
     The separation distance  879  is larger than the depth of focus of the lithography apparatus  169  for an averaged optical spectrum that has a single primary wavelength. The depth of focus may be defined for a dose value (an amount of optical energy provided to the wafer) as the range of focus along the z direction at which that dose provides a feature size that is within an acceptable range of feature sizes for the process that is being applied to the wafer  170 . The process  500  is able to increase the depth of focus of the lithography exposure apparatus  169  by providing more than one distinct aerial image at the wafer  170  during a single exposure pass. This is because the plurality of aerial images are each able to expose the wafer at a different location in the z direction with features that are within the acceptable range of feature sizes. In other words, the process  500  is able to provide the lithography exposure apparatus  169  with a greater rage of depth of focus during a single exposure pass. As discussed above, the operator of the lithography exposure apparatus  169  may control various parameters of the exposure process through the recipe file  259 . In some implementations, the operator of the lithography exposure apparatus  169  may receive information from a simulation program, such as the Tachyon Source-Mask Optimization (SMO) available from Brion, an ASML Company, and this information may be used to program or otherwise specify the parameters of the recipe file  259 . For example, the operator of the lithography exposure apparatus  169  may know that an upcoming lot is not going to require as much depth of focus as previously exposed lot. In this example, the operator may specify a depth of focus and a dose variation to the simulation program, and the simulation program returns the value of the spectral separation  703  to achieve the desired parameters. The operator may then specify the value of the spectral separation  703  for the upcoming lot by programing the recipe file  259  through the I/O interface  253 . In some implementations, the operator may use the simulation to determine whether or not a greater depth of focus (such as is possible by exposing the wafer  170  with a plurality of aerial images at distinct planes) is needed for a particular exposure pass. In instances in which the greater depth of focus is not required to form a particular portion of the semiconductor component, the recipe file  259  may be structured so that, for example, the exposure pass used to form that particular portion of the semiconductor component has an averaged optical spectrum that includes a single primary wavelength. 
     Moreover, the operator and/or simulator may receive information about the formed three-dimensional component as measured by the metrology system  172  or by another sensor. For example, the metrology system  172  may provide data relating to a sidewall angle of the formed 3D semiconductor component and the data may be used to program parameters in the recipe file  259  for a subsequent exposure pass. 
       FIG.  8 B  shows the aerial image  873   a  in the x-y plane (looking into the page in  FIG.  8 A ) at the plane  878   a . The aerial images  873   a  and  873   b  are generally two-dimensional intensity patterns that are formed in the x-y plane. The nature of the intensity pattern depends on the characteristics of the mask  174 . The first and second planes  878   a ,  878   b  are portions of the wafer  170 . As illustrated in  FIG.  8 B , the first plane  878   a  may be only a small portion of the entire wafer  170 . 
     The value of the separation distance  879  depends on the spectral separation  703  and on properties of the optical system  275 . For example, the value of the separation distance  879  may depend on the focal length, aberration, and other properties of lenses and other optical elements in the optical system  275 . For a scanner lens with a chromatic aberration C, the separation distance  879  may be determined from Equation 1: 
       Δ D=C*Δλ.   Equation (1),
 
     where ΔD is the separation distance  879  in nanometers (nm), C is the chromatic aberration (defined as the distance the focal plane moves in the propagation direction for a wavelength change), and Δλ, is the spectral separation  873  in picometers. The separation distance  875  may be, for example, 5000 nm (5 μm), and the spectral separation  873  may be about 200-300 fm. 
     Moreover, due to variations in manufacturing and installation processes and/or modifications made by end users, different primary wavelengths may be required to achieve a desired separation distance  879  for a particular instance of a certain type of exposure apparatus  169 . As discussed above, a recipe or process control program  259  may be stored on the electronic storage  252  of the control system  250 . The recipe  259  may be modified or programmed to be customized to a particular exposure apparatus or a type of exposure apparatus. The recipe  259  may be programmed when the lithography system  200  is manufactured and/or the recipe  259  may be programmed via, for example, the I/O interface  253 , by an end user or other operator familiar with the performance of the system  200 . 
     The recipe  259  also may specify a different separation distance  879  for different exposure passes used to expose different areas of the wafer  170 . Additionally or alternatively, the recipe  259  may specify the separation distance  879  on a per-lot or per-layer basis or on a per-wafer basis. A lot or a layer is a group of wafers that are processed by the same exposure apparatus under the same nominal conditions. The recipe  259  also allows specification of other parameters related to the aerial images  873   a ,  873   b , such as the dose provided by each image. For example, the recipe  259  may specify a ratio of the number of pulses in the N pulses that have the first primary wavelength  702 _ 1  to the number of pulses that have the second primary wavelength  702 _ 2 . These other parameters also may be specified on a per-field, per-lot (or per-layer), and/or per-wafer basis. 
     Moreover, the recipe  259  may specify that some layers are not exposed with the first primary wavelength  702 _ 1  and the second primary wavelength  702 _ 2  and are instead exposed with a pulse that has an optical spectrum that includes a single primary wavelength. Such an optical spectrum may be used, for example, when a planar semiconductor component is to be formed instead of a three-dimensional semiconductor component. The I/O interface  253  allows an end-user and/or manufacturer to program or create the recipe to specify the number of primary wavelengths, including a scenario in which a single primary wavelength is used, for example, for a particular layer or lot. 
     Additionally, although the example above discusses the average optical spectrum  701  having two primary wavelengths, in other examples, the average optical spectrum  701  may have more than two primary wavelengths (for example, three, four, or five primary wavelengths), each of which are separated from the nearest other primary wavelength by a spectral separation and a region such as the region  704 . The I/O interface  253  allows an end-user and/or manufacturer to program or create the recipe to specify these parameters. 
     A three-dimensional (3D) semiconductor component is formed ( 540 ).  FIG.  9 A  shows a cross-sectional view of an example of a 3D semiconductor component  995 .  FIG.  9 B  shows the wafer  170  and the component  995  in the x-y plane at the first plane  878   a . The 3D semiconductor component  995  may be a complete component or a portion of a larger component. The 3D semiconductor component  995  may be any type of semiconductor component that has features that are not all formed at one z location in the wafer  170 . For example, the 3D semiconductor component may be a device that includes recesses or openings that extend along the z axis. The 3D semiconductor component may be used for any type of electronic application. For example, the 3D semiconductor component may be all or part of a 3D NAND flash memory component. A 3D NAND flash memory is a memory in which memory cells are stacked along the z axis in layers. 
     In the example of  FIG.  9 A , the 3D semiconductor component  995  includes a recess  996  that is formed in a periphery  999 . The recess  996  includes a floor  997  and a sidewall  998 , which extends generally along the z axis between the periphery  999  and the floor  997 . The floor  997  is formed by exposing photoresist at the plane  878   b  with light that is in the second aerial image  873   b  ( FIG.  8 A ). Features on the periphery  999  are formed using light that is in the first aerial image  873   a  ( FIG.  8 A ). 
     Using the process  500  also may result in a sidewall angle  992  being equal to 90° or closer to 90° than is possible with other processes. The sidewall angle  992  is the angle between the floor  997  and the sidewall  998 . If the sidewall  998  extends in the x-z plane and the floor extends in the x-y plane, the sidewall angle  992  is 90° and may be considered vertical in this example. A vertical sidewall angle is desirable because, for example, such a sidewall allows for more well-defined features in a 3D semiconductor component. The process  500  achieves a sidewall angle  992  that is equal or close to 90° because the locations of the first aerial image  873   a  and the second aerial image  873   b  (the first plane  878   a  and the second plane  878   b , respectively) are separate images that are in different parts of the wafer  170 . Forming separate aerial images in a single exposure pass allows the quality of each of the images to be improved resulting in a more defined feature that is more vertically oriented as compared to a feature formed by a single aerial of lower quality. 
       FIGS.  10 A and  10 B  are examples of simulated data relating to the process  500 .  FIG.  10 A  shows three plots  1001 ,  1002 ,  1003  of aerial image intensity versus mask position along the y axis ( FIG.  9 A ). Each of the plots  1001 ,  1002 ,  1003  represents intensity versus mask position for one aerial image. In  FIG.  10 A , the plot  1001  represents a simulation of an average optical spectrum that forms two aerial images during a single exposure pass, such as discussed above with respect to  FIG.  5   . The plot  1002  represents a simulation of a situation in which the wafer stage is tilted according to ASML&#39;s EFESE technique, which is a procedure for increasing the depth of focus to facilitate the printing of three-dimensional features (such as vias and holes) on a wafer. In the EFESE technique, the wafer stage is tilted at an angle to scan the aerial image through the focus while exposing the wafer. The EFESE technique generally results in a greater depth of focus. In  FIG.  10 A , only the plot  1002  represents data simulated using the EFESE technique. The remaining data shown on  FIG.  10 A  did not employ the EFESE technique. The plot  1003  represents data from a simulation of a best focus based on dose. 
     The aerial image intensity as a function of mask position shown in  FIG.  10 A  illustrates that forming two or more aerial images in a single exposure pass may produce similar contrast as tilting the wafer stage. A greater contrast indicates that the three-dimensional features that are at different locations along the z axis ( FIG.  8 A ) are more likely to be properly formed. 
       FIG.  10 B  shows three plots  1004 ,  1005 ,  1006  of critical dimension as a function of the focus position for three different aerial images, with each aerial image averaged over an exposure pass. In  FIG.  10 B , the plot  10004  represents data from a simulation in which no EFESE technique was applied and a single aerial image was formed. The plot  1005  represents data from a simulation in which the EFESE technique was applied. As shown, the EFESE technique increases the depth of focus as compared to the no-EFESE simulation because the critical dimension value remains the same for a further distance from zero focus. The plot  1005  represents data from a simulation in which two aerial images were generated in a single exposure pass and no EFESE technique was employed. The depth of focus for the no-EFESE simulations using multiple aerial images are on par or better than the EFESE technique. Thus, the process  500  may be used to achieve a greater depth of focus in a single exposure pass without relying on a technique such as EFESE. 
     The embodiments may further be described using the following clauses: 
     1. A method of forming a three-dimensional semiconductor component using a photolithography system, the method comprising: 
     directing a pulsed light beam along a direction of propagation toward a mask, the pulsed light beam comprising a plurality of pulses of light; 
     passing a set of the pulses of light in the light beam through the mask toward a wafer during a single exposure pass; 
     generating, during the single exposure pass, at least a first aerial image and a second aerial image on the wafer based on pulses of light in the set of pulses that pass through the mask, the first aerial image being at a first plane on the wafer and the second aerial image being at a second plane on the wafer, the first plane and the second plane being separated from each other by a separation distance along the direction of propagation; and 
     patterning, in photoresist, the three-dimensional semiconductor component based on an interaction between light in the first aerial image and a material in a first portion of the wafer and an interaction between light in the second aerial image and a material in a second portion of the wafer, wherein 
     at least one of the pulses in the set of pulses has a first primary wavelength and at least one of the other pulses in the set of pulses has a second primary wavelength that is different from the first primary wavelength, such that spectra of the first and second set of pulses are spectrally distinct and the separation distance is based on the difference between the first primary wavelength and the second primary wavelength. 
     2. The method of clause 1, wherein at least one of the pulses in the set of pulses that passes through the mask during the single exposure pass comprises more than one primary wavelength of light.
 
3. The method of clause 2, wherein each primary wavelength is separated by a spectral separation of 200 femtometers (fm) to 500 picometers (pm) from the nearest other primary wavelength.
 
4. The method of clause 1, wherein the separation distance between the first aerial image and the second aerial image changes during the single exposure pass.
 
5. The method of clause 1, wherein the single exposure pass is a first exposure pass, and the method further comprising: passing a second set of pulses of light in the light beam through the mask toward the wafer during a second exposure pass and after the first exposure pass is completed, and wherein the separation distance between the first aerial image and the second aerial image is different during the first exposure pass and the second exposure pass.
 
6. The method of clause 1, wherein the separation distance between the first aerial image and the second aerial image is set prior to the single exposure pass, and the separation distance does not change during the single exposure pass.
 
7. The method of clause 6, wherein the separation distance between the first aerial image and the second aerial image is set to accommodate one or more features of the photolithography system.
 
8. The method of clause 1, wherein the set of pulses comprises a first group of pulses of light and a second group of pulses of light, each pulse in the first group of pulses of light has the first primary wavelength, each pulse in the second group of pulses has the second primary wavelength, and the method further comprising:
 
     controlling a property of the first group of pulses to thereby control an amount of light in the first aerial image; and 
     controlling a property of the second group of pulses to thereby control an amount of light in the second aerial image. 
     9. The method of clause 8, wherein the property of the first group comprises a count of pulses in the first group, and the property of the second group comprises a count of pulses in the second group.
 
10. The method of clause 9, wherein controlling the count of pulses in the first group comprises determining, before the single exposure pass begins, a first number of pulses to include in the first group of pulses, and controlling the count of pulses in the second group of pulses comprises determining, before the single exposure pass, a second number of pulses to include in the second group of pulses.
 
11. The method of clause 10, wherein determining the first number of pulses and the second number of pulses comprises one or more of: (a) receiving input from an operator and (b) accessing a pre-defined setting associated with the photolithography system.
 
12. The method of clause 8, wherein the property of the first group of pulses comprises an intensity of each pulse in the first group, and the property of the second group of pulses comprises an intensity of each pulse in the second group.
 
13. The method of clause 1, wherein the first plane on the wafer and the second plane on the wafer are planes that are substantially perpendicular to the direction of propagation.
 
14. The method of clause 9, wherein the first group of pulses and the second group of pulses comprise all of the pulses that pass through the mask in the single exposure pass.
 
15. The method of clause 1, wherein
 
     a first feature of the three-dimensional semiconductor is formed at the first plane, 
     a second feature of the three-dimensional semiconductor is formed at the second plane, and the first and second features are displaced from each other by a sidewall that extends substantially parallel to the direction of propagation. 
     16. The method of clause 1, wherein the three-dimensional semiconductor component comprises a three-dimensional NAND flash memory component.
 
17. The method of clause 1, wherein the first plane corresponds to a first focal plane and the second plane corresponds to a second focal plane, and the separation distance between the first plane and the second plane is based on a difference between one or more wavelengths in a pulse of light that passes through the mask or a difference between a wavelengths among discrete pulses in the set of pulses.
 
18. A photolithography system comprising:
 
     a light source; 
     a lithography scanner apparatus comprising: 
     a mask positioned to interact with a pulsed light beam from the light source, and 
     a wafer holder; and 
     a control system coupled to the light source, the control system configured to cause the light source to emit the pulsed light beam toward the lithography scanner apparatus during a single exposure pass such that, during the single exposure pass, at least a first aerial image and a second aerial image are formed on a wafer received at the wafer holder based on pulses of light in a set of pulses of light that pass through the mask along a direction of propagation, the first aerial image being at a first plane on the wafer and the second aerial image being at a second plane on the wafer, the first plane and the second plane being separated from each other by a separation distance along the direction of propagation, and a three-dimensional semiconductor component is formed based on an interaction between light in the first aerial image and a material in a first portion of the wafer and an interaction between light in the second aerial image and a material in a second portion of the wafer, wherein 
     at least one of the pulses in the set of pulses has a first primary wavelength, 
     at least one of the other pulses in the set of pulses has a second primary wavelength that is different from the first primary wavelength such that spectra of the first and second set of pulses are spectrally distinct, and 
     the separation distance is based on the difference between the first primary wavelength and the second primary wavelength. 
     19. The photolithography system of clause 18, wherein the control system comprises a computer-readable storage medium, one or more electronic processors coupled to the computer-readable storage medium, and an input/output interface, and a recipe related to the photolithography system is stored on the computer-readable storage medium.
 
20. The photolithography system of clause 19, wherein the recipe specifies the separation distance.
 
21. The photolithography system of clause 20, wherein the recipe specifies the separation distance on a per-wafer or per-lot basis.
 
22. The photolithography system of clause 18, wherein the light source comprises a krypton fluoride (KrF) gain medium or a argon fluoride (ArF) gain medium.
 
     Other implementations are within the scope of the claims.