EP4634970A1

EP4634970A1 - Duv photolithography electrode fabrication method and electrode produced using the method

Info

Publication number: EP4634970A1
Application number: EP23837405.2A
Authority: EP
Inventors: Tobias Kippenberg; Zihan LI
Original assignee: Ecole Polytechnique Federale de Lausanne EPFL
Current assignee: Ecole Polytechnique Federale de Lausanne EPFL
Priority date: 2022-12-12
Filing date: 2023-12-11
Publication date: 2025-10-22
Also published as: WO2024127225A1

Abstract

DUV photolithography compatible electrode fabrication method comprising providing at least one material surface to which at least one electrode is to be attached; providing at least one undercut non-photoresist material or support between the at least one material surface and a photoresist material or layer provided on the at least one undercut non-photoresist material or support; providing or depositing at least one electrode or lift-off material on the photoresist material or layer and on the at least one material surface to form at least a portion of the electrode; and removing or stripping the photoresist material or layer from the at least one undercut non-photoresist material or support to remove the at least one electrode or lift-off material from the at least one undercut non-photoresist material or support.

Description

DUV PHOTOLITHOGRAPHY ELECTRODE FABRICATION METHOD AND ELECTRODE PRODUCED USING THE METHOD

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to European patent application EP22212960.3 filed on December 12th, 2022, the entire contents thereof being herewith incorporated by reference.

FIELD OF THE INVENTION

The present disclosure relates to method for fabricating one or more electrodes on a material or a device, for example, an optical device such as an electro-optic modulator, for example, a lithium niobate optical device or modulator. The present disclosure relates to an electrode fabrication method that is deep ultraviolet (DUV) photolithography compatible. More particularly, the present disclosure relates to a photolithography electrode fabrication method, for example, an ultraviolet or a deep ultraviolet (DUV) lithography electrode fabrication method. The present disclosure relates to a method for electrode fabrication via, for example, DUV lithography and lift-off permitting to provide electrodes of larger or more significant thickness and a method that is compatible with standardized photolithography material processing methods and systems.

BACKGROUND

Electrodes of large thickness for optical waveguides, for example for lithium niobate waveguides used in electro-optic modulators, may be produced using UV photolithography and UV photoresist, However, this approach frequently results in a misalignment between the electrodes and the waveguide. Such misalignment can be significant and result in degraded device operation and strong optical loss.

An alternative approach is to use a DUV stepper (a deep ultraviolet lithography stepper exposure, or step and scan system) and DUV photoresist for electrode fabrication. However, this approach is unable to produce electrodes of large or significant thickness.

A lift-off process performed with a DUV stepper imposes the use of certain photoresists and to assure a high resolution of the stepper, the thickness of the coatings of these photoresists must be kept thin. As a result, the material which needs to be structured by lift-off (lift-off material), cannot be deposited too thickly and typically is restricted to a thickness or the order of 100 nm. Changing the available photoresists in the stepper system to assure thicker coatings requires the system parameters, such as exposure dose and defocus, to be reoptimized once again.

The article entitled “A negative tone lift-off method for small metal holes using PMMA/SiO2 double layer” by BAEK and LEE, published in Bull. Mater. Sci. (2021 )44:275, discloses a method to fabricate small apertures in a metal film using polymethyl methacrylate (PMMA). A PMMA/SIO2 double layer is employed to perform a negative tone lift-off with SiO? as a sacrificial layer. The method, however, is relatively complex and uses electron beam lithography to expose the PMMA resulting in slower and lower productivity in relation to device production.

SUMMARY

It is therefore one aspect of the present disclosure to address the above-mentioned inconveniences by providing a DUV photolithography electrode fabrication method or a DUV photolithography compatible electrode fabrication method.

Preferably, the method may comprise providing at least one material surface to which at least one electrode is to be attached; and providing at least one undercut non-photoresist material or support between the at least one material surface and a photoresist material or layer provided on the at least one undercut non-photoresist material or support.

The method may comprise providing or depositing at least one electrode or lift-off material on the photoresist material or layer and on the at least one material surface to form at least a portion of the electrode; and removing or stripping the photoresist material or layer from the at least one undercut non-photoresist material or support to remove the at least one electrode or lift-off material from the at least one undercut non-photoresist material or support.

According to another aspect, the removing or stripping of the photoresist material or layer may be carried out by the photoresist material or layer being directly acted upon by a photoresist removal or stripper product to detach the photoresist material or layer from a non-photoresist material of the at least one undercut non-photoresist material or support, the non-photoresist material remaining attached to the at least one material surface.

According to yet another aspect, detachment of the photoresist material or layer from a nonphotoresist material results in a lift-off of the at least one electrode or lift-off material located on the at least one undercut non-photoresist material or support.

According to yet another aspect, the at least one undercut non-photoresist material or support may be formed by deep ultraviolet photolithography by exposing a photoresist material or layer, provided on a non-photoresist material, to ultra-violet light and by carrying out development to remove photoresist material to expose at least one or a plurality of upper portions of the underlying non-photoresist material and define at least one pattern or mask on the non-photoresist material. The ultra-violet light may have a wavelength , in a wavelength range 300nm >X> 190nm.

According to yet another aspect, the at least one undercut non-photoresist material or support may be formed by removing at least a portion of exposed non-photoresist material provided or deposited on the at least one material surface using dry etching to expose at least one or a plurality of lateral walls or lateral surfaces of the non-photoresist material; and removing at least a portion of the exposed at least one lateral wall or walls or lateral surface or surfaces using wet etching to form at least one or a plurality of overhanging portions of photoresist material or layer and form at least one or a plurality of undercuts.

According to yet another aspect, the dry etching can be carried out to remove non-photoresist material in a thickness direction towards the at least one material surface, and wet etching is carried out to remove non-photoresist material in a lateral direction, the lateral direction extending substantially orthogonally to the thickness direction.

The exposed non-photoresist material may be exposed by the at least one photoresist pattern or mask in zones where the photoresist material or layer has been removed.

The at least one undercut non-photoresist material or support can be formed by the at least one wet etched lateral wall or walls or lateral surface or surfaces of the non-photoresist material and a portion photoresist material or layer attached on top of the non-photoresist material.

According to yet another aspect, the method may include removing the at least one undercut non-photoresist material or support after removing or stripping the photoresist material or layer from the at least one undercut non-photoresist material or support.

The at least one undercut non-photoresist material or support may include a plurality of undercuts.

The at least one undercut non-photoresist material or support may include a first undercut and a second undercut, wherein the first undercut is located opposite the second undercut.

According to yet another aspect, providing at least one undercut non-photoresist material or support between the material surface and a photoresist material or layer provided on the undercut non-photoresist material or support may comprise providing or depositing a nonphotoresist material on the at least one material surface to which the at least one electrode is to be attached; providing or depositing the photoresist material or layer on the at least one non-photoresist material; removing at least a portion of the photoresist material or layer and at least a portion of the non-photoresist material underlying the photoresist material or layer to expose at least one or a plurality of lateral walls or lateral surfaces of the non-photoresist material underlying the photoresist material or layer; and removing at least a portion of the exposed at least one lateral wall or walls or lateral surface or surfaces to form at least one or a plurality of undercuts and form the at least one undercut non-photoresist material or support. A portion of the photoresist material or layer on the non-photoresist material may remain attached to the non-photoresist material after the step of the removal of the portion of the nonphotoresist material underneath the photoresist material or layer that exposes the at least one lateral wall or lateral surface of the non-photoresist material.

According to yet another aspect, at least one pattern or mask formed by deep ultraviolet photolithography of the photoresist material or layer may define or comprise the portion of the photoresist material or layer located on the non-photoresist material that remains attached to the non-photoresist material following the removal of the portion of the non-photoresist material underneath the photoresist material or layer.

The non-photoresist material underlying the photoresist material or layer may be partially removed to leave a portion of the non-photoresist material on the material surface and form a protective covering on or for the material surface to which at least one electrode is to be attached.

The at least one undercut or each undercut may define defines a protrusion distance d permitting the at least one electrode or lift-off material forming the at least one electrode to be deposited without direct lateral contact to a neighboring element.

The photoresist material or layer provided or deposited on the non-photoresist material may be exposed to ultra-violet light and developed to remove photoresist material to expose at least one or a plurality of portions of the underlying non-photoresist material and define at least one pattern or mask on the non-photoresist material.

The exposed portions of the underlying non-photoresist material following removal of the photoresist material or layer may be etched to expose the at least one lateral wall or walls or lateral surface or surfaces of the non-photoresist material.

The exposed at least one lateral wall or walls or lateral surface or surfaces of the nonphotoresist material may be etched to form the at least one undercut.

According to yet another aspect, removing or stripping the photoresist material or layer from the at least one undercut non-photoresist material or support may remove the at least one electrode or lift-off material from only the at least one undercut non-photoresist material or support.

According to yet another aspect, the undercut non-photoresist material defines a notch under the photoresist material or layer, or an over-hanging portion of the photoresist material or layer over-hanging the material surface.

A thickness H1 of the undercut non-photoresist material or support or of the non-photoresist material may be at least X times a thickness t of the at least one electrode or lift-off material, where X has a value between 1.5 and 2.5. According to yet another aspect, the at least one material surface is defined by a non-organic material surface, or a non-organic material surface of a device made of or consisting of non- organic material.

According to yet another aspect, the electrode fabrication method may be an optical device electrode fabrication method, or optical waveguide device electrode fabrication method.

The optical device may include a ridge waveguide, or the optical device may be an optical modulator. The optical device may include or be an electro-optic modulator. The optical device may be a lithium niobate modulator, or the ridge waveguide may include lithium niobate.

According to yet another aspect, the optical device may include the at least one material surface, or the at least one undercut non-photoresist material or support may be provided on at least one material surface of the optical device.

The ridge waveguide may include the at least one material surface, or the at least one undercut non-photoresist material or support may be provided on at least one material surface of the ridge waveguide to enclose lateral side walls and a lateral wall interconnecting ceiling of the ridge waveguide.

According to yet another aspect, the at least one undercut non-photoresist material or support may be provided on at least one material surface of a portion of an optical device, and the at least one electrode or lift-off material provided or deposited on the at least one material surface may form at least a portion of at least one or a plurality of electrodes of the optical device.

The optical device may be or comprise an electro-optic modulator or a lithium niobate modulator. The at least one undercut non-photoresist material or support may be provided on a modulation region of an electro-optic modulator or a lithium niobate modulator.

According to yet another aspect, a thickness t of the electrode or lift-off material provided or deposited may be in the range of 10Onm < t <5p.m.

A further aspect of the present disclosure concerns an electrode formed on at least one material surface fabricated using the DUV photolithography electrode fabrication method. The electrode fabrication method of the present disclosure permits to overcome the electrode thickness limitation of traditional lift-off methods, in particular, methods based on DUV stepper. Additionally, in comparison to the E-beam lithography lift-off, a high productivity and precise alignment lift-off method is assured by the electrode fabrication method of the present disclosure which, moreover, can be used for commercial electrode fabrication, for example, for commercial electrode fabrication for lithium niobate modulators.

Advantageously, the method of the present disclosure can be used on any lift-off related process with a high alignment requirement, such as, integrated electrode fabrication.

Advantageously, the method of the present disclosure is compatible with current DUV stepper exposure, and a lift-off process assures the provision of a thick metal layer (for example, >1 |im) while simultaneously benefiting from the stepper system’s high productivity and alignment.

The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description with reference to the attached drawings showing some preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate the presently preferred embodiments of the invention, and together with the general description given above and the detailed description given below, serve to explain features of the invention.

Figures 1A to 1 H schematically show exemplary steps of an electrode fabrication method of the present disclosure.

Figure 2 schematically shows a further example of exemplary steps of an electrode fabrication method of the present disclosure.

Figures 3A to 3D show SEM images at different stages during the fabrication method of the present disclosure, where Figure 3A shows an undercut of a dielectric layer, Figure 3B shows the deposition of an exemplary thickness of 500 nm of aluminum on the wafer, Figure 3C is a cross sectional view and Figure 3D is a top view of the lift-off result.

Figures 4A to 4C are photos of fabricated devices produced with the lift-off method of the present disclosure, where Figure 4A shows a wafer, Figure 4B shows a chip comprising lithium niobate modulators, and Figure 4C is a microscopy image of devices including gold electrodes. Figure 5 shows a further step of the exemplary electrode fabrication method schematically illustrated in Figures 1 A to 1 H in which a photoresist material or layer is provided or deposited on a non-photoresist material prior to being patterned by photolithography.

Herein, identical reference numerals are used, where possible, to designate identical elements that are common to the Figures. Also, the images are simplified for illustration purposes and may not be depicted to scale.

DETAILED DESCRIPTION OF SEVERAL EMBODIMENTS

Figures 1 and 2 schematically show exemplary embodiments of the electrode fabrication method of the present disclosure. The specific materials and their thicknesses indicated in Figures 1 and 2 are non-limiting exemplary materials and thicknesses. The electrode fabrication method can be, for example, a deep ultraviolet (DUV) photolithography compatible electrode fabrication method, or a deep ultraviolet (DUV) photolithography electrode fabrication method.

A deep ultraviolet wavelength in the deep ultraviolet DUV wavelength range can be, for example, a wavelength in the ultra violet wavelength range that has a wavelength > 190nm. For example, a wavelength in the wavelength range 280nm >7> 190nm, or the range 300nm >X> 190nm, or the range 300nm >A> 190nm.

Figures 1A to 1 H show an example of the electrode fabrication method of the present disclosure in which a device or substrate 3 includes a patterned or structured outer layer 4 comprising or defining a surface 5. The outer layer 4 may, for example, comprise or define an optical structure 7, for example, a ridge waveguide. The ridge waveguide includes, for example, lateral side walls WL1 , WL2 and a lateral wall interconnecting ceiling CL (see for example Figure 1A). The lateral side walls WL1 , WL2 and the lateral wall interconnecting ceiling CL that extend longitudinally or in a light guiding direction.

The outer layer 4 may comprise or consist of, for example, lithium niobate (or a layer provided on lithium niobate material), and the electrode fabrication method permits to provide one or more electrodes 9 (of large thickness) for the lithium niobate waveguide device, for example, for a lithium niobate electro-optic modulator. The one or more electrodes 9 permit device operation.

Figures 2A to 2F show another example of the electrode fabrication method of the present disclosure in which a layer or substrate 3 (for example, a silicon substrate) includes or defines an outer surface 5 upon which one or more electrodes 9 are provided or deposited by the electrode fabrication method of the present disclosure.

The electrodes 9 may directly or indirectly contact the surface 5.

The electrode fabrication method concerns or is a photolithography or optical lithography electrode fabrication method. The photolithography electrode fabrication method is, for example, preferably an ultraviolet lithography electrode fabrication method, or a deep or extreme ultraviolet lithography electrode fabrication method. The fabrication method is, for example, compatible with and can be carried out using a stepper or stepper device, or a step and scan system. That is, a system or device in which a reticle or mask is used during light exposure of the wafer or material surface to transfer a pattern to a photoresist deposited on the wafer or material surface, and stepped or repeatedly moved across the wafer surface or material surface to expose the photoresist thereon. The relative displacement of the wafer surface or material surface with respect to the reticle or mask may be carried out be displacing the wafer or material by, for example, a wafer stage.

The electrode fabrication method comprises providing at least one material surface 5 to which at least one or a plurality of electrodes 9 are to be attached (see, for example, Figures 1 A and 2A). The material surface 5 may, for example, be defined by or included in a layer or substrate 3.

At least one layer 5 and at least one substrate 3 to which the at least one layer is attached or provided on may be provided, the layer 5 comprising or defining the at least one material surface 5. Alternatively, at least one substrate 3 may be provided, the substrate 3 comprising or defining the at least one material surface 5.

The at least one layer 5 or substrate 3 may, for example, be patterned or include at least one structure 7, as for example, shown in Figure 1A. The structure 7 may comprise or define an optical device OD, for example, an optical waveguide, or an optically active element of an optical device, the element being operated by one or more electrical signals applied to the deposited electrode(s) 9.

In the non-limiting exemplary embodiment of Figures 1A to 1 H, the layer 5 comprises or consists of lithium niobate, and the substrate 3 comprises or consists of silicon. Intermediate layers may also be present, for example, thermal oxide layers as shown in Figures 1 A to 1 H.

At least one undercut non-photoresist material (or undercut support) 11 or a plurality of undercut non-photoresist material (or supports) 11 are provided between the material surface 5 and a photoresist material or layer 15 that is provided or deposited on the undercut nonphotoresist material 11 (see, for example, Figures 1 E and 2D).

The (or each) undercut non-photoresist material or support 11 includes at least one undercut UC or a plurality of undercuts UC, for example, a first undercut UC1 and a second undercut UC2. The first undercut UC1 can be located, for example, opposite the second undercut UC2 (see, for example, Figures 1 E and 2D). The undercut UC is defined or delimited by a protruding base portion BS of the photoresist material or layer 15 and a lateral wall LW defining or comprising a lateral surface SW of a support material 17 (preferably, a non-photoresist material) that is located underneath the photoresist material or layer 15 (Figure 1 E).

At least one electrode or lift-off material 19 is provided or deposited on the photoresist material or layer 15 and on the material surface 5 (see, for example, Figures 1 F and 2E) to form the electrode or electrodes 9 (or at least a portion of the electrode or electrodes 9). The photoresist material or layer 15, that receives the electrode or lift-off material 19, forms a part of a pattern or mask PS formed, for example, by deep ultraviolet photolithography of an initially deposited photoresist material or layer (15).

A plurality of different electrode or lift-off materials 19 may be deposited. The electrode or liftoff material 19 may, for example, be deposited by material evaporation.

A thickness t of the electrode or lift-off material 19 provided or deposited is that intended for the thickness of the electrode 9. A thickness t of the electrode or lift-off material 19 provided or deposited may, for example, be >100nm or >200nm or >400nm or >500nm, or be, for example, a thickness t between 1 nm and 5pm, or a thickness t <5|a.m. A thickness t of the electrode or lift-off material 19 can thus for example, be in the range 100nm or 200nm < t <5|im.

The electrode or lift-off material 19 may, for example, comprise or consist of a metal or a dielectric material for example SiO? or MgF or indeed any material that can be deposited by evaporator. The metal comprises or consists of, for example, aluminum, or gold or titanium, or chromium, or any other metal used as an electrode material.

For example, a first material that is a dielectric material may be deposited followed by a second material that is a metal deposited on the deposited dielectric material.

The undercut non-photoresist material or support 11 defines a notch under the photoresist material or layer 15, or an over-hanging portion NO of the photoresist material or layer 15 over-hanging the material surface 5. The notch or over-hanging portion NO shelters the material surface 5 underneath the notch or over-hanging portion NO during the deposition of the electrode or lift-off material 19. As a result, at least one depression or trench DP is formed (for example, for each undercut UC) and located between the undercut non-photoresist material or support 11 and the electrode or lift-off material 19 on the material surface 5 (see, for example, Figure 1 F and 2E). The depression or trench DP is widened or enlarged if and when the non-photoresist material 17 is removed (see, for example, Figure 1 H).

The fabrication method of the present disclosure further includes removing or stripping the photoresist material or layer 15 (of the pattern or mask PS) from the undercut non-photoresist material or support 11 to remove the electrode or lift-off material 19 from the undercut nonphotoresist material or support 11 (see, for example, Figures 1 H and 2F).

Removal may, for example, be carried out using a photoresist remover depending on the photoresist used. For example, 1165 photoresist remover for M35G photoresist 15 which is a deep ultraviolet photoresist.

Removing or stripping of the photoresist material or layer 15 (of the pattern or mask PS) is, for example, carried out by the photoresist material or layer 15 being directly acted upon by a photoresist removal or stripper product to detach the photoresist material or layer 15 from the non-photoresist material 17 of the undercut non-photoresist material or support 11. While the photoresist material or layer 15 is removed, the non-photoresist material 17 remains attached to the material surface 5.

The removal of the photoresist material or layer 15 from the undercut non-photoresist material or support 11 removes the electrode or lift-off material 19 from only the undercut nonphotoresist material or support 11 . That is, the electrode or lift-off material 19 and electrode 9 remain attached to the material surface 5. The detachment of the photoresist material or layer 15 from a non-photoresist material 17 results in a lift-off of the electrode or lift-off material 19 located on the non-photoresist material 17 and on the undercut non-photoresist material or support 11.

As a result, the material surface 5 includes one or more electrodes attached (directly or indirectly) to it, and one or more undercut non-photoresist material or supports 11. For example, in Figure 1G, the optical structure 7 has a first electrode 9A and a second electrode 9B located on opposite sides of the optical structure 7. The electrodes 9A, 9B may extend, for example, in elongated manner along a direction of extension of the optical structure 7 (for example, a light propagation direction), as seen for example in Figures 3B and 4C. The one or the plurality of undercut non-photoresist materials or supports 11 (or the remaining support material 17 (Figure 1G)) may, for example, be removed after the removal or stripping of the photoresist material or layer 15 from the undercut non-photoresist material or support 11 . Figure 1 H shows an example where the undercut non-photoresist material or support 11 has been removed to expose the outer surface of the optical structure 7 and a portion of the material surface 5.

The removal of the undercut non-photoresist materials or supports 11 (or the remaining support material 17 (Figure 1G)) can be carried out, for example, by etching, for example, by dry or wet etching. For example, one can use the wet etchant that is buffe red HF to etch SiO?, and XeF2, KOH to etch Si.

The provision or deposition of the undercut non-photoresist material or undercut support 11 between the material surface 5 and the photoresist material or layer 15 provided on the undercut non-photoresist material or support 11 comprises providing or depositing the nonphotoresist material 17 on the material surface 5 to which the electrode 9 is to be attached, as for example, shown in Figures 1 B and 2A.

The non-photoresist material 17 may, for example, comprise or consist of a dielectric material, for example, amorphous silicon or silicon dioxide (SiO?) or amorphous Si from plasma- enhanced chemical vapor deposition (PECVD). The silicon dioxide material may be deposited using, for example, plasma enhanced chemical vapor deposition (PECVD).

The undercut non-photoresist material 17 may comprises or consists of a deep ultraviolet lithography compatible material.

A thickness H1 of the non-photoresist material 17 (and/or the undercut non-photoresist material or support 11 ) is at least X times a thickness t of the electrode or lift-off material 19, where X has a value between 1 .25 or 1 .5 and 2.5, for example, 1 .5 or 2 times. In other words, the thickness of the non-photoresist material 17 is, for example, 2 times or twice the thickness of the electrode or lift-off material 19 (that is or will be deposited).

The photoresist material or layer 15 is then provided or deposited on the non-photoresist material 17 (see, for example, Figure 5). This provides an initially deposited or non-patterned photoresist material or layer 15. The thickness t1 of the photoresist material or layer 15 deposited is such that the non-photoresist material 17 may be etched fully through or etched to a desired depth into the non-photoresist material 17.

The thickness t1 of the photoresist material or layer 15 is such that a quantity of photoresist material or layer 15 covers or remains on top of the non-photoresist material 17 after etching of the non-photoresist material 17 in a direction towards the material surface 5 to expose lateral walls LW1 (see, for example, Figure 1 D), as explained below in further detail. Idem, for etching to form lateral walls LW and undercut UC (see, for example, Figure 1 E), as explained below in further detail.

The photoresist material or layer 15 may, for example, comprise or consist of a DUV stepper photoresist such as JSR Micro NV M108Y, or JSR Micro NV M35G

The photoresist material or layer 15 (the initially deposited or non-patterned photoresist material or layer 15) is then patterned (see, for example, Figures 1C and 2B) using photolithographic exposure and development (for example, using a DUV stepper). The photoresist material or layer 15 is, for example, exposed to ultra-violet light and developed, using a developer, to remove photoresist material, expose underlying non-photoresist material 17 and define a pattern or mask PS on the non-photoresist material 17.

Deep ultraviolet (DUV) photolithography is used and the photoresist material or layer 15, provided on a non-photoresist material 17, is exposed to or irradiated with ultra-violet light and development is carried out (using a photoresist developer) to remove photoresist material 15 to expose one or more upper portions, areas, or surfaces S1 , S2 of the underlying nonphotoresist material 17. This defines the photoresist pattern or mask PS on the nonphotoresist material 17. The ultra-violet light has, for example, a wavelength A in a wavelength range 300nm >A> 190nm.

The pattern or mask PS includes photoresist material structures ST attached to and covering non-exposed areas of the non-photoresist material or layer 17 (Figure 1C). The nonphotoresist material or layer 17 includes exposed areas or portions P1 , P2 where the initially deposited photoresist material 15 has been removed to expose the non-photoresist material or layer 17.

This produces a pattern or structure PS covering or concealing one or more zones or regions ZR of the material surface 5 where the electrode 9 is non-formed. A ACS200 GEN3 coater/developer platform (by SUSS) may for example be used to produce the pattern or structure PS.

One or a plurality of portions of the photoresist material or layer 15 are thus removed to expose a portion (and an area or a surface S1 , S2 thereof) or a plurality of portions (and areas or surfaces S1 , S2 thereof) of the non-photoresist material 17 underlying the photoresist material or layer 15 (see, for example, Figure 1C).

A portion (or material column) or a plurality of portions (material columns) (for example, portions/columns underlying the surfaces S1 , S2 in Figure 1 C) of the non-photoresist material 17 underlying the removed portion or portions P1 , P2 of the photoresist material or layer 15 are then removed to expose at least one or a plurality of lateral walls LW1 defining or comprising lateral surfaces SW1 of the non-photoresist material 17 underlying the photoresist material or layer 15 (Figures 1 D and 2C). The removal of the non-photoresist material 17 can be carried out using dry etching, for example, by plasma etching. For example, C4F8 or CF4 plasma etching may be used. Etching is, for example, performed in a direction of the material surface 5. The portion or material column removed define a volume of material extending a certain thickness into the non-photoresist material 17 in a direction of the material surface 5. In the example shown in Figure 1 D, elongated material strips of non-photoresist material 17 are removed to form a protruding structure extending away from the material surface 5 and capped by photoresist material 15.

The non-photoresist material 17 underlying the (now removed) photoresist material or layer 15 may be fully removed (for example, by etching to the material surface 5) to expose the underlying material surface 5. Alternatively, the non-photoresist material 17 underlying the (now removed) photoresist material or layer 15 may be partially removed to leave a portion or film of the non-photoresist material 17 on the material surface 5. For example, etching of nonphotoresist material 17 is stopped to leave between 5% and 15% (for example 10%) of the thickness H1 of the non-photoresist material 17 on the material surface 5. This forms a protective covering on or for the material surface 5 to which electrode 9 is to be attached.

This removal of the non-photoresist material 17 exposes or forms the lateral wall or walls LW1 of the non-photoresist material 17. The lateral wall or walls LW1 extend, for example, from the remaining photoresist material or layer 15 towards the material surface 5. The exposed lateral wall or walls LW1 (or the lateral surface or surfaces SW1) or at least a portion thereof is removed to form the undercut (or undercuts) UC and form the undercut nonphotoresist material or support 11.

The exposed underlying non-photoresist material 17 or the exposed lateral wall or walls LW1 are, for example, etched to expose the (inner) lateral wall or walls LW (or (inner) lateral surface or surfaces SW) and to form the undercut or undercuts UC. Etching can be carried out, for example, using wet etching. For example, hydrofluoric acid may be used to create the undercut UC. Etching can be carried out, for example, in a direction orthogonal or non-parallel to the direction of extension of the lateral wall or walls LW1 , or in a direction that removes material 17 underneath the remaining photoresist material 15.

In other words, the undercut non-photoresist material or support 11 can be formed by removing one or more portions or volumes of exposed non-photoresist material 17 provided or deposited on the material surface 5 using dry etching to expose one or more lateral walls LW1 or lateral surfaces SW1 of the non-photoresist material 17; and removing material at least a portion of the exposed lateral wall or walls LW1 or lateral surface or surfaces SW1 using wet etching to form one or more overhanging portions of photoresist material or layer 15 and form one or more undercuts UC.

The dry etching can be carried out to remove non-photoresist material 17 in a thickness direction towards the material surface 5, and wet etching can be carried out to remove nonphotoresist material 17 in a lateral direction. The lateral direction extends, for example, substantially orthogonally to the thickness direction.

The exposed non-photoresist material 17, that undergoes drying etching to etch into the body of non-photoresist material 17, is exposed by the pattern or mask PS defined in the photoresist material 15, or defined by zones in the photoresist material 15 where the photoresist material or layer 15 has been removed.

The undercut non-photoresist material or support 11 is formed by the wet etched lateral wall or walls LW or lateral surface or surfaces SW of the non-photoresist material (17) and a portion or capping of photoresist material or layer 15 is attached on top of the non-photoresist material 17 of the undercut non-photoresist material or support 11.

This undercutting process uses both dry and wet etching in the undercutting process. This advantageously allows to increase the allowable thickness of the lift-off materials and also allows to maintain the precise pattern, obtained via DUV photolithography, for small structures and accurate alignment.

As a result, and as mentioned previously, the undercut DC is defined or delimited by the protruding base portion BS of the photoresist material or layer 15 and the lateral wall LW defining or comprising the lateral surface SW of a support material 17 (Figure 1 E).

The undercut UC or the overhanging portion NO defines an undercut or protrusion distance d (see, for example, Figure 1 E) which allows the electrode or lift-off material 19 to be deposited onto the material surface 5 without direct lateral contact to a neighboring element, such as the lateral wall LW of the non-photoresist material 17. The undercut or protrusion distance d may, for example, between 1 % and 10% of a total width W of the etched non-photoresist material 17 (Figure 1 D). The undercut or protrusion distance d may, for example, be between 100nm and 500nm, for example, 400nm.

A portion (or capping of photoresist material 15) P3 (Figure 1 E) of the photoresist material or layer 15 is still present after removal of part of the non-photoresist material 17 underneath the photoresist material or layer 15 to expose the lateral wall or walls LW (or lateral surfaces SW) of the non-photoresist material 17. That is, the thickness of the photoresist material or layer 15 deposited on the non-photoresist material 17 is such that a portion or block P3 (Figure 1 E) of the photoresist material or layer 15 remains after removal of part of the non-photoresist material 17 underneath the photoresist material or layer 15 to expose the lateral wall or walls LW (or lateral surfaces SW) of the non-photoresist material 17.

The undercut non-photoresist material or support 11 and the photoresist material or layer 15 form a bi-material defining one or more undercuts that assure the provision of electrodes 9 of large thickness.

The remaining portion P3 includes the overhanging portions NO and the base BS of the undercut UC. An upper surface S4 receives the electrode or lift-off material 19 and shelters the directly underneath material surface 5 from receiving deposition of the electrode or lift-off material 19.

Figures 2A to 2F show another non-limiting exemplary embodiment illustrating how electrodes 9 of high thickness may be provided on a material surface 5 that is the surface of a silicon layer or substrate using the fabrication method of the present disclosure. As previously mentioned, the fabrication method is compatible with current DUV stepper exposure. A dielectric material 17 is used to create the undercuts UC for lift-off. The dielectric material is, for example, silicon dioxide (SiO?), but as already mentioned, other compatible materials with the stepper process can be used.

A thick dielectric layer 17 is deposited on the surface 5 of the substrate 3. This layer 17 is, for example, 1 .5 times thicker or at least 1 .5 times thicker than the expected thickness of the liftoff material 19.

As mentioned, the dielectric layer 17 can be SiO? or alternatively could be amorphous Si. For example, aluminum (Al) can be used as the lift-off material to demonstrate the working of the present fabrication method. 1 |_im of SiO2 was, for example, deposited using the equipment Oxford PlasmaLab 100 PECVD which permitted to lift off 500 nm of the Al material 19.

Coating, exposure and development was carried out using a known and widely used DUV stepper photoresist. The thickness of the photoresist is chosen to be a thickness permitting to etch through the dielectric layer 17. For example, 2 .m of AZ10XT20 using a Suss ACS200 Gen3 was deposited, which has the same vertical sidewall as DUV PR.

Etching of the dielectric layer 17 was carried out using dry etching. A thin slab or film, for example 100 nm, may be preserved on the material surface 5 of the substrate 3 to protect the substrate 3. For example, 900 nm of SIO? was etched by plasma etching, as detailed previously above.

The undercuts UC were created on the dielectric layer 17 using wet etching. The width of the undercut UC can vary from 100 nm to 500 nm, depending on the thickness of the lift-off material 19. For example, the substrate/wafer was placed in buffer hydrofluoric acid for 2 minutes 30 seconds to create an undercut UC of about 400 nm, as shown in the scanning electron microscopy (SEM) image of Figure 4A.

The lift-off material 19 is then deposited, and deposited thickness is the expected resulting thickness of that of the electrode 9. The lift-off material 19 can be any metal or dielectric material which the evaporator can deposit. For example, 500 nm of Al was for example deposited by the material evaporation machine Alliance-Concept EVA 451 , and the resulting deposited metal can be seen, for example, in Figure 3C. The resist 15 is removed or stripped using known resist remover, for example, of the stepper process. For example, the photoresist 15 is stripped using 1165 Remover, as seen in Figures 3D and 3E. The dielectric layer 17 may then be selectively removed.

The electrode fabrication method may, for example, be an optical device OD electrode fabrication method. The optical device OD may, for example, include a ridge waveguide 7. The optical device OD, may, for example, be an optical modulator, such as, an electro-optic modulator. The optical device OD may, for example, be a lithium niobate modulator, or the ridge waveguide 7 may include lithium niobate.

The optical device OD thus includes the material surface 5. The undercut non-photoresist material or support 11 is, for example, provided on the material surface 5 of the optical device OD, or is provided on the material surface 5 of the ridge waveguide 7 to enclose lateral side walls WL1 , WL2 and a lateral wall interconnecting ceiling CL of the ridge waveguide 7. The undercut non-photoresist material or support 11 , may for example be provided on a modulation region of an electro-optic modulator or a lithium niobate modulator.

In the exemplary embodiment, as for example shown in Figures 1 A to 1 H, the undercut nonphotoresist material or supports 11 are provided on at least one material surface 5 of a portion of a device, for example, a (lithium niobate) modulator, or more precisely provided on an optical waveguide 7 of a (lithium niobate) modulator. The electrode or lift-off material 19 is provided or deposited on the material surface 5 to form at least a portion of the electrodes 9 of the device or of the (lithium niobate) modulator.

The one or more material surfaces 5, may be defined by a non-organic material surface, or may be defined by a non-organic material surface of a device made of or consisting of non- organic material.

As mentioned, Figures 3A to 3D show SEM images at different stages during the fabrication method. Figure 3A shows an undercut UC of a dielectric layer. Figure 3B shows the deposition of an exemplary thickness of 500 nm of an electrode or lift-off material 19 that is aluminum on the wafer. Figure 3C is a cross sectional view and Figure 3D is a top view of the lift-off result.

Figures 4A to 4C are photos of fabricated devices produced with the electrode fabrication method of the present disclosure. Figure 4A shows a wafer and Figure 4B shows a chip comprising lithium niobate modulators. Figure 4C is a microscopy image of devices including gold electrodes. A further aspect of the present disclosure concerns one or more electrodes 9 formed on at least one material surface 5 fabricated using the photolithography electrode fabrication method.

Implementations described herein are not intended to limit the scope of the present disclosure but are just provided to illustrate possible realizations.

While the invention has been disclosed with reference to certain preferred embodiments, numerous modifications, alterations, and changes to the described embodiments, and equivalents thereof, are possible without departing from the sphere and scope of the invention. Accordingly, it is intended that the invention not be limited to the described embodiments and be given the broadest reasonable interpretation in accordance with the language of the appended claims.

The features of any one of the above described embodiments may be included in any other embodiment described herein.

Claims

1. Deep ultraviolet photolithography compatible electrode fabrication method comprising: providing at least one material surface (5) to which at least one electrode (9) is to be attached; providing at least one undercut non-photoresist material or support (11 ) between the at least one material surface (5) and a photoresist material or layer (15) provided on the at least one undercut non-photoresist material or support (11); providing or depositing at least one electrode or lift-off material (19) on the photoresist material or layer (15) and on the at least one material surface (5) to form at least a portion of the electrode (9); and removing or stripping the photoresist material or layer (15) from the at least one undercut non-photoresist material or support (11 ) to remove the at least one electrode or lift-off material (19) from the at least one undercut non-photoresist material or support (11 ).

2. Method according to claim 1 , wherein the removing or stripping of the photoresist material or layer (15) is carried out by the photoresist material or layer (15) being directly acted upon by a photoresist removal or stripper product to detach the photoresist material or layer (15) from a non-photoresist material (17) of the at least one undercut non-photoresist material or support (11 ), the non-photoresist material (17) remaining attached to the at least one material surface (5).

3. Method according to claim 2, wherein detachment of the photoresist material or layer (15) from a non-photoresist material (17) results in a lift-off of the at least one electrode or lift-off material (19) located on the at least one undercut non-photoresist material or support (11).

4. Method according to any one of the previous claims, wherein the at least one undercut nonphotoresist material or support (11 ) is formed by deep ultraviolet photolithography by exposing a photoresist material or layer (15), provided on a non-photoresist material (17), to ultra-violet light and by carrying out development to remove photoresist material (15) to expose at least one or a plurality of upper portions (S1 , S2) of the underlying non-photoresist material (17) and define at least one pattern or mask (PS) on the non-photoresist material (17).

5. Method according to the previous claim, wherein the ultra-violet light has a wavelength in a wavelength range 300nm >X> 190nm.

6. Method according to any one of the previous claims, wherein the at least one undercut nonphotoresist material or support (11 ) is formed by: removing at least a portion of exposed non-photoresist material (17) provided or deposited on the at least one material surface (5) using dry etching to expose at least one or a plurality of lateral walls (LW1 ) or lateral surfaces (SW1 ) of the non-photoresist material (17); and removing at least a portion of the exposed at least one lateral wall or walls (LW1 ) or lateral surface or surfaces (SW1 ) using wet etching to form at least one or a plurality of overhanging portions of photoresist material or layer (15) and form at least one or a plurality of undercuts (UC).

7. Method according to the previous claim, wherein the dry etching is carried out to remove non-photoresist material (17) in a thickness direction towards the at least one material surface (5), and wet etching is carried out to remove non-photoresist material (17) in a lateral direction, the lateral direction extending substantially orthogonally to the thickness direction.

8. Method according to the previous claim 6 or 7, wherein the exposed non-photoresist material (17) is exposed by the at least one photoresist pattern or mask (PS) in zones where the photoresist material or layer (15) has been removed.

9. Method according to any one of the previous claims 6 to 8, wherein the at least one undercut non-photoresist material or support (11) is formed by the at least one wet etched lateral wall or walls (LW) or lateral surface or surfaces (SW) of the non-photoresist material (17) and a portion photoresist material or layer (15) attached on top of the non-photoresist material (17).

10. Method according to any one of the previous claims, further including removing the at least one undercut non-photoresist material or support (11 ) after removing or stripping the photoresist material or layer (15) from the at least one undercut non-photoresist material or support (11 ).

11. Method according to any one of the previous claims, wherein the at least one undercut non-photoresist material or support (11 ) includes a plurality of undercuts (UC).

12. Method according to any one of the previous claims, wherein the at least one undercut non-photoresist material or support (11 ) includes a first undercut (UC1 ) and a second undercut (UC2), wherein the first undercut (UC1) is located opposite the second undercut (UC2).

13. Method according to any one of the previous claims, wherein providing at least one undercut non-photoresist material or support (11) between the material surface (5) and a photoresist material or layer (15) provided on the undercut non-photoresist material or support (11 ) comprises: providing or depositing a non-photoresist material (17) on the at least one material surface (5) to which the at least one electrode (9) is to be attached; providing or depositing the photoresist material or layer (15) on the at least one nonphotoresist material (17); removing at least a portion of the photoresist material or layer (15) and at least a portion of the non-photoresist material (17) underlying the photoresist material or layer (15) to expose at least one or a plurality of lateral walls (LW1 ) or lateral surfaces (SW1 ) of the non-photoresist material (17) underlying the photoresist material or layer (15); and removing at least a portion of the exposed at least one lateral wall or walls (LW1 ) or lateral surface or surfaces (SW1 ) to form at least one or a plurality of undercuts (UC) and form the at least one undercut non-photoresist material or support (11 ).

14. Method according to the previous claim, wherein a portion of the photoresist material or layer (15) on the non-photoresist material (17) remains attached to the non-photoresist material (17) after the step of the removal of the portion of the non-photoresist material (17) underneath the photoresist material or layer (15) that exposes the at least one lateral wall (LW1 ) or lateral surface (SW1 ) of the non-photoresist material 17.

15. Method according to the previous claim, wherein at least one pattern or mask (PS) formed by deep ultraviolet photolithography of the photoresist material or layer (15) defines or comprises the portion of the photoresist material or layer (15) located on the non-photoresist material (17) that remains attached to the non-photoresist material (17) following the removal of the portion of the non-photoresist material (17) underneath the photoresist material or layer (15).

16. Method according to any one of claims 13 to 15, wherein the non-photoresist material (17) underlying the photoresist material or layer (15) is partially removed to leave a portion of the non-photoresist material (17) on the material surface (5) and form a protective covering on or for the material surface (5) to which at least one electrode is to be attached.

17. Method according to any one of claims 13 to 16, wherein the at least one undercut (UC) or each undercut (UC) defines a protrusion distance (d) permitting the at least one electrode or lift-off material (9) forming the at least one electrode (9) to be deposited without direct lateral contact to a neighboring element.

18. Method according to any one of the previous claims 13 to 17, wherein the photoresist material or layer (15) provided or deposited on the non-photoresist material (17) is exposed to ultra-violet light and developed to remove photoresist material (15) to expose at least one or a plurality of portions of the underlying non-photoresist material (17) and define at least one pattern or mask (PS) on the non-photoresist material (17).

19. Method according to the previous claim, wherein the exposed portions of the underlying non-photoresist material (17) following removal of the photoresist material or layer (15) are etched to expose the at least one lateral wall or walls (LW1 ) or lateral surface or surfaces (SW1) of the non-photoresist material (17).

20. Method according to the previous claim, wherein the exposed at least one lateral wall or walls (LW1 ) or lateral surface or surfaces (SW1 ) of the non-photoresist material (17) is etched to form the at least one undercut (UC).

21. Method according to any one of the previous claims, wherein removing or stripping the photoresist material or layer (15) from the at least one undercut non-photoresist material or support (11 ) removes the at least one electrode or lift-off material (19) from only the at least one undercut non-photoresist material or support (11 ).

22. Method according to any one of the previous claims, wherein the undercut non-photoresist material (17) defines a notch (NO) under the photoresist material or layer (15), or an overhanging portion (NO) of the photoresist material or layer (15) over-hanging the material surface (5).

23. Method according to any one of the previous claims, wherein a thickness (H1) of the undercut non-photoresist material or support (11) or of the non-photoresist material (17) is at least X times a thickness (t) of the at least one electrode or lift-off material (19), where X has a value between 1 .5 and 2.5.

24. Method according to any one of the previous claims, wherein the at least one material surface (5) is defined by a non-organic material surface, or a non-organic material surface of a device made of or consisting of non-organic material.

25. Method according to any one of the previous claims, wherein the electrode fabrication method is an optical device electrode fabrication method, or optical waveguide device electrode fabrication method.

26. Method according to the previous claim, wherein the optical device includes a ridge waveguide (7), or the optical device is an optical modulator.

27. Method according to the previous claim 25 or 26, wherein the optical device includes or is an electro-optic modulator.

28. Method according to any one of the previous claims 25 to 27, wherein the optical device is a lithium niobate modulator, or the ridge waveguide (7) includes lithium niobate.

29. Method according to any one of the previous claims 25 to 28, wherein the optical device includes the at least one material surface (5), or the at least one undercut non-photoresist material or support (11 ) is provided on at least one material surface (5) of the optical device.

30. Method according to any one of the previous claims 25 to 29, wherein the ridge waveguide (7) includes the at least one material surface (5), or the at least one undercut non-photoresist material or support (11 ) is provided on at least one material surface (5) of the ridge waveguide (7) to enclose lateral side walls and a lateral wall interconnecting ceiling of the ridge waveguide (7).

31. Method according to any one of the previous claims, wherein the at least one undercut non-photoresist material or support (11) is provided on at least one material surface (5) of a portion of an optical device, and the at least one electrode or lift-off material (19) provided or deposited on the at least one material surface (5) forms at least a portion of at least one or a plurality of electrodes (9) of the optical device.

32. Method according to the previous claim, wherein the optical device is or comprises an electro-optic modulator or a lithium niobate modulator.

33. Method according to the previous claim 31 or 32, wherein the at least one undercut nonphotoresist material or support (11) is provided on a modulation region of an electro-optic modulator or a lithium niobate modulator.

34. Method according to any one of the previous claims, wherein a thickness t of the electrode or lift-off material (19) provided or deposited is in the range of 100nm < t <5pm.