CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claim priority to U.S. Application No. 62/915,182, filed October 15, 2019 and titled LITHOGRAPHIC APPARATUS AND DEVICE MANUFACTURING METHOD, which is incorporated herein in its entirety by reference.

TECHNICAL FIELD

[0002] The present disclosure relates to a lithographic apparatus and a method for manufacturing a device.

BACKGROUND

[0003] Extreme ultraviolet (EUV) radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm, and may be produced using a plasma. A radiation system for producing EUV radiation may include a laser for exciting a fuel (also called a target material) to provide the plasma, and a source collector apparatus for containing the plasma and collecting the EUV radiation. The plasma may be created, for example, by directing a laser beam at a fuel, such as particles, e.g., liquid droplets, of a suitable material (e.g. tin), or a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector. The radiation collector may be a mirrored normal incidence radiation collector, which receives the radiation and focuses the radiation into a beam. The source collector apparatus may include an enclosing structure or chamber arranged to provide a vacuum environment to support the plasma. Such a radiation system is typically termed a laser produced plasma (LPP) source. [0004] One application of an EUV radiation source is in lithography. A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. [0005] In order to reduce the minimum printable size, imaging may be performed using radiation having a short wavelength. It has therefore been proposed to use an EUV radiation source providing EUV radiation within the range of 13-14 nm, for example. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Such radiation is termed extreme ultraviolet radiation or soft x-ray radiation.

[0006] In addition to the desired EUV radiation, the EUV radiation source generates non- EUV, out-of-band (such as deep ultraviolet (DUV)) radiation. This out-of-band radiation may be transmitted to the substrate, and can negatively affect the resultant image as the resist may be sensitive to this out-of-band radiation. It is desirable to mitigate the effect of the out-of- band source radiation during a lithographic process.

SUMMARY

[0007] The following presents a simplified summary of one or more embodiments in order to provide a basic understanding of the embodiments. This summary is not an extensive overview of all contemplated embodiments, and is not intended to identify key or critical elements of all embodiments nor delineate the scope of any or all embodiments. Its sole purpose is to present some concepts of one or more embodiments in a simplified form as a prelude to the more detailed description that is presented later.

[0008] According to an aspect of an embodiment, there is disclosed a lithographic apparatus comprising a source, the source comprising an illumination system configured to condition a radiation beam, the radiation beam comprising both EUV radiation and non-EUV radiation and a controller adapted to control a dose of radiation delivered to a substrate by the illumination system at least partially on the basis of a ratio of a magnitude of energy of the non-EUV radiation to a magnitude of energy of the EUV radiation. The non-EUV radiation may be DUV radiation. The apparatus may further comprise a sensor, i.e., one or more sensors, arranged to measure a magnitude of energy of the DUV radiation. The sensor may be located to measure the magnitude of energy of the DUV radiation at the substrate. The sensor may be located to measure the magnitude of energy of the DUV radiation at the illumination system. The apparatus may include a module configured to infer a magnitude of energy of the DUV radiation from a plurality of operating parameters of the source. The controller may be adapted to control a dose of radiation delivered to a substrate by the illumination system at least partially on the basis of a product of the ratio and a calibration factor. [0009] According to another aspect of an embodiment, there is disclosed a lithographic apparatus comprising an illumination system configured to condition a radiation beam, the radiation beam comprising both EUV radiation and non-EUV radiation, a first module for generating a first signal indicative of a magnitude of energy of the EUV radiation, a second module for generating a second signal indicative of a magnitude of energy of the non-EUV radiation, a third module configured to multiply the second signal by a calibration factor to obtain a third signal, a fourth module configured to add the first signal and the third signal to obtain a fourth signal, and a controller arranged to receive the fourth signal and adapted to control a dose of radiation delivered to a substrate by the radiation beam at least partially on the basis of a sum of a magnitude of energy of the EUV radiation and a product of a calibration factor and a magnitude of an energy of the non-EUV radiation. The non-EUV radiation may be DUV radiation. The second module may comprise a, i.e., at least one sensor arranged to measure a magnitude of energy of the non-EUV radiation. The sensor may be located to measure the magnitude of energy of the non-EUV radiation at the substrate. The sensor may be located to measure the magnitude of energy of the non-EUV radiation at the illumination system. The second module may be configured to infer a magnitude of DUV radiation from a plurality of operating parameters of the source.

[0010] According to another aspect of an embodiment, there is disclosed a method of manufacturing a device, comprising using a radiation source including an illumination system to generate a radiation beam, the radiation beam comprising both EUV radiation and non- EUV radiation and controlling a dose of radiation delivered to a substrate by the radiation beam at least partially on the basis of a ratio of a magnitude of energy of the non-EUV radiation to a magnitude of energy of the EUV radiation. The non-EUV radiation may be DUV radiation. The method may include measuring the magnitude of energy of the DUV radiation at the substrate or at the illumination system. The method may include inferring the magnitude of energy of the DUV radiation from a plurality of operating parameters of the source. Controlling a dose of radiation delivered to a substrate by the radiation beam at least partially on the basis of a ratio of a magnitude of energy of the DUV radiation to a magnitude of energy of the EUV radiation may be performed at least partially on the basis of a product of the ratio and a calibration factor.

[0011] According to another aspect of an embodiment, there is disclosed a method of manufacturing a device, comprising generating a radiation beam, the radiation beam comprising both EUV radiation and non-EUV radiation, generating a first signal indicative of a magnitude of energy of the EUV radiation, generating a second signal indicative of a magnitude of energy of the non-EUV radiation, multiplying the second signal by a calibration factor to obtain a third signal, adding the first signal and the third signal to obtain a fourth signal, and supplying the fourth signal to a dose controller, the dose controller controlling a dose of radiation delivered to a substrate by the radiation beam at least partially on the basis of a sum of a magnitude of energy of the EUV radiation and a product of a calibration factor and a magnitude of an energy of the non-EUV radiation. The non-EUV radiation may be DUV radiation. The method may include sensing a magnitude of energy of the non-EUV radiation which may comprise measuring the magnitude of energy of the non-EUV radiation at the substrate. Generating a second signal indicative of a magnitude of energy of the non- EUV radiation may comprise inferring a magnitude of the non-EUV radiation from a plurality of operating parameters of the source.

[0012] Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. It is noted that the present invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the present invention and to enable a person skilled in the relevant art(s) to make and use the present invention.

[0014] FIG. 1 depicts schematically a lithographic apparatus having reflective projection optics.

[0015] FIG. 2 is a more detailed view of the apparatus of FIG. 1.

[0016] FIG. 3 is a schematic depiction of a dose control arrangement according to an aspect of an embodiment.

[0017] FIG. 4 is a flowchart depicting a method of operation of the embodiment of FIG. 3 according to an aspect of an embodiment.

[0018] FIG. 5 is a schematic depiction of a dose control arrangement according to an aspect of an embodiment.

[0019] FIG. 6 is a flowchart depicting a method of operation of the embodiment of FIG. 5 according to an aspect of an embodiment. [0020] FIG. 7 is a flowchart showing a method of determining a calibration constant according to an aspect of an embodiment.

[0021] The features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements.

DETAILED DESCRIPTION

[0022] This specification discloses one or more embodiments that incorporate the features of this invention. The disclosed embodiment(s) merely exemplify the present invention. The scope of the present invention is not limited to the disclosed embodiment(s). The present invention is defined by the claims appended hereto.

[0023] The embodiment(s) described, and references in the specification to “one embodiment”, “an embodiment”, “an example embodiment”, etc., indicate that the embodiment(s) described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is understood that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

[0024] In the description that follows and in the claims the terms “up,” “down,” “top,” “bottom,” “vertical,” “horizontal,” and like terms may be employed. These terms are intended to show relative orientation only and not any orientation with respect to gravity. Similarly, terms such as left, right, front, back, etc., are intended to give only relative orientation.

[0025] Before describing embodiments in more detail, it is instructive to present an example environment in which embodiments of the present invention may be implemented.

[0026] FIG. 1 schematically depicts a lithographic apparatus 100 including a source module SO according to one embodiment of the invention. The apparatus comprises: an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. EUV radiation), a support structure (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask or a reticle) MA and connected to a first positioner PM configured to accurately position the patterning device; a substrate table (e.g. a wafer table) WT constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate; and a projection system (e.g. a reflective projection system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W. [0027] The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation. [0028] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be a frame or a table, for example, which may be fixed or movable as required. The support structure may ensure that the patterning device is at a desired position, for example with respect to the projection system.

[0029] The term “patterning device” should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. The pattern imparted to the radiation beam may correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

[0030] The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix. [0031] The projection system, like the illumination system, may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of a vacuum. It may be desired to use a vacuum for EUV radiation since other gases may absorb too much radiation. A vacuum environment may therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps. [0032] As here depicted, the apparatus is of a reflective type (e.g. employing a reflective mask).

[0033] The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such “multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.

[0034] Referring to FIG. 1, the illuminator IL receives an extreme ultra violet radiation beam from the source module SO. Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV range. In one such method, often termed laser produced plasma ("LPP") the required plasma can be produced by irradiating a fuel, such as a droplet, stream or cluster of material having the required line-emitting element, with a laser beam. The source module SO may be part of an EUV radiation system including a laser, not shown in Figure 1, for providing the laser beam exciting the fuel. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector, disposed in the source module. The laser and the source module may be separate entities, for example when a C02 laser is used to provide the laser beam for fuel excitation.

[0035] In such cases, the radiation beam is passed from the laser to the source module with the aid of a beam delivery system comprising, for example, suitable directing mirrors and/or a beam expander. An EUV source may be an integral part of the source module, for example when the EUV source is a discharge produced plasma EUV generator, often termed as a DPP source.

[0036] The radiation beam B is incident on the patterning device (e.g., mask) MA, which is held on the support structure (e.g., mask table) MT, and is patterned by the patterning device. After being reflected from the patterning device (e.g. mask) MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor PS2 (e.g. an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor PS 1 can be used to accurately position the patterning device (e.g. mask) MA with respect to the path of the radiation beam B. Patterning device (e.g. mask) MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks PI, P2. [0037] The depicted apparatus could be used in at least one of the following modes: 1. In step mode, the support structure (e.g. mask table) MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e. a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed. 2. In scan mode, the support structure (e.g. mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e. a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure (e.g. mask table) MT may be determined by the (de- )magnification and image reversal characteristics of the projection system PS. 3. In another mode, the support structure (e.g. mask table) MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.

[0038] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[0039] FIG. 2 shows an embodiment of the lithographic apparatus in more detail, including a radiation system 42, the illumination system IL, and the projection system PS. The radiation system 42 as shown in Figure 2 is of the type that uses a laser-produced plasma as a radiation source. EUV radiation may be produced using the laser to convert a gas, vapor, or liquid (continuous stream or droplets), for example Xe gas, Li vapor or Sn vapor or liquid droplets in which a very hot plasma is created to emit radiation in the EUV range of the electromagnetic spectrum. The very hot plasma is created by causing an at least partially ionized plasma by, for example, optical excitation using C02 laser light. Partial pressures of, for example, 10 Pa of Xe, Li, Sn vapor or any other suitable gas or vapor may be required for efficient generation of the radiation. In an embodiment, Sn is used to create the plasma in order to emit the radiation in the EUV range.

[0040] The radiation system 42 embodies the function of source SO in the apparatus of FIG.

1. Radiation system 42 comprises a source chamber 47, in this embodiment not only substantially enclosing a source of EUV radiation, but also collector 50 which, in the example of FIG. 2, is a normal-incidence collector, for instance a multi-layer mirror.

[0041] As part of an LPP radiation source, a laser system 61 is constructed and arranged to provide a laser beam 63 which is delivered by a beam delivering system 65 through an aperture 67 provided in the collector 50. Also, the radiation system includes a target material 69, such as Sn or Xe, which is supplied by target material supply 71. The beam delivering system 65, in this embodiment, is arranged to establish a beam path focused substantially upon a desired plasma formation position 73.

[0042] In operation, the target material 69, which may also be referred to as fuel, is supplied by the target material supply 71 in the form of droplets. When such a droplet of the target material 69 reaches the plasma formation position 73, the laser beam 63 impinges on the droplet and an EUV radiation-emitting plasma forms inside the source chamber 47. In the case of a pulsed laser, this involves timing the pulse of laser radiation to coincide with the passage of the droplet through the position 73. As mentioned, the fuel may be for example xenon (Xe), tin (Sn) or lithium (Li). These create a highly ionized plasma with electron temperatures of 30 to 50 eV. Higher energy EUV radiation may be generated with other fuel materials, for example Tb and Gd. The energetic radiation generated during de-excitation and recombination of these ions includes the wanted EUV which is emitted from the plasma at position 73. The plasma formation position 73 and the aperture 52 are located at first and second focal points of collector 50, respectively and the EUV radiation is focused by the normal-incidence collector mirror 50 onto the intermediate focus point IF.

[0043] The beam of radiation emanating from the source chamber 47 traverses the illumination system IL via so-called normal incidence reflectors 53, 54, as indicated in Figure 2 by the radiation beam 56. The normal incidence reflectors direct the beam 56 onto a patterning device (e.g. reticle or mask) positioned on a support (e.g. reticle or mask table)

MT. A patterned beam 57 is formed, which is imaged by projection system PS via reflective elements 58, 59 onto a substrate carried by wafer stage or substrate table WT. More elements than shown may generally be present in illumination system IL and projection system PS. [0044] For example there may be one, two, three, four or even more reflective elements present than the two elements 58 and 59 shown in Figure 2. Radiation collectors similar to radiation collector 50 are known from the prior art.

[0045] As the skilled reader will know, reference axes X, Y and Z may be defined for measuring and describing the geometry and behavior of the apparatus, its various components, and the radiation beams 55, 56, 57. At each part of the apparatus, a local reference frame of X, Y and Z axes may be defined. The Z axis broadly coincides with the direction of optical axis O at a given point in the system, and is generally normal to the plane of a patterning device (reticle) MA and normal to the plane of substrate W. In the source module (apparatus) 42, the X axis coincides broadly with the direction of fuel stream (69, described below), while the Y axis is orthogonal to that, pointing out of the page as indicated. On the other hand, in the vicinity of the support structure MT that holds the reticle MA, the X axis is generally transverse to a scanning direction aligned with the Y axis. For convenience, in this area of the schematic diagram Figure 2, the X axis points out of the page, again as marked. These designations are conventional in the art and will be adopted herein for convenience. In principle, any reference frame can be chosen to describe the apparatus and its behavior.

[0046] In addition to the wanted EUV radiation, the plasma produces other wavelengths of radiation, for example in the visible, UV and DUV range. There is also IR (infrared) radiation present from the laser beam 63. The non- EUV wavelengths are not wanted in the illumination system IL and projection system PS and various measures may be deployed to block the non- EUV radiation. As schematically depicted in FIG. 2, a transmissive spectral filter (SPF) may be applied upstream of the virtual source point IF. It will be understood by one of ordinary skill in the art that the transmissive SPF may alternatively be provided downstream of the virtual source point IF. Alternatively or in addition to such a filter, filtering functions can be integrated into other optics. Filters for DUV and other unwanted wavelengths may thus be provided at one or more locations along the paths of beams 55, 56, 57, within source module (radiation system 42), the illumination system IL and/or projection system PS and/or above wafer table (WT). Despite these measures there may still be residual DUV radiation in the radiation beam.

[0047] To deliver the fuel, which for example is liquid tin, a droplet generator or target material supply 71 is arranged within the source chamber 47, to fire a stream of droplets towards the plasma formation position 73. In operation, laser beam 63 may be delivered in a synchronism with the operation of target material supply 71, to deliver impulses of radiation to turn each fuel droplet into a plasma. The frequency of delivery of droplets may be several kilohertz, or even several tens or hundreds of kilohertz. In practice, laser beam 63 may be delivered by a laser system 61 in at least two pulses: a pre pulse (PP)with limited energy is delivered to the droplet before it reaches the plasma location, in order to expand the fuel material to a disk target or vaporize the fuel material into a small cloud, and then a main pulse (MP) of laser energy is delivered to the cloud at the desired location, to generate the plasma. In a typical example, the diameter of the plasma is about 300um to 800um. A trap 72 is provided on the opposite side of the enclosing structure 47, to capture fuel that is not, for whatever reason, turned into plasma.

[0048] Laser system 61 is may be for example of the MOP A (Master Oscillator Power Amplifier) type. Such a laser system 61 includes a “master” laser or “seed” laser, followed by a power amplifier system PA, for firing a main pulse of laser energy towards an expanded disk target or droplet cloud, and a pre pulse laser for firing a pre pulse of laser energy towards a droplet. A beam delivery system 65 is provided to deliver the laser energy 63 into the source chamber 47. In practice, the pre-pulse element of the laser energy may be delivered by a separate laser. Laser system 61, target material supply 71 and other components can be controlled by a controller (not shown separately). The controller performs many control functions, and has sensor inputs and control outputs for various elements of the system. Sensors may be located in and around the elements of radiation system 42, and optionally elsewhere in the lithographic apparatus. In some embodiments of the present invention, the main pulse and the pre pulse are derived from a same laser. In other embodiment of the present invention, the main pulse and the pre-pulse are derived from different lasers which are independent from each other but controlled to operate synchronously. In order to block as much contamination as possible, a contamination trap 80 of some sort may be provided between the plasma formation site 73 and optical elements of the beam delivery system 65. [0049] As noted, both LPP and discharge produced plasma (DPP) EUV sources emit a broad spectrum of wavelengths, comprising the desired EUV radiation (at 13.5 nm) and other out- of-band wavelengths. Out-of-band wavelengths in this context may comprise deep ultraviolet (DUV) radiation (at around 130 nm to 400 nm) and beyond. This DUV radiation is emitted from the low-density low-temperature part of the plasma when the target material being used is tin. The DUV portion of the emitted light is a by-product of the EUV plasma emission and can, in principle, propagate through the illuminator and the projection optics to the wafer and affect imaging performance by contributing to the exposure in photoresist. This is because the photoresist at the wafer is not only sensitive for the 13.5 nm EUV light, but also for the other out-of-band wavelengths. At the scanner wafer level, a typical chemically activated resist (CAR) for EUV is very sensitive to DUV.

[0050] The actual EUV imaging performance is affected adversely by the non-EUV out-of- band content in the spectrum. This non-EUV part of the spectrum contains only wavelengths that are far too long to be able to resolve the features of interest at the mask (MA) on the wafer (W), and therefore only reduces the image contrast. As consequence, the imaging performance (e.g., critical dimension uniformity (CDU), image placement) is affected especially for the edges and comers of abutted dies, and imaging and optical process correction (OPC) (e.g. for matching between two different lithographic tools) is compromised. Parasitic DUV radiation propagating together with inband EUV radiation reaching wafers thus has an impact on the aerial images used to monitor and control the lithographic process.

[0051] The presence of DUV radiation also affects dose control, that is, the control of the amount of radiation supplied to the wafer during an exposure. This control is complicated by the fact that there are intrinsic drifts in beam characteristics in LPP EUV sources, such as drifts due to drive laser cold-hot thermal transition, main pulse / seed pulse laser degradation over time, drive laser gain command variation (from control modules), etc. Also, during production, the Sn plasma emission from time to time is not kept the same, as indicated by the fact that conversion efficiency (CE) is not constant. It also means that the ratio of the magnitude of the energy of the DUV radiation with respect to the magnitude of the energy of the EUV radiation (the DUV/EUV ratio) from the plasma and at the wafer level is varying. [0052] Also, the imaging performance may be affected adversely by the variations in the sensitivity of the resist in cases where relative sensitivity of resist to non- EUV vs EUV depends on the type of resist. Controlling the overall effective dose, including the non-EUV component, creates the ability to mitigate the effects of wavelength-dependent resist sensitivity.

[0053] Conventionally dose control, that is, control of the amount of energy (per unit area) that the photoresist is subjected to upon exposure by a lithographic exposure system, is based on measurements from the inband EUV metrology, which ensures constant inband EUV energy at the wafer level. As a consequence, however, the DUV/EUV ratio may drift, and DUV energy at wafer level may drift as function of time. Drift of DUV energy can be expected to cause critical dimension (CD) drifts on the product wafer. For a constant inband EUV dose but with drifting, e.g. increasing DUV energy, the photoresist will experience a higher effective dose, which will result in a smaller or larger CD depending on the tone of photoresist.

[0054] In other words, from a CD perspective, the net effective dose will drift if the DUV energy is permitted to drift. From this perspective, the effective dose is a combination of in- band (EUV) energy and out-of-band (DUV) energy. One approach to modeling the effective dose is as the sum of the EUV energy and some constant times the DUV energy. For some applications it is desirable to control (e.g., keep constant) this effective dose rather than just the EUV dose at the wafer level.

[0055] One option to control effective dose is to add a DUV/EUV ratio control loop to the dose control algorithm to make sure the DUV/EUV ratio is kept constant so that the effective dose is constant at the wafer level. Again, the effective dose DE can be determined as follows: DE = EEUV (1 + K* EDUV/EEUV)

[0056] where DE is effective dose, EEUV is the inband EUV energy, EDUV is the out-of-band DUV energy, and K is a constant determined, for example, empirically or by simulations. Or, if

R = DUV/EUV ratio = EDUV/EEUV

[0057] then

DE = EEUV (1 + KR)

[0058] The effective dose can be controlled, for example, by controlling operation of the prepulse laser or main pulse beam size at the target (for example by controlling the pressure in the pressurized pre-pulse module). In various implementations of a dose control technique, numerical simulations can be used for modelling, calculating, or estimating EUV emission and non-EUV emission by hot dense plasmas.

[0059] An example of a system implementing such a dose control method is shown schematically in FIG. 3. In the arrangement depicted in FIG. 3, a dose controller 310 includes a module 320 configured to control an EUV dose generated by an EUV source 300. The amount of the EUV radiation is measured by a sensor 330 which generates an output that is applied as a feedback input to the dose controller 310. At the same time, an amount of DUV radiation is sensed by sensor 340 and is supplied to a control module 330 which acts as another control loop in the controller 310. Control module 330 controls the effective dose on the basis of the DUV/EUV ratio. An example of an EUV sensor, in this context, is a sensor configured to measure power in the electromagnetic spectrum between, for example, 13.2- 13.8 nm, 13-14 nm, 10-15 nm, 5-20 nm, 10-30 nm, or other ranges of wavelength suitable for an EUV photolithographic process. An example of an DUV sensor, in this context, is a sensor configured to measure power in the electromagnetic spectrum in a range between, for example, approximately 14 nm, approximately 15 nm, approximately 30 nm, approximately 40 nm, approximately 50 nm, approximately 100 nm, approximately 130 nm, or approximately 200 nm (or other wavelengths relevant as lower bounds for out-of-band radiation in an EUV photolithographic process) and approximately 200 nm, approximately 250 nm, approximately 300 nm, approximately 350 nm, or approximately 400 nm, for example.

[0060] FIG. 4 is a flowchart describing a mode of operation of the system shown in FIG. 3.

In a step S 100 the amount of EUV energy at the substrate is input to a dose controller. In a step SI 10 the amount of DUV energy at the substrate is input into the dose controller. In a step S120 the dose controller controls total effective dose based on both inputs as contributions from both EUV energy and DUV energy. As will be apparent to one of ordinary skill in the art, the steps S100 and the steps SI 10 may occur concurrently.

[0061] Another approach would be to use a conventional current dose control algorithm to control effective EUV dose (instead of inband EUV dose only), using a calibrated effective dose (EEUV + K*EDUV) as input. An example of a system implementing such a dose control method is shown schematically in FIG. 5. In the arrangement depicted in FIG. 5, a dose controller 310 includes a module 320. The amount of the EUV radiation is measured by a sensor 330 which generates an output. At the same time, an amount of DUV radiation is sensed by a sensor 340. The sensor 340 generates an output signal which is multiplied by a constant K in module 350 and then added to the signal from the EUV sensor 330 at a summing junction 360. The resulting signal is supplied to the control module 320.

[0062] FIG. 6 is a flowchart describing a mode of operation of the system shown in FIG. 5.

In a step S200 a signal is generated indicating the amount of EUV energy at the substrate. In a step S210 signals generated indicating the amount of DUV energy at the substrate. In a step S240 the DUV energy signal is multiplied a constant K. In a step S250 the multiplied DUV energy signal is summed with the EUV energy signal. The resulting sum is then supplied to a dose controller which executes dose control based on the sum. As will be apparent to one of ordinary skill in the art, the steps S200 and S210 may occur concurrently.

[0063] DUV energy could be obtained in any one of several ways. For example, one or more DUV sensors configured to measure the magnitude of the DUV energy at the wafer level or in the illumination system could be used. That is, a sensor operable to directly sense the spectral content of the out-of-band radiation may be provided on the lithographic tool. Such a sensor 90 is shown in FIG. 2. Sensor 90 can then be used to directly perform the spectral measurement of step SI 10 or step S210. Such a sensor may be employed at wafer level (for example on wafer table WT), so that any spectral measurement takes into account the illumination and projection optics and any transmission effects before the wafer stage. However, the sensor may be placed elsewhere. Such a sensor may be part of the lithographic apparatus, or may be a stand-alone sensor which is inserted (for example close to the wafer table WT) only when a test is to be executed at a location. The sensor may operate in the spectral range of 10 to 400 nm, but is not restricted to this range. The illumination characteristics can then be adapted based on the spectral data recorded using the sensor. [0064] Alternately or in combination a DUV indicator may be derived by determining how DUV energy varies with measured parameters and then inferring the DUV energy from those parameters. For example if DUV energy is a function of a few key source operating parameters such as conversion efficiency, main pulse energy, main pulse beam size, and/or target size, then the effective dose can be derived from measurements of these parameters. In this case, there is no need to measure DUV energy directly.

[0065] As noted, K may be a calibrated parameter, that is, one that is measured in a calibration procedure. The calibration procedure could be one such as that shown in FIG. 7.

In the method shown in FIG. 7, in a step S300, a wafer with a targeting line or contact hole structure is exposed while measuring EUV pulse energy and DUV pulse energy. The exposed wafer is processed in a step S310. After the wafer has been processed then in a step S340 CD may be measured as a function of wafer location, which may then be mapped to timing of exposure, and timing of EUV pulse energy and DUV pulse energy. In a step S350 a mathematical fit such as a linear fit may be applied to the measurements of CD, EUV energy, and DUV energy, for example using the modelling discussed above with regard to effective dose, to obtain a value for K. A Design-of- Experiment with multiple wafer exposures may be used to evaluate the factors that control the value of K to cancel out contributions to this calibration procedure from time-dependent contributors to CD drift (e.g. mirror heating, process impact, etc.).

[0066] As mentioned, the relative sensitivity of resist to non- EUV vs EUV radiation may depend on the type of resist. The above calibration procedure creates the ability to mitigate the effects of wavelength-dependent resist sensitivity by selection of an appropriate calibration parameter.

[0067] Software functionalities of a computer system involve programming, including executable code, may be used to implement the above described methods. The software code is executable by the general-purpose computer. In operation, the code and possibly the associated data records are stored within a general-purpose computer platform. At other times, however, the software may be stored at other locations and/or transported for loading into the appropriate general-purpose computer systems. Hence, the embodiments discussed above involve one or more software products in the form of one or more modules of code carried by at least one machine-readable medium. Execution of such code by a processor of the computer system enables the platform to implement the catalog and/or software downloading functions, in essentially the manner performed in the embodiments discussed and illustrated herein.

[0068] The computer readable medium may be located in the scanner portion of the lithographic device, or located in the source portion of the lithographic device, or may be distributed between several systems of the lithographic device. The computer readable medium could be a portable medium.

[0069] As used herein, terms such as computer or machine “readable medium” refer to any medium that participates in providing instructions to a processor for execution. Such a medium may take many forms, including but not limited to, non-volatile media, volatile media, and transmission media. Non-volatile media include, for example, optical or magnetic disks, such as any of the storage devices in any computer(s) operating as one of the server platform, discussed above. Volatile media include dynamic memory, such as main memory of such a computer platform. Physical transmission media include coaxial cables; copper wire and fiber optics, including the wires that comprise a bus within a computer system. Carrier- wave transmission media can take the form of electric or electromagnetic signals, or acoustic or light waves such as those generated during radio frequency (RF) and infrared (IR) data communications. Common forms of computer-readable media therefore include, for example: a floppy disk, a flexible disk, hard disk, magnetic tape, any other magnetic medium, a CD- ROM, DVD, any other optical medium, less commonly used media such as punch cards, paper tape, any other physical medium with patterns of holes, a RAM, a PROM, and EPROM, a FLASH-EPROM, any other memory chip or cartridge, a carrier wave transporting data or instructions, cables or links transporting such a carrier wave, or any other medium from which a computer can read programming code and/or data. Many of these forms of computer readable media may be involved in carrying one or more sequences of one or more instructions to a processor for execution.

[0070] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion”, respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

[0071] While specific embodiments of the present invention have been described above, it will be appreciated that the present invention may be practiced other than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the present invention as described without departing from the scope of the claims that follow.

[0072] It is to be appreciated that the Detailed Description section, and not the Summary and Abstract sections, is intended to be used to interpret the claims. The Summary and Abstract sections may set forth one or more but not all exemplary embodiments of the present invention as contemplated by the inventor(s), and thus, are not intended to limit the present invention and the appended claims in any way.

[0073] The present invention has been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed.

[0074] Various references to signals and operations on those signals are illustrative only; it is to be understood that the various signals may be analog or digital, current-based or wireless. Operations on those signals (such as addition and multiplication) contemplate operations that apply to the information carried by those signals.

[0075] The foregoing description of the specific embodiments will so fully reveal the general nature of the present invention that others can, by applying knowledge within the skill of the art, readily modify and/or adapt for various applications such specific embodiments, without undue experimentation, without departing from the general concept of the present invention. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by the skilled artisan in light of the teachings and guidance.

[0076] Other aspects of the invention are set out in the following numbered clauses.

1. A lithographic apparatus comprising: a conditioning system configured to condition a radiation beam, the radiation beam comprising both EUV radiation and non-EUV radiation; and a controller adapted to control a dose of radiation delivered to a substrate by the conditioning system at least partially on the basis of a ratio of a magnitude of energy of the non-EUV radiation to a magnitude of energy of the EUV radiation.

2. The lithographic apparatus of clause 1 wherein the non-EUV radiation is DUV radiation and the controller is adapted to control a dose of radiation delivered to a substrate by the conditioning system at least partially on the basis of a ratio of a magnitude of energy of the DUV radiation to magnitude of energy of the EUV radiation.

3. The lithographic apparatus of clause 2 comprising at least one sensor arranged to measure a magnitude of energy of the DUV radiation.

4. The lithographic apparatus of clause 3 wherein the at least one sensor is located to measure the magnitude of energy of the DUV radiation at the substrate.

5. The lithographic apparatus of clause 3 wherein the at least one sensor is located to measure the magnitude of energy of the DUV radiation at the conditioning system.

6. The lithographic apparatus of clause 5 wherein the lithographic apparatus comprises an illuminator, and wherein the conditioning system is comprised in the illuminator.

7. The lithographic apparatus of clause 5 wherein the lithographic apparatus is a radiation source, and wherein the conditioning system is comprised in the radiation source.

8. The lithographic apparatus of clause 2 further comprising a module configured to determine a magnitude of energy of the DUV radiation from a plurality of operating parameters of the source.

9. The lithographic apparatus of clause 2 wherein the controller is adapted to control a dose of radiation delivered to a substrate by the conditioning system at least partially on the basis of a product of the ratio and a calibration factor.

10. A lithographic apparatus comprising: an conditioning system configured to condition a radiation beam, the radiation beam comprising both EUV radiation and non-EUV radiation; a first module configured for generating a first signal indicative of a magnitude of energy of the EUV radiation; a second module configured for generating a second signal indicative of a magnitude of energy of the non-EUV radiation; a third module configured to multiply the second signal by a calibration factor to obtain a third signal; a fourth module configured to add the first signal and the third signal to obtain a fourth signal; and a controller arranged to receive the fourth signal and adapted to control a dose of radiation delivered to a substrate by the radiation beam at least partially on the basis of a sum of a magnitude of energy of the EUV radiation and a product of a calibration factor and a magnitude of an energy of the non-EUV radiation.

11. The lithographic apparatus of clause 10 wherein the non-EUV radiation is DUV radiation and the controller is adapted to control a dose of radiation delivered to a substrate by the conditioning system at least partially on the basis of a sum of a magnitude of energy of the EUV radiation and a product of a calibration factor and a magnitude of an energy of the DUV radiation.

12. The lithographic apparatus of clause 10 wherein the second module comprises at least one sensor arranged to measure a magnitude of energy of the non-EUV radiation.

13. The lithographic apparatus of clause 12 wherein the at least one sensor is located to measure the magnitude of energy of the non-EUV radiation at the substrate.

14. The lithographic apparatus of clause 12 wherein the at least one sensor is located to measure the magnitude of energy of the non-EUV radiation at the conditioning system.

15. The lithographic apparatus of clause 10 wherein the second module is configured to infer a magnitude of DUV radiation from a plurality of operating parameters of the source.

16. A method of manufacturing a device, comprising: generating a radiation beam comprising both EUV radiation and non-EUV radiation; and controlling a dose of radiation delivered to a substrate by the radiation beam at least partially on the basis of a ratio of a magnitude of energy of the non-EUV radiation to a magnitude of energy of the EUV radiation.

17. The method of clause 16 wherein the non-EUV radiation is DUV radiation and controlling a dose of radiation delivered to a substrate by the radiation beam at least partially on the basis of a ratio of a magnitude of energy of the non-EUV radiation to a magnitude of energy of the EUV radiation comprises controlling a dose of radiation delivered to a substrate by the radiation beam at least partially on the basis of a ratio of a magnitude of energy of the DUV radiation to a magnitude of energy of the EUV radiation. 18. The method of clausel6 comprising sensing a magnitude of energy of the DUV radiation.

19. The method of clause 18 wherein sensing a magnitude of energy of the DUV radiation comprises measuring the magnitude of energy of the DUV radiation at the substrate.

20. The method of clause 18 wherein sensing a magnitude of energy of the DUV radiation comprises measuring the magnitude of energy of the DUV radiation at the conditioning system.

21. The method of clause 16 further comprising determining a magnitude of energy of the DUV radiation from a plurality of operating parameters of the source.

22. The method of clause 16 wherein controlling a dose of radiation delivered to a substrate by the radiation beam at least partially on the basis of a ratio of a magnitude of energy of the DUV radiation to a magnitude of energy of the EUV radiation is performed at least partially on the basis of a product of the ratio and a calibration factor.

23. A method of manufacturing a device, comprising: generating a radiation beam using a radiation source, the radiation beam comprising both EUV radiation and non-EUV radiation; generating a first signal indicative of a magnitude of energy of the EUV radiation; generating a second signal indicative of a magnitude of energy of the non-EUV radiation; multiplying the second signal by a calibration factor to obtain a third signal; adding the first signal and the third signal to obtain a fourth signal; and supplying the fourth signal to a dose controller, the dose controller controlling a dose of radiation delivered to a substrate by the radiation beam at least partially on the basis of a sum of a magnitude of energy of the EUV radiation and a product of a calibration factor and a magnitude of an energy of the non-EUV radiation.

24. The method of clause 23 wherein the non-EUV radiation is DUV radiation and the dose controller controls a dose of radiation delivered to a substrate by the conditioning system at least partially on the basis of a sum of a magnitude of energy of the EUV radiation and a product of a calibration factor and a magnitude of an energy of the DUV radiation.

25. The method of clause 23 comprising sensing a magnitude of energy of the non-EUV radiation.

26. The method of clause 25 wherein sensing a magnitude of energy of the non-EUV radiation comprises measuring the magnitude of energy of the non-EUV radiation at the substrate. 27. The method of clause 25 wherein sensing a magnitude of energy of the non-EUV radiation comprises measuring the magnitude of energy of the non-EUV radiation at the conditioning system.

28. The method of clause 25 wherein generating a second signal indicative of a magnitude of energy of the non-EUV radiation comprises determining a magnitude of the non-EUV radiation from a plurality of operating parameters of the source.

29. A method comprising: receiving a first signal indicative of a power of in-band EUV radiation in a radiation beam comprising EUV radiation; receiving a second signal indicative of a power of out-of-band radiation in the radiation beam; generating a third signal, wherein the third signal is based on the first and second signals; and supplying the third signal to a dose controller, the dose controller configured to control a power of the radiation beam based at least in part on the third signal.

30. The method of clause 29, wherein the second signal is based on a measurement of a combined power of the radiation beam, wherein the combined power includes the power of in-band EUV radiation in the radiation beam and the power of out-of-band radiation in the radiation beam.

31. A tangible nontransitory computer-readable medium having encoded thereon instructions executable by a processor to perform a method comprising: receiving a first signal indicative of a power of in-band EUV radiation in a radiation beam comprising EUV radiation; receiving a second signal indicative of a power of out-of-band radiation in the radiation beam; generating a third signal, wherein the third signal is based on the first and second signals; and supplying the third signal to a dose controller, the dose controller configured to control a power of the radiation beam based at least in part on the third signal.

[0077] The breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.