WO2025219014A1

WO2025219014A1 - Optical measurement device and method of manufacturing same

Info

Publication number: WO2025219014A1
Application number: PCT/EP2025/057934
Authority: WO
Inventors: Parag Vinayak KELKAR; Andrew Robert KLUMPH; Matthew Lipson; John D. Martin
Original assignee: ASML Netherlands BV
Current assignee: ASML Netherlands BV
Priority date: 2024-04-19
Filing date: 2025-03-24
Publication date: 2025-10-23
Anticipated expiration: 2026-10-19

Abstract

In one aspect of the present disclosure, there is provided a method comprising: placing a reflective mirror into a chamber, the reflective mirror comprising a substrate and a reflective coating on the substrate; generating a vacuum within the chamber; providing a bias to the reflective mirror placed in the chamber; introducing plasma gasses into the chamber; and generating a reactive plasma with the gasses in the chamber while the bias is provided to the reflective mirror in order to form an oxide layer on the reflective coating to passivate the reflective coating. In another aspect of the present disclosure, there is provided a mirror comprising: a substrate; a reflective coating on the substrate; an oxide layer formed on the reflective coating, the oxide layer having an average thickness of greater than 3 nm and a thickness non-uniformity of 0.12 nm or less.

Description

OPTICAL MEASUREMENT DEVICE AND METHOD OF MANUFACTURING SAME

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63/636,446 which was filed on April 19, 2024 and which is incorporated herein in its entirety by reference.

TECHNICAL FIELD

[0002] The present disclosure relates to optical measurement systems in photolithography systems.

BACKGROUND

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern (also often referred to as "design layout" or "design") of a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer).

[0004] As semiconductor manufacturing processes continue to advance, the dimensions of circuit elements have continually been reduced while the amount of functional elements, such as transistors, per device has been steadily increasing over decades, following a trend commonly referred to as 'Moore's law'. To keep up with Moore's law the semiconductor industry is chasing technologies that enable to create increasingly smaller features. To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which are patterned on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm and 13.5 nm. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within a range of 4 nm to 20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0005] In photolithography systems, precise measurement of wafer height relative to the height of the optical exposure system is of utmost importance for the quality of the lithography process. Optical techniques to measure wafer height often require mirrored surfaces and are extremely sensitive to the thickness and uniformity of oxide films that grow naturally on those surfaces. A typical metal used as a mirror surface is aluminum but multiple other metals are known to those skilled in the art.

[0006] For LS systems, measuring wafer height typically involves directing a light beam, often from a laser, towards the wafer's surface. The aluminum-coated mirror is used to either direct this measuring light beam towards the wafer or to reflect the beam back after it hits the wafer surface. By analyzing the angle of incidence and reflection of polarized light, the system can calculate the height and topography of the wafer with high precision. This measurement is used for aligning the wafer correctly relative to the exposure optics, ensuring that the patterns are accurately transferred onto the wafer surface at the intended dimensions and positions.

[0007] In the case of UVLS systems, which specifically use ultraviolet (UV) light for measurement, the principle is similar but tailored to work effectively with UV wavelengths. UV light can offer advantages in terms of resolution and measurement sensitivity. The aluminum-coated mirrors in these systems are designed to reflect UV light efficiently towards the wafer surface and back to the detector. The properties of the aluminum coating are optimized for minimal absorption, high reflectivity for polarized light and uniform retardance at UV wavelengths which are used for accurate height determination.

[0008] Both systems rely on the precise fabrication and positioning of the mirrors to ensure that the light paths are correctly established and maintained. The accuracy of the level measurement directly impacts the focus and depth of field of the photolithography process, which are critical parameters for achieving the desired patterns on the wafer with very fine accuracy.

[0009] The base material for the mirror used in level sensors for determining wafer height in photolithography scanners is typically made of a substrate that offers excellent optical qualities, such as glass composite, glass ceramics or fused silica. These materials are chosen for their thermal stability, low thermal expansion, and ease of polishing to achieve very low surface roughness.

[0010] The process of applying the aluminum coating to this base material is commonly achieved through physical vapor deposition (PVD). PVD is a vacuum deposition method used to produce thin films and coatings. During the PVD process, the aluminum is vaporized in a vacuum environment and then condensed on the surface of the substrate to form a thin, uniform metallic layer. This is achieved by either evaporating the aluminum (thermal evaporation) or by sputtering it from a target material (sputtering).

[0011] After physical vapor deposition of aluminum coating on the mirror surface, the aluminum surface undergoes a natural oxidation process in ambient environment creating a layer of natural oxide (NatOx), for example in the range of 2-5 nm, over span of hundreds of days or so. The NatOx prevents indefinite oxidation growth beyond 2-5 nm, but often varies in thickness on sub-nanometer scale over the surface of the mirror. The thickness variation in the oxide layer creates high retardance variation across the mirror surface, particularly when it is used at high angle of incidence (for example, exceeding 30 degrees).

[0012] In level sensor (LS) and UV level sensor (UVLS) systems that are used for determining wafer height prior to exposure in a photolithography scanner, the accuracy in wafer height measurement (Height process dependency HPD) is directly proportional to the retardance gradient. The nonuniformity in NatOx growth or accumulation of small amounts of contamination can result in impactful inaccuracy in wafer height measurements. Just for example, contamination layer as small as 0.1 nm thick can lead to 0.2 deg retardance change. If the contamination layer or NatOx layer or their combination has variation of 0.1 nm over 1 cm on the mirror surface, it can result in up to 5 nm inaccuracy in wafer height. This would constitute a substantial part of the focus budget allowed for UVLS systems.

[0013] It is also possible to intentionally deposit a layer of oxide material on top of aluminum surface using physical vapor deposition process. Such an oxide layer can be used to quickly protect aluminum surface from further oxidation but has an even larger thickness variation up to 1-5 nm across the mirror surface, making the mirror mostly unusable.

[0014] Historically, natural oxide growth has been preferred over deposition of additional oxide layer because of a factor of 10 less in thickness variation in NatOx and extreme sensitivity of UVLS system to the mirror retardance gradient resulting from oxide layer thickness variation.

[0015] The non-uniformity in the NatOx thickness is a result of natural oxidation processes. NatOx typically reaches its final thickness of about 5 nm over hundreds of days in ambient environment. The NatOx growth rate depends on the environment it is exposed to in its lifetime and level of contamination on the surface. This adds spatial variability in oxide thickness over time creating retardance gradient and loss of performance as measured by wafer height accuracy.

[0016] From the above, it can be seen that two problems are (1) the time required for reaching terminal oxide thickness under ambient conditions can be very long (e.g., can be several years) and (2) the uniformity of such oxide growth cannot be sufficiently controlled for various reasons, such as the fact that local conditions on the mirror surface cannot be predicted or controlled under operational conditions.

SUMMARY

[0017] In one aspect of the present disclosure, there is provided a method comprising: placing a reflective mirror into a chamber, the reflective mirror comprising a substrate and a reflective coating on the substrate; generating a vacuum within the chamber; providing a bias to the reflective mirror placed in the chamber; introducing plasma gasses into the chamber; and generating a reactive plasma with the gasses in the chamber while the bias is provided to the reflective mirror in order to form an oxide layer on the reflective coating to passivate the reflective coating.

[0018] In another aspect of the present disclosure, the reactive plasma is oxygen plasma.

[0019] In another aspect of the present disclosure, the method further comprises introducing a fluorine based gas within the reactive plasma to assist formation of aluminum-oxy-fluoride or fluorocarbon compounds on the mirror surface.

[0020] In another aspect of the present disclosure, the fluorine-based gas comprises CHF3, CF4 or SF6.

[0021] In another aspect of the present disclosure, the reactive plasma comprises oxygen based ions.

[0022] In another aspect of the present disclosure, the reactive plasma bombards the reflective mirror surface with ions with 50- 500 eV energy.

[0023] In another aspect of the present disclosure, the reflective coating comprises aluminum. [0024] In another aspect of the present disclosure, the substrate comprises glass or ceramic.

[0025] In another aspect of the present disclosure, the bias is provided to a mirror support configured to support the reflective mirror while the reactive plasma is generated.

[0026] In another aspect of the present disclosure, the reflective mirror comprises a non-planar surface shape on a side thereof opposite the reflective coating, and wherein the mirror support comprises a tool having a tool surface that corresponds to the non-planar surface shape of the reflective mirror, such that the reactive plasma is evenly applied across the reflective coating.

[0027] In another aspect of the present disclosure, the tool comprises a separate structure that is removable from a base of the mirror support.

[0028] In another aspect of the present disclosure, wherein formed oxide layer has a resulting average thickness of between 3 nm to 6nm.

[0029] In another aspect of the present disclosure, the formed oxide layer has a thickness nonuniformity of 0.12 nm or less.

[0030] In another aspect of the present disclosure, there is provided a mirror comprising: a substrate; a reflective coating on the substrate; an oxide layer formed on the reflective coating, the oxide layer having an average thickness of greater than 3 nm and a thickness non-uniformity of 0.12 nm or less. [0031] In another aspect of the present disclosure, the oxide layer comprises aluminum oxide.

[0032] In another aspect of the present disclosure, the oxide layer has a thickness non-uniformity of 0.10 nm or less.

[0033] In another aspect of the present disclosure, the oxide layer has a thickness non-uniformity of 0.05 nm or less.

[0034] In another aspect of the present disclosure, wherein the oxide layer has an average thickness of less than 6nm.

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The accompanying drawings, which are incorporated in and constitute a part of this specification, show certain aspects of the subject matter disclosed herein and, together with the description, help explain some of the principles associated with the disclosed implementations. In the drawings,

[0036] Figure 1 depicts a schematic overview of a lithographic apparatus;

[0037] Figure 2 depicts a schematic overview of a level or height sensor;

[0038] Figures 3 and 4 depicts schematic side views of different mirrors used in different embodiments in accordance with the disclosure;

[0039] Figure 5 depicts a schematic overview of one embodiment of a plasma treatment system in accordance with one embodiment of the disclosure;

[0040] Figure 6 is similar to Figure 3 and depicts the angle of incidence referred to in this disclosure; [0041] Figures 7-9 depict a mirror with a non-planar bottom surface and the effect this would have on oxide thickness and retardance in accordance with this disclosure of not accommodated for;

[0042] Figures 10-12 depict the mirror of Figure 7, now with a tool in place to accommodate for the non-planar bottom surface in accordance with one embodiment of this disclosure, and the resulting uniformity of oxide formation and retardance.

DETAILED DESCRIPTION

[0043] Figure 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus LA includes an illumination system (also referred to as illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation or EUV radiation), a mask support (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA in accordance with certain parameters, a substrate support (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 support in accordance with certain parameters, and a projection system (e.g., a refractive projection lens 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.

[0044] In operation, the illumination system IL receives a radiation beam from a radiation source SO, e.g., via a beam delivery system BD. The illumination system IL may include various types of optical components, such as refractive, reflective, electromagnetic, and/or other types of optical components, or any combination thereof, for directing, shaping, and/or controlling radiation. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross section at a plane of the patterning device MA.

[0045] The term “projection system” PS used herein should be broadly interpreted as encompassing various types of projection system, including refractive, reflective, catadioptric, anamorphic, and/or electromagnetic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, and/or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system” PS.

[0046] The lithographic apparatus LA may be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the projection system PS and the substrate W - which is also referred to as immersion lithography. More information on immersion techniques is given in U.S. 6,952,253, which is incorporated herein by reference.

[0047] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also named “dual stage”). In such “multiple stage” machine, the substrate supports WT may be used in parallel, and/or steps in preparation of a subsequent exposure of the substrate W may be carried out on the substrate W located on one of the substrate support WT while another substrate W on the other substrate support WT is being used for exposing a pattern on the other substrate W.

[0048] In addition to the substrate support WT, the lithographic apparatus LA may comprise a measurement stage. The measurement stage is arranged to hold a sensor. The sensor may be arranged to measure a property of the projection system PS or a property of the radiation beam B. The measurement stage may hold multiple sensors. The measurement stage may move beneath the projection system PS when the substrate support WT is away from the projection system PS.

[0049] In operation, the radiation beam B may be incident on the patterning device, e.g., mask, MA which is held on the mask support MT, and is patterned by the pattern (design layout) present on patterning device MA. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which may focus the beam onto a target portion C of the substrate W or onto a sensor arranged at a stage. With the aid of the second positioner PW and a position measurement system PMS, the substrate support WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B at a focused and aligned position. Similarly, the first positioner PM and possibly another position sensor (which is not explicitly depicted in Figure 1) may be used to accurately position the patterning device MA with respect to the path of the radiation beam B. Patterning device MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2. Although the substrate alignment marks Pl, P2 as illustrated occupy dedicated target portions, they may be located in spaces between target portions. Substrate alignment marks Pl, P2 are known as scribe-lane alignment marks when these are located between the target portions C. Substrate alignment marks Pl, P2 may also be arranged in the target portion C area as in-die marks. These in-die marks may also be used as metrology marks, for example, for overlay measurements.

[0050] To clarify the present disclosure, in some instances a Cartesian coordinate system may be used. The Cartesian coordinate system has three axis, i.e., an X-axis, a Y-axis and a Z-axis. Each of the three axis is orthogonal to the other two axis. A rotation around the x-axis is referred to as an Rx-rotation. A rotation around the Y-axis is referred to as an Ry-rotation. A rotation around the Z-axis is referred to as an Rz-rotation. The X-axis and the Y-axis define a horizontal plane, whereas the Z-axis is in a vertical direction. The Cartesian coordinate system is not limiting the disclosure and is used for clarification only. Instead, another coordinate system, such as a cylindrical or spherical coordinate system, may be used. The orientation of the Cartesian coordinate system may be different, for example, such that the Z-axis has a component along the horizontal plane.

[0051] A topography measurement system, level sensor or height sensor, and which may be integrated in the lithographic apparatus, is arranged to measure a topography of a top surface of a substrate (or wafer). A map of the topography of the substrate, also referred to as height map, may be generated from these measurements indicating a height of the substrate as a function of the position on the substrate. This height map may subsequently be used to correct the position of the substrate during transfer of the pattern on the substrate, in order to provide an aerial image of the patterning device in a properly focus position on the substrate. It will be understood that “height” in this context refers to a dimension broadly out of the plane to the substrate (also referred to as Z-axis). Typically, the level or height sensor performs measurements at a fixed location (relative to its own optical system) and a relative movement between the substrate and the optical system of the level or height sensor results in height measurements at locations across the substrate.

[0052] An example of a level or height sensor LS as known in the art is schematically shown in Figure 2, which illustrates only the principles of operation. In this example, the level sensor LS comprises an optical system, which includes a projection unit LSP and a detection unit LSD. The projection unit LSP comprises a radiation source LSO providing a beam of radiation LSB, which is imparted by a projection grating PGR of the projection unit LSP. The radiation source LSO may be, for example, a narrowband or broadband radiation source, such as a supercontinuum light source, polarized or non-polarized, pulsed or continuous, such as a polarized or non-polarized laser beam. The radiation source LSO may include a plurality of radiation sources having different colors, or wavelength ranges, such as a plurality of LEDs. The radiation source LSO of the level sensor LS is not restricted to visible radiation, but may additionally or alternatively encompass UV and/or IR radiation and any range of wavelengths suitable to reflect from a surface of a substrate W or from a layer at the substrate W.

[0053] The projection grating PGR is a grating comprising, for example, a periodic structure resulting in a beam of radiation BE1 having a periodically varying intensity. The beam of radiation BE1 with the periodically varying intensity is directed towards a measurement location MLO on a substrate W having an angle of incidence ANG with respect to an axis perpendicular (Z-axis) to the incident substrate surface between 0 degrees and 90 degrees, typically between 70 degrees and 80 degrees. At the measurement location MLO, the patterned beam of radiation BE1 is reflected by the substrate W (indicated by arrows BE2) and directed towards the detection unit LSD.

[0054] In order to determine the height level at the measurement location MLO, the level sensor LS further comprises a detection unit LSD comprising a detection grating DGR, a detector DET and a processing unit (not shown) for processing an output signal of the detector DET. The detection grating DGR may be identical to the projection grating PGR. The detector DET produces a detector output signal indicative of the light received, for example indicative of the intensity of the light received, such as a photodetector, or representative of a spatial distribution of the intensity received, such as a camera. The detector DET may comprise any combination of one or more detector types.

[0055] By means of triangulation techniques, the height level at the measurement location MLO can be determined. The detected height level is typically related to the signal strength as measured by the detector DET, the signal strength having, for example, a periodicity that depends, amongst others, on the design of the projection grating PGR and the (oblique) angle of incidence ANG.

[0056] The projection unit LSP and/or the detection unit LSD may include further optical elements, such as lenses and/or mirrors, along the path of the patterned beam of radiation between the projection grating PGR and the detection grating DGR. [0057] In an embodiment, the detection grating DGR may be omitted, and the detector DET may be placed at the position where the detection grating DGR is located. Such a configuration provides a more direct detection of the image of the projection grating PGR.

[0058] In order to cover the surface of the substrate W effectively, a level sensor LS may be configured to project an array of measurement beams BE1 onto the surface of the substrate W, thereby generating an array of measurement areas (e.g., measurements at several measurement locations) or spots covering a larger measurement range.

[0059] Various height sensors of a general type are disclosed for example in U.S. 7,265,364 and U.S. 7,646,471, both incorporated by reference. A height sensor using UV radiation instead of visible or infrared radiation is disclosed in U.S. 2010/233600 Al, incorporated by reference. In WO 2016/102127 Al , incorporated by reference, a compact height sensor is described which uses a multi-element detector to detect and recognize the position of a grating image, without needing a detection grating. The combinations and sub-combinations of the elements disclosed herein constitute separate embodiments and are provided as examples only. Also, 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 as described without departing from the scope of the claims set out below.

[0060] Figure 3 depicts a side view of one embodiment of a reflective mirror 300 configured to form part of an optical measurement system (e.g., level sensor LS), and Figure 4 depicts a side view of an alternative embodiment of a reflective mirror 400 that is also configured to be able to form part of such an optical measurement system for measuring topology of a surface of a substrate (e.g., wafer W) such as depicted in Figure 2. The optical measurement system can include a light projector LSO (e.g., radiation source LSO) configured to project light beam LSB (e.g., beam of radiation LSB) to the surface. The optical measurement system can also include a light detector (e.g., detector DET) configured to receive light beam LSB from light projector LSO that has been reflected from the surface. Any of the optical measurement systems disclosed herein can also include additional mirrors to direct the light beam to/from the surface, a mirror relay, light projector, and/or a light detector.

[0061] In the embodiment of Figure 3, the mirror 300 has a substrate material 302, and a coating layer 304. In the embodiment of Figure 4, the mirror 400 has a substrate material 402 and a coating layer 404. In some embodiments, the substrate material 302 and 402 for mirrors 300 or 400 can be made from glass or ceramic. In some embodiments, the coating layers 304 or 404 is a highly reflective aluminum. [0062] The embodiment of substrate 302 is essentially flat, with a planar top and bottom. The embodiment of substrate 402 has a non-planar bottom surface (e.g., the mirror substrate can have a frustoconical shape, just for example). These different mirrors can be used for different level sensing applications and/or at different parts of the level lensing system. In addition, while the upper surface of both mirror substrates 302 and 402 are shown as flat or planar, it should be understood that the upper surface can also be curved (e.g., concave or convex), and the coatings 304 and 404 would have a corresponding curved shape as well in such cases. [0063] To manufacture the mirrors 300 and/or 400, the respective substrates 302 or 402 are placed into a vacuum chamber (not shown). The aluminum coating 304 or 404 is applied to the underlying substrate 302 or 402 in the vacuum chamber, for example by physical vapor deposition (PVD).

[0064] The aluminum in the vacuum chamber at that point is pure aluminum, with virtually no oxidation formed thereon. However, as soon as the mirror is removed from the vacuum chamber and sees air, oxygen, and/or water vapor, the passivation process shall commence.

[0065] The formed mirror 300 or 400 is immediately placed in a plasma treatment system 500. For example, as illustrated in Figure 5, mirror 300 has been placed into the plasma treatment system 500.

[0066] In one embodiment, the plasma treatment system 500 can be an inductively coupled plasma (ICP) system and optionally an integrated or separate reactive ion etching (RIE) system.

[0067] In one embodiment, the plasma treatment system can have (although not limited to) the following components: vacuum chamber 502, upper electrode 504, lower electrode 506 that is integrated into or connected to a mirror base or support structure 530, RF power source 508, shower head 510, gas sources 512, a vacuum pump 514, and a controller 540. In one embodiment, the controller 540 comprises one or more computer processors and operatively connected with one or more of the gas sources (specifically the control valves associated therewith for controlling gas flow into the chamber 502), the vacuum pump 514, and the RF power source 508. The controller 540 can be programmed to control operation of each of these components connected therewith. As an overview of the method in accordance with an embodiment of the present disclosure in which the optional etching process is utilized in advance of oxide formation, the controller 540 can control the vacuum pump 514 to pull a vacuum within the chamber 502. The controller 540 further controls the gas source 512 and the RF power source 508 such that reactive ions are generated within the chamber 502, via the gas sources 512 emitted from shower head 510, and by activation of the electrodes 504 and 506 to create reactive ions that begin to etch the top surface of the aluminum coating to remove whatever has grown on the aluminum surface prior to transfer into chamber 502.

[0068] In the initial stages of exposure to the reactive etching ions, at the molecular level contamination accumulated on the surface prior to the mirror being transferred into chamber 502 is removed, exposing a fresh aluminum surface. The fresh aluminum surface enables uniform oxidation of the surface with energetic atomic oxygen in the following steps. It should be appreciated that the initial contamination removal cleaning phase and oxidation phase can be a separate processes (e.g., in two different systems, two different chambers, or separate in the sense of sequence, with removal and cleaning phases being serial) or one continuous process (meaning taking place in the same chamber and/or taking place simultaneously in that same chamber. Argon, oxygen or combination of them can be used in the cleaning phase while oxygen plasma can be used for growing oxide thickness. Argon used during the cleaning phase sputters off the contamination or the naturally grown oxide layer. Oxygen plasma during cleaning phase removes volatile contamination from the surface. The cleaning phase is typically short, for example, from 10 s to 2 min. The oxidation step can be 1 min to 20 min long, for example.

[0069] Specifically, after the etching of the top surface is complete in one example embodiment, the controller 540 is operated such that reactive gasses are removed from the chamber 502 by the vacuum pump 514, and oxidating plasma (e.g, oxygen plasma) gasses are then provided from the gas sources 512 to the chamber 502 via the shower head 510. Providing this passivating plasma shortens the time needed to grow terminal oxide thickness.

[0070] In another alternative embodiment, fluorinated gas is introduced simultaneously alongside the oxidating plasma gasses, or after oxidating plasma gasses are introduced. For example, CF3, CF4, SF6, C2F6, or other fluorinated or reactive gas is introduced to make the mirror surface more hydrophobic to substantially reduce further oxidation. In other words, introducing the reactive gas can be part of the plasma process by introducing fluorinated gasses during the plasma treatment, throughout the process, or at the end of the plasma treatment process. This would be done in the same chamber 502, or in alternative embodiment can be done in a separate chamber.

[0071] When fluorinated gases are introduced into the plasma environment, they dissociate into highly reactive fluorine-containing species. These species can then react with the aluminum surface of the mirror. This fluorination of the surface reduces its surface energy, making it more hydrophobic. A hydrophobic surface repels water, which is beneficial for preventing moisture -related degradation and contamination.

[0072] The fluorine-terminated surface also acts as a barrier to oxidation. By replacing reactive sites on the surface with fluorine, the availability of sites for oxygen to bind and form oxides is reduced and helps maintain the integrity of the surface and underlying layers by preventing further oxidation.

[0073] In one embodiment, by controlling various parameters such as power, pressure, flow rate of gases, density of the plasma, and/or the accelerating voltages for the plasma ions, better oxide thickness uniformity can be achieved. The mirror can be treated with passivating plasma gas (e.g„ oxygen plasma) from between 1 minute to about 20 minutes, for example. A bias is provided to the aluminum mirror substrate 302 via electrode/support 506/530 while exposing them to plasma source so that oxygen ions bombard the aluminum surface with 50- 500 eV energy, thus accelerating oxide growth.

[0074] It should be noted that he mean free path of the ions in the chamber can be changed by controlling the pressure, power, power density, and/or distance between the electrodes, to thereby control uniformity of oxide thickness formed on the mirror surface 302. In addition, a partial pressure of oxygen plasma gas can be used in conjunction with some other gases also in the chamber (from sources 512) to further help control making the oxide layer more uniform.

[0075] In another embodiment, the plasma treatment system described here can be described as a selfbiasing system wherein the fast electrons formed in the gas plasma charge (i.e. bias) the surface. The bias in turn determines the local plasma density which determines oxide growth.. [0076] It should be noted that in one embodiment, the reactive etching steps is optional, especially in arrangements where an accommodation is made to significantly reduce or eliminate exposure of the substrate to air or other gasses during the period between deposition of the aluminum coating onto the substrate and transfer to the plasma vacuum chamber 502. For example, in one embodiment, it is contemplated to keep the mirror in a vacuum environment during such transfer, although this is not necessary.

[0077] In other embodiments, other systems can also be used, such as capacitively coupled plasma system to generate a unform oxide layer.

[0078] It should be appreciated that the system 500 generates plasma 520 by using the radio frequency (RF) power source 508 applied through electrodes 504, 506 to create an oscillating electromagnetic field inside a vacuum chamber 502. Controller 540 is operated so that oxygen gas is introduced into the chamber from gas sources 512, and the energy from the electromagnetic field generated by the RF power source 508 ionizes the oxygen, creating oxygen plasma.

[0079] The mirrors 300 or 400 are exposed to oxygen plasma for specified time to achieve the desired oxide layer thickness, as longer exposure time will lead to higher thickness levels (at least up to a point). The inductive coupling of the system is accomplished by energy being transferred from the RF coil of the RF power source 508 to the gas through the electrodes 504 and 506, allowing for the generation of plasma at relatively low pressures and high densities. Alternatively, the plasma can be directly generated between the two electrodes 504 and 506 in the chamber 502, or a combination of both methods can be used.

[0080] The mirror 300 or 400 within the chamber 502 is exposed to the oxygen plasma 520 generated in the chamber 502. The highly reactive oxygen species (including ions, electrons, and neutral atoms) in the plasma interact with the surface of the mirror 300.

[0081] The energetic environment of the system 500 allows for a highly uniform generation of plasma, ensuring that the reactive oxygen species are evenly distributed and uniform across the mirror surface. The process parameters, including RF power, chamber pressure, oxygen flow rate, and exposure time, can be precisely controlled to adjust the thickness and uniformity of the oxide layer.

[0082] Reactive ion etching (RIE), which is a plasma-based etching process can further enhance the uniformity and quality of oxide layers on the mirror surface substrate by precisely removing contamination on the surface and top layer of oxide layer through an anisotropic etching process before oxidation process is started.

[0083] The RIE aspect of the system 500 utilizes both chemical and physical etching mechanisms. The plasma generated in the system 500 creates reactive species that chemically react with the substrate surface material. Additionally, the ions in the plasma are accelerated towards the substrate by electric field generated by self-biasing via electrodes 504 and 506, and RF power source 508. This ion bombardment provides a directional, or anisotropic, etching capability, allowing for the precise patterning of the surface. [0084] After an oxide layer is grown onto the mirror surface via the plasma treatment system 500, the RIE functionality of the system 500 system can also be used to etch back the oxide layer with high precision. This step can help in achieving an extremely uniform and smooth oxide surface. By carefully controlling the etching rate, one can remove any non-uniformities or roughness that might have been introduced during the deposition phase, resulting in a more uniformly thin oxide layer across the entire substrate.

[0085] The ability to finely tune the etch depth, rate, and profile through RIE parameters (such as gas composition, pressure, and RF power) complements the deposition capabilities of ICP, offering a comprehensive toolkit for oxide layer management.

[0086] The plasma gas sources 512 will supply the specific gases needed to create the plasma for controlling the oxidation process and chemical nature of the surface or to conduct the etching process. The choice of gas depends on the desired oxidation layer properties and the desired etching properties. For example, , CHF3, CF4 or SF6 can be used within the oxygen plasma to assist formation of aluminum-oxy-fluoride or fluorocarbon compounds on the surface of the aluminum coating while Ar can be introduced in the plasma to assist etching.

[0087] The RIE functionality relies on the shower head 510 to distribute the etching gases evenly across the surface of mirror 300. This ensures a uniform reaction on the surface of the material being etched. The upper electrode 504 disposed above the shower head the lower electrode is disposed on (or forming part of) a chuck or mirror holder/support 530 that holds the mirror 300 in place during the etching process. The RF power source 508 is connected to the two electrodes 504, 506 and supplies radio frequency (RF) power to the electrodes, and used to maintain the plasma state. The RF power excites the gas molecules, ionizing them and creating the plasma. Specifically, applying an electric field between the upper and lower electrodes will function to ionize the gas, turning it into plasma.

[0088] It should be appreciated that in one embodiment, the uniformity of the oxide layer variance should be with +/- 0.1 nm, especially for mirrors intended to be used at higher angles of incidences 0 (e.g., see Figure 6) greater than 55 degrees. For mirrors intended to reflect light at angles of incidence that are lower, such as between 30 to 55 degrees, then higher amounts of oxide thickness variations can be -accommodated, such as up to 1.0 nm.

[0089] For example, in one embodiment, the retardance variance is engineered to be less than one degree per centimeter between 30 to 55 degree angles of incidence, and in embodiments where the angles of incidence are above 55 degrees the variance of thickness is engineered to be less than 0.1 degree per centimeter. Thus, as angle of incidence increases, greater uniformity of aluminum oxide growth is desired.

[0090] While reactive oxygen is used as mentioned above, other reactive species can be used. For example, reactive water (water with argon) can be used (i.e., water plasma). In addition, in some embodiments, instead of using oxygen atoms to form a passivation layer, hydroxyl radicals can be used instead. [0091] Further, while inductively coupled plasma (ICP) systems can be used, other capacitively coupled systems could be used as well.

[0092] Further, in some embodiments, other techniques for rapid oxidation can be used, such as the use of UVO or UV ozone can be used in place of oxygen plasma.

[0093] In another embodiment, oxide growth can be further accelerated by heating of the mirror. For example, in one embodiment, a heater can be placed in the mirror support 506, with the heater coupled to the controller 540 to control the temperature of the mirror.

[0094] As mentioned previously, in some embodiments for certain applications, the substrate may have a non-planar bottom surface 700, such as shown in FIG 7. For these applications, certain portions of the bottom surface would not contact the support/electrode (506,530) as shown. This, in turn, would potentially affect the plasma distribution and plasma uniformity, which would translate to the greater variance in oxide thickness (e.g., aluminum oxide) created on the surface (with greater thickness being formed at the periphery of the aluminum coating in comparison with central portions as shown in FIG. 8), and hence increased retardance from the aluminum oxide layer in central portions of the mirror (as shown in FIG. 9) if not taken into account. Specifically, more oxide growth will take place where the lower surface 700 is closer to the plasma 520, and where it’s further away less oxide growth will occur. Otherwise stated, non-uniform plasma will create non-uniform retardance, resulting in uneven oxide formation, and the shape of the plasma (and hence the formed oxide) can depend on the substrate shape. Thus, in cases in which the backside of the mirror is not planar, it may be desirable to design a tool to accommodate such nonplanar backside. Such tool can facilitate uniform contact of the mirror and tool to the bottom electrode resulting in uniform bias and plasma density across mirror surface which is essential in creating uniform oxide thickness, as will be described in connection with the description of FIG. 10 below.

[0095] For some embodiments, and some applications, the non-uniformity or oxide formation and retardance across the surface of the mirror may be acceptable. For more exacting applications, such as those with higher angles on incidence (as discussed with respect to Fig. 6), so additional accommodation may be desirable, as also discussed with respect to Figure 10.

[0096] In one embodiment, as illustrated in FIG. 10, the mirror 400 with substrate 402 with a nonplanar bottom surface is accommodated by a designed tool 1000 that is configured and shaped to complement the non-planar underside 700 of the substrate 402 and take up the otherwise empty space 702 (see FIG. 7) between the bottom surface 700 of the substrate and the support/electrode 506,530.

[0097] The distance of the substrate 402 from the plasma 520 depends on the electrical impedance that substrate 402 provides. Thus, there is a dark region between the substrate and the plasma that is determined by the shape of the substrate and its electrical properties. As a result of the non-planar surface 700, different electrical properties exist at the edge of the substrate compared to the center of the substrate. [0098] To counteract this effect, the tool 1000 is provided with an upper support surface 1002 that is designed to match the shape of the underside surface 700 (or at least a portion of the surface 700) of the substrate 402sits. This enables the substrate to present more uniform electrical properties to the plasma. In one embodiment, the substrate has a generally frustoconical male shape, and the tool 1000 has a correspondence female mating shape. In one embodiment, the tool 1000 has an opening therethrough (e.g., at its center 1004) such that portions of the substrate 402 contact the electrode/support 506, 530 directly, while in another embodiment, the tool 1000 has no openings and is simply thinner in certain regions (e.g. at the center 1004 thereof) than others.

[0099] In one embodiment, the tool is made from the same material as the substrate of the mirror. This material of the tool occupies the volume between the backside of the mirror substrate and the chuck electrode so as to present a flat, unform thickness structure to the plasma.

[00100] The same functionality can also be achieved with tools made of materials different from the mirror substrate, so long as such materials have the same electrical properties (e.g., impedance, conductance, resistance) as the mirror substrate.

[00101] In one or more embodiments, the mirror substrate and tool substrate are made of an insulating material, such as glass or ceramic. In one embodiment, the material for the tool and/or the mirror substrate is a thermoplastic polymer, such polyether ether ketone (PEEK), or another semicrystalline thermoplastic.

[00102] In one embodiment, the tool is fixed to the mirror chuck. In another embodiment, the tool simply rests of the mirror chuck.

[00103] As shown in Figures 11 and 12, the aluminum oxide thickness uniformity and retardance uniformity has substantially improved as a result of use of the tool 1000.

[00104] In some embodiments, terminal thickness is reached with very short exposure ( < 10 min) to oxygen plasma, with or without reactive etching. In one embodiment, the temperature of substrate does not exceed 80 C during oxide formation.

[00105] In some embodiments, after oxide formation (e.g., aluminum oxide) on the mirror, up to 10X smaller accumulated thickness change at the beginning of mirror life can be achieved, and up to 10X slower growth in oxide thickness can also be achieved over the life of a mirror.

[00106] Various mirror samples were processed using features of the present disclosure. Oxide thickness and non-uniformity was measured before and after exposure to etching and oxidation methods described above. With bias applied to the bottom electrode (using the reactive etching) maximum oxide growth was observed. In various embodiments, the oxide layer reached a terminal thickness of over 3.0 nm. In some embodiments, the oxide layer reached terminal thickness of 3.0 nm to 6.0 nm while the thickness non-uniformity was 0.12 nm or lower. In some embodiments, thickness uniformity was 0.10 nm or lower. In some embodiments, thickness uniformity was 0.05 nm or lower. In some embodiments, thickness uniformity was 0.04 nm or lower. The table below shows different parameters utilizing ICP/RIE and resulting oxide thickness. In the chart below, The pressure below is measured in milli Torr, time in minutes, power in watts, and thickness in nanometers.

[00107] In the results above, oxygen gas was used to form an oxygen plasma, so that aluminum oxide was formed on the mirror surface. Average oxide thickness and thickness non-uniformity was calculated using ellipsometric measurements. Ellipsometric data was fitted with known refractive index values of the materials, and thickness was calculated using a best fit method.

[00108] Various measurement techniques can be employed. For example, polarized light reflected off the oxide surface undergoes changes in polarization state that are indicative of the oxide's thickness and optical properties. The beam of polarized light can be directed onto the oxide-coated mirror surface at chosen angle of incidence to enhance measurement sensitivity. Changes in the polarization state of the reflected light, can be detected by a detector and analyzed. By adopting known refractive index values of the oxide generated, a regression analysis to fit measured ellipsometric parameters to a theoretical model of the oxide layer can be used. This model fitting, performed through advanced computational software, can be used to minimize discrepancies between measured and calculated values of the parameters pertaining to change in polarization. The 'best fit' method iteratively adjusts of the theoretical oxide thickness in the model, aligning the calculated ellipsometric parameters with the experimental observations until an optimal match is achieved.

[00109] The embodiments may further be described using the following clauses:

1. An optical fabrication method comprising: placing a reflective mirror into a chamber, the reflective mirror comprising a substrate and a reflective coating on the substrate; generating a vacuum within the chamber; providing a bias to the reflective mirror placed in the chamber; introducing plasma gasses into the chamber; generating a reactive plasma with the gasses in the chamber while the bias is provided to the reflective mirror in order to form an oxide layer on the reflective coating to passivate the reflective coating.

2. The optical fabrication method of clause 1, wherein the reactive plasma is oxygen plasma.

3. The optical fabrication method of clause 1 or 2, further comprising introducing a fluorine based gas within the reactive plasma to assist formation of aluminum-oxy-fluoride or fluorocarbon compounds on the mirror surface.

4. The optical fabrication method of clause 3, wherein the fluorine-based gas comprises CHF3, CF4 or SF6.

5. The optical fabrication method of any of the above clauses, wherein the reactive plasma comprises oxygen based ions.

6. The optical fabrication method of any of the above clauses, wherein the reactive plasma bombards the reflective mirror surface with ions with 50- 500 eV energy.

7. The optical fabrication method of any of the above clauses, wherein the reflective coating comprises aluminum.

8. The optical fabrication method of any of the above clauses, wherein the substrate comprises glass or ceramic.

9. The optical fabrication method of any of the above clauses, wherein the bias is provided to a mirror support configured to support the reflective mirror while the reactive plasma is generated.

10. The optical fabrication method of clause 9, wherein the reflective mirror comprises a non-planar surface shape on a side thereof opposite the reflective coating, and wherein the mirror support comprises a tool having a tool surface that corresponds to the non-planar surface shape of the reflective mirror, such that the reactive plasma is evenly applied across the reflective coating.

11. The optical fabrication method of clause 10, wherein the tool comprises a separate structure that is removable from a base of the mirror support.

12. The optical fabrication method of any of the above clauses, wherein formed oxide layer has a resulting average thickness of between 3 nm to 6 nm.

13. The optical fabrication method of any of the above clauses, wherein the formed oxide layer has a thickness non-uniformity of 0.12 nm or less.

14. A mirror comprising: a substrate; a reflective coating on the substrate; an oxide layer formed on the reflective coating, the oxide layer having an average thickness of greater than 3 nm and a thickness non-uniformity of 0.12 nm or less.

15. The mirror of clause 14, wherein the substrate comprises glass or ceramic.

16. The mirror of clauses 14 or 15, wherein the reflective coating comprises aluminum.

17. The mirror of any one of clauses 14 to 16, wherein the oxide layer comprises aluminum oxide. 18. The mirror of any one of clauses 14 to 17, wherein the oxide layer has a thickness nonuniformity of 0.10 nm or less

19. The mirror of any one of clauses 14 to 17, wherein the oxide layer has a thickness nonuniformity of 0.05 nm or less.

20. The mirror of any one of clauses 14 to 19, wherein the oxide layer has an average thickness of less than 6 nm.

[00110] While the above description primarily discusses the use of an aluminum based reflective coating, the principles of the present disclosure can be applied to other reflective materials that can be used in mirrors.

[00111] Although specific reference may be made in this text to the use of a lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquidcrystal displays (LCDs), thin-film magnetic heads, etc.

[00112] Although specific reference may have been made above to the use of embodiments in the context of optical lithography, it will be appreciated that, where the context allows, the subject matter of the present disclosure is not limited to optical lithography and may be used in other applications, for example imprint lithography, e-beam lithography, or directed self-assembly.

[00113] Where the context allows, embodiments of the present disclosure, such as the controller 540, may be implemented in hardware (e.g, one or more processors), firmware, software, or any combination thereof. Embodiments of the present disclosure may also be implemented as instructions stored on a machine -readable medium, which may be read and executed by one or more processors. A machine- readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.

[00114] While specific embodiments of the present disclosure have been described above, it will be appreciated that various embodiments may be practiced otherwise 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 various embodiments as described without departing from the scope of the claims set out below.

Claims

2. The optical fabrication method of claim 1 , wherein the reactive plasma is oxygen plasma.

3. The optical fabrication method of claims 1 or 2, further comprising introducing a fluorine based gas within the reactive plasma to assist formation of aluminum-oxy-fluoride or fluorocarbon compounds on the mirror surface.

4. The optical fabrication method of claim 3, wherein the fluorine-based gas comprises CHF3, CF4 or SF6.

5. The optical fabrication method of any of the above claims, wherein the reactive plasma comprises oxygen based ions.

6. The optical fabrication method of any of the above claims, wherein the reactive plasma bombards the reflective mirror surface with ions with 50- 500 eV energy.

7. The optical fabrication method of any of the above claims, wherein the reflective coating comprises aluminum.

8. The optical fabrication method of any of the above claims, wherein the substrate comprises glass or ceramic.

9. The optical fabrication method of any of the above claims, wherein the bias is provided to a mirror support configured to support the reflective mirror while the reactive plasma is generated.

10. The optical fabrication method of claim 9, wherein the reflective mirror comprises a non-planar surface shape on a side thereof opposite the reflective coating, and wherein the mirror support comprises a tool having a tool surface that corresponds to the non-planar surface shape of the reflective mirror, such that the reactive plasma is evenly applied across the reflective coating.

11. The optical fabrication method of claim 10, wherein the tool comprises a separate structure that is removable from a base of the mirror support.

12. The optical fabrication method of any of the above claims, wherein formed oxide layer has a resulting average thickness of between 3 nm to 6 nm.

13. The optical fabrication method of any of the above claims, wherein the formed oxide layer has a thickness non-uniformity of 0.12 nm or less.