WO2024211339A1

WO2024211339A1 - Hard mask for grey-tone (gt) photoresist thickness control

Info

Publication number: WO2024211339A1
Application number: PCT/US2024/022732
Authority: WO
Inventors: Yongan Xu; Jin Xu; Wenhui Wang; Jhenghan YANG; Ludovic Godet
Original assignee: Applied Materials Inc
Current assignee: Applied Materials Inc
Priority date: 2023-04-04
Filing date: 2024-04-03
Publication date: 2024-10-10
Anticipated expiration: 2025-10-04

Abstract

Methods for forming waveguides having structures with different depths. In one example, a method includes disposing a film stack over a substrate, the film stack including a patterned mask, a hardmask layer, a grey-tone resist layer, and a top anti-reflective coating (TARC) layer, forming a thickness profile in the grey-tone resist layer by exposing the substrate to light of a varying intensity via a lithography process, the varying intensity corresponding to the thickness profile to be formed, and developing the grey-tone resist layer to form the thickness profile. The method further includes forming the thickness profile in the hardmask layer via a transfer etch process, and performing an etch process to remove exposed segments of the substrate exposed by the patterned mask to form a waveguide with waveguide structures, each of the waveguide structures having a depth corresponding to the thickness profile.

Description

HARD MASK FOR GREY-TONE (GT) PHOTORESIST THICKNESS CONTROL

BACKGROUND

Field

[0001] Embodiments of the present disclosure generally relate to waveguides for augmented, virtual, mixed reality. More specifically, embodiments described herein provide forming waveguides having structures with different depths.

Description of the Related Art

[0002] Virtual reality is generally considered to be a computer-generated simulated environment in which a user has an apparent physical presence. A virtual reality experience can be generated in 3D and viewed with a headmounted display (HMD), such as glasses or other wearable display devices that have near-eye display panels as lenses to display a virtual reality environment that replaces the actual environment.

[0003] Augmented reality, however, enables an experience in which a user can still see through the display lenses of the glasses or other HMD device, or handheld device, to view the surrounding environment, yet also see images of virtual objects that are generated in the display and appear as part of environment. Augmented reality can include any type of input, such as audio and haptic inputs, as well as virtual images, graphics, and video that enhances or augments the environment that the user experiences. As an emerging technology, there are many challenges and design constraints with augmented reality.

[0004] One such challenge is grey-tone thickness which is vital for waveguide structures depth accuracy and color uniformity. Accordingly, what is needed in the art are improved methods of forming waveguides having structures with different depths. SUMMARY

[0005] In one embodiment, a method is provided. The method includes disposing a film stack over a substrate, the film stack including a patterned mask disposed over the substrate, the patterned mask exposing segments of the substrate, a hardmask layer disposed over the patterned mask, a grey-tone resist layer disposed over the hardmask layer, and a top anti-reflective coating (TARC) layer disposed over the grey-tone resist layer. The method further includes forming a thickness profile in the grey-tone resist layer, the forming the thickness profile including exposing the substrate to light of a varying intensity via a lithography process, the varying intensity corresponding to the thickness profile to be formed, removing the TARC layer, and developing the grey-tone resist layer to form the thickness profile in the grey-tone resist layer. The method further includes forming the thickness profile in the hardmask layer via a transfer etch process, the transfer etch process removing the grey-tone resist layer, and performing an etch process, the etch process removing the hardmask layer and exposed segments of the substrate exposed by the patterned mask, the removing the exposed segments forming a waveguide with waveguide structures, each of the waveguide structures having a depth corresponding to the thickness profile.

[0006] In another embodiment, a method is provided. The method includes disposing a film stack over a substrate, the film stack including a patterned mask disposed over the substrate, the patterned mask exposing segments of the substrate, a hardmask layer disposed over the patterned mask, and a greytone resist layer disposed over the hardmask layer. The method further includes forming a thickness profile in the grey-tone resist layer, the forming the thickness profile including exposing the substrate to light of a varying intensity via a lithography process, the varying intensity corresponding to the thickness profile to be formed, and developing the grey-tone resist layer to form the thickness profile in the grey-tone resist layer. The method further includes forming the thickness profile in the hardmask layer via a transfer etch process, the transfer etch process removing the grey-tone resist layer, and performing an etch process, the etch process removing the hardmask layer and exposed segments of the substrate exposed by the patterned mask, the removing the exposed segments forming a waveguide with waveguide structures, each of the waveguide structures having a depth corresponding to the thickness profile.

[0007] In another embodiment, a method is provided. The method includes disposing a film stack over a substrate, the film stack including a patterned mask disposed over the substrate, the patterned mask exposing segments of the substrate, a hardmask layer disposed over the patterned mask, a grey-tone resist layer disposed over the hardmask layer, and a top anti-reflective coating (TARC) layer disposed over the grey-tone resist layer. The method further includes forming a thickness profile in the grey-tone resist layer, the forming the thickness profile including exposing the substrate to light of a varying intensity via a lithography process, the varying intensity corresponding to the thickness profile to be formed, removing the TARC layer, and developing the grey-tone resist layer to form the thickness profile in the grey-tone resist layer. The method further includes forming the thickness profile in the hardmask layer via a transfer etch process, the transfer etch process removing the grey-tone resist layer, and performing a reactive ion (RIT) etch process, the RIT etch process removing the hardmask layer and exposed segments of the substrate exposed by the patterned mask, the removing the exposed segments forming a waveguide with waveguide structures, each of the waveguide structures having a depth corresponding to the thickness profile.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only exemplary embodiments of the present disclosure and are therefore not to be considered limiting of its scope, and may admit to other equally effective embodiments.

[0009] Figure 1A is a perspective, frontal view of a waveguide according to embodiments described herein. [0010] Figure 1 B is a cross-sectional view of a waveguide according to embodiments described herein.

[0011] Figure 1 C is a cross-sectional view of a waveguide according to embodiments described herein.

[0012] Figure 2 is a flow diagram of a method of fabricating a waveguide according to embodiments described herein.

[0013] Figure 3A-3E are schematic, cross-sectional views of a substrate during a method of fabricating a waveguide, according to embodiments described herein.

[0014] Figure 4 is a graph illustrating swing curves.

[0015] To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation.

DETAILED DESCRIPTION

[0016] Embodiments of the present disclosure relate to waveguides for augmented, virtual, mixed reality. More specifically, embodiments described herein provide forming waveguides having structures with different depths.

[0017] Figure 1A is a perspective, frontal view of a waveguide 100. It is to be understood that the waveguide 100 described herein is an exemplary waveguide and that other waveguides may be used with or modified to accomplish aspects of the present disclosure. The waveguide 100 includes a plurality of waveguide structures 110. The waveguide structures 110 may be disposed over, under, or on a top surface 105 of a substrate 101 , or disposed in the substrate 101 . The waveguide structures 110 are nanostructures have a sub-micron critical dimension, e.g., a width less than 1 micrometer. Regions of the waveguide structures 110 correspond to one or more gratings 104. In one embodiment, which can be combined with other embodiments described herein, the waveguide 100 includes at least a first grating 104a corresponding to an input coupling grating and a third grating 104c corresponding to an output coupling grating. In another embodiment, which can be combined with other embodiments described herein, the waveguide 100 further includes a second grating 104b. The second grating 104b corresponds to a pupil expansion grating or a fold grating.

[0018] Figure 1 B is a cross-sectional view of the waveguide 100. The waveguide 100 has the waveguide structures 110 disposed in the substrate 101. Adjacent waveguide structures 110 have a trench 111 therebetween. Each waveguide structure 110 has a depth 113 from a top surface 105 of the waveguide 100 to a bottom surface 106 of the trench 111. The depth 113 of each waveguide structure 110 varies. The depths of the waveguide structures 110 range from 0 nm to 400 nm. Each of the waveguide structures has two sidewalls 115.

[0019] Figure 1 B illustrates the first grating 104a, the second grating 104b, and the third grating 104c. In the first grating 104a of the waveguide 100, the waveguide structures 110 are binary waveguide structures 120. In the second grating 104b, the waveguide structures 110 are binary waveguide structures 120. For the binary waveguide structures 120, the bottom surface 106 of the adjacent trenches 111 are parallel to the top surface 105 of the waveguide 100. The sidewalls 115 of the binary waveguide structures 120 are oriented normal to a major axis of the waveguide 100. In third grating 104c of the waveguide, the waveguide structures 110 are angled waveguide structures 130. The sidewalls 115 of the angled waveguide structures 130 are slanted at an angle relative to the top surface 105 of the waveguide 100. The angles of the angled waveguide structures 130 range from greater than 0 degrees to less than or equal to 80 degrees. As shown in Figure 1 B, in some embodiments, the waveguide 100 has the binary waveguide structures 120 and the angled waveguide structures 130 at different angles. In some embodiments, the waveguide 100 has all binary waveguide structures 120 or all angled waveguide structures 130 at a single angle. [0020] As shown in Figure 1 B, the waveguide structures 110 are etched into the substrate 101 of the waveguide 100. The substrate 101 may be selected from any suitable material, provided the waveguide 100 can adequately transmit a suitable amount of light of a desired wavelength or wavelength range, such as one or more wavelengths from about 380 nm to about 700 nm. The substrate 101 can include substrate of any suitable material, including but not limited to, amorphous dielectrics, non-amorphous dielectrics, crystalline dielectrics, silicon oxide, polymers, transparent materials, or any combinations thereof. Suitable examples may include, but are not limited to, an oxide, sulfide, phosphide, telluride, or combinations thereof. In one example, the waveguide includes silicon (Si), silicon dioxide (SiC>2), germanium (Ge), silicon germanium (SiGe), indium phosphide (InP), gallium arsenide (GaAs), gallium nitride (GaN), fused silica, quartz, sapphire, and high-index transparent materials (refractive index greater than 2.0) such as glass, or combinations thereof.

[0021] Figure 1 C is a cross-sectional view of the waveguide 100. The waveguide structures 110 are shown formed in a device layer 102. The device layer 102 may be selected from any suitable material, provided the waveguide 100 can adequately transmit a suitable amount of light of a desired wavelength or wavelength range, such as one or more wavelengths from about 380 nm to about 700 nm. The device layer 102 includes, but is not limited to, one or more of silicon oxycarbide (SiOC), titanium dioxide (TiO2), silicon dioxide (SiO2), vanadium (IV) oxide (VOx), aluminum oxide (AI2O3), aluminum-doped zinc oxide (AZO), indium tin oxide (ITO), tin dioxide (SnO2), zinc oxide (ZnO), tantalum pentoxide (Ta2Os), silicon nitride (Si3N4), zirconium dioxide (ZrO2), niobium oxide (Nb20s), cadmium stannate (Cd2SnO4), or silicon carbon-nitride (SiCN) containing materials. Figure 2 is a flow diagram of a method 200 of fabricating a waveguide 100. Figure 3A-3E are schematic, cross-sectional views of a waveguide layer 301 during the method 200 of fabricating a waveguide 100. At operation 201 , as shown in Figure 3A, a film stack 305 is disposed over a waveguide layer 301. In some embodiments as shown in Figure 3A, the waveguide layer 301 is the substrate 101. In other embodiments, the waveguide layer 301 is the device layer 102. When the waveguide layer 301 is the device layer 102, the substrate 101 is disposed under the waveguide layer 301 . The film stack 305 includes a patterned mask 310, a hardmask layer 302, a grey-tone resist layer 303, and a top anti-reflective coating (TARC) layer 304.

[0022] The patterned mask 310 is disposed over the waveguide layer 301 . The patterned mask 310 exposes segments of the waveguide layer 301. The exposed segments of the waveguide layer 301 will be etched. The nonexposed portions of the waveguide layer 301 will not be etched. The patterned mask 310 includes, but is not limited to, chromium (Cr), silicon nitride (SiN), and silicon oxide (SiOx).

[0023] The hardmask layer 302 is disposed over the patterned mask 310. The hardmask layer 302 is an optically active layer in the film stack. Therefore, the hardmask layer 302 is designed with a thickness and a refractive index. The refractive index of the hardmask layer 302 is designed to reduce the reflection between the grey-tone resist layer 303 and the hardmask layer 302. The thickness of the hardmask layer 302 also is designed to reduce the reflection between the grey-tone resist layer 303 and the hardmask layer 302.

[0024] The combined refractive index of the film stack 305 is described in the following equation.

Where Rtotai is the combined refractive index, p-12 is the reflection between the grey-tone resist layer 303 and the hardmask layer 302, P23 is the reflection between the hardmask layer 302 and the waveguide layer 301 , and TD is the internal transmittance of the hardmask layer 302. Both p-12 and p23 are described in the following equations. ni — n₂ l 2 — ni + n₂

Where m is the refractive index of the grey-tone resist layer 303, n2 is the refractive index of the hardmask layer 302, and ns is the refractive index of the waveguide layer 301. The internal transmittance TD is described in the following equation.

T_D = _e ~ik2D₂

Where D2 is the thickness of the hardmask layer 302. Based on the above equations, the combined refractive index of the film stack 305 is a function of both the refractive index of the hardmask layer 302 and the thickness of the hardmask layer 302.

[0025] The combined refractive index of the film stack 305 reduces the reflection of light in the grey-tone resist layer 303. When the grey-tone resist layer 303 is developed into the thickness profile 313 in operation 202, the reduction of the reflection increases the thickness accuracy of the thickness profile 313. The hardmask layer 302 could have a refractive index of 1.8 - 0.178i depending on the film stack 305, where 0.178i is the imaginary part of the refractive index, which is responsible for the absorption of a material. The hardmask layer 302 can have a thickness ranging from less than 100 nm to 400 nm depending on what materials are used.

[0026] The hardmask layer 302 includes an inorganic material. The inorganic material includes, but is not limited to, tin oxides (TiOx), silicon oxynitrides (SiNxOy), or combinations thereof. The hardmask layer 302 of the inorganic material has a more stable etch rate than conventional resists or organic films. The etch rate of resists and organic films may vary with temperature and etch cycle.

[0027] The grey-tone resist layer 303 is disposed over the hardmask layer 302. The grey-tone resist layer 303 is made of a photoresist material. The photoresist material may include, but is not limited to, light-sensitive polymer containing materials.

[0028] The TARC layer 304 is disposed over the grey-tone resist layer 303. The TARC layer 304 may include, but is not limited, to an organic material. The material of the TARC layer 304 allows the TARC layer 304 to be removed easily when the grey-tone resist layer 303 is developed. The TARC layer 304 minimizes the amount of light that would reflect off the grey-tone resist layer 303. In some embodiments, the TARC layer 304 has a thickness d of less than 100 nm depending on which exposing wavelength is used. The thickness d is described in the following equation.

A d —— 4n

Where A is the wavelength of light and n is the refractive index of the TARC layer 304.

[0029] At operation 202, as shown in Figure 3B, a thickness profile 313 is formed in the grey-tone resist layer 303. The thickness profile 313 has a changing thickness. Light of a varying intensity is exposed to the film stack 305 and the waveguide layer 301 via a lithography process. The light has a wavelength ranging from 193 nm to 365 nm and in some embodiments other wavelength ranges are used. The varying intensity of the light corresponds to the changing thickness of the thickness profile 313 to be formed. The greytone resist layer 303 is developed to form the thickness profile 313. The developing of the grey-tone resist layer 303 causes the TARC layer 304 to be evaporated and removed from the grey-tone resist layer 303. The inorganic material of the hardmask layer 302 may be selected to minimize the total reflection of light back into the grey-tone resist layer 303 which contributes to accurate thickness control of the grey-tone resist layer 303.

[0030] At operation 203, as shown in Figure 3C, the thickness profile 313 is formed in the hardmask layer 302. A transfer etch process is used to form the thickness profile 313 in the hardmask layer 302. The transfer etch process removes the grey-tone resist layer 303 and forms the thickness profile 313 in the hardmask layer 302 via the etch selectivity of the grey-tone resist layer 303 to the hardmask layer 302. The etch selectivity of the grey-tone resist layer 303 to the hardmask layer 302 is about 1 :1 to about 10:1. In some embodiments, the transfer etch process may be a sysm3 process.

[0031] At operation 204, waveguide structures 110 are formed. The waveguide structures 110 are formed by an etch process. In some embodiments, the etch process is a reactive ion (RIT) etch process. The hardmask layer 302 is removed by the etch process. The exposed segments of the waveguide layer 301 exposed by the patterned mask 310 are removed by the etch process. The removing of the exposed segments of the waveguide layer 301 forms the waveguide 100 with the waveguide structures 110. Each of the waveguide structures 110 have a depth corresponding to the thickness profile 313. Figure 3D illustrates operation 204 with the waveguide layer 301 being the substrate 101. Figure 3D depicts the waveguide structures 110 formed in the substrate 101 in the first grating 104a, the second grating 104b, and the third grating 104c. The patterned mask 310 is positioned on each of the waveguide structures 110. The pattern mask 310 is removed, forming the waveguide 100 shown in Figure 1 B.

[0032] Figure 3E illustrates operation 204 with the waveguide layer 301 being in the device layer 102 with the substrate 101 being disposed below the waveguide layer 301 . Figure 3E depicts the waveguide structures 110 formed in the device layer 102 in the first grating 104a, the second grating 104b, and the third grating 104c. The patterned mask 310 is positioned on each of the waveguide structures 110. The pattern mask 310 is removed, forming the waveguide 100 shown in Figure 1 C.

[0033] Figure 4 is a graph 400 illustrating swing curves. Swing curve 401 shows the resist thickness verses the intensity of light in a process where only a resist is used to etch the waveguide layer 301 . Swing curve 402 shows the resist thickness verses the intensity of light in a process where a resist and a hardmask are used to etch the waveguide layer 301 as shown in some embodiments. Swing curve 403 shows the resist thickness verses the intensity of light in a process where a resist, a hardmask, and a TARC are used to etch the waveguide layer 301 as shown in some embodiments.

[0034] Ideally, light at a particular intensity should produce a single resist thickness. In swing curve 401 for resist thicknesses less than 200 nm, a single intensity of light may produce two different resist thicknesses. For example, as shown in Figure 4, when the light hitting the resist has an intensity of 0.6 the resulting resist thickness could be either around 50 nm or 5 nm making the thickness profile hard to predict and control. In swing curve 402 for resist thicknesses less than 75 nm, a single intensity of light may produce two different resist thicknesses. In swing curve 403 at all resist thicknesses the intensity of light produces one resist thickness. Figure 4 shows that the hardmask layer 302 under the grey-tone resist layer 303 with the TARC layer 304 above the grey-tone resist layer 303 achieves accurate resist control from a resist thickness of 0 pm and above such as 10 pm and above.

[0035] In summation, the film stack 305 containing the hardmask layer 302 is used to accurately control resist thickness during the forming of the thickness profile 313 in the grey-tone resist layer 303. The hardmask layer 302 and the TARC layer 304 reduce the swing curve causing a single intensity of light to produce a single resist thickness. The hardmask layer 302 and the TARC layer 304 therefore improve etch depth accuracy and color uniformity of the waveguide 100. The hardmask layer 302 also has a more stable etch rate.

[0036] While the foregoing is directed to embodiments of the present disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Claims

What is claimed is:

1 . A method, comprising: disposing a film stack over a waveguide layer, the film stack comprising: a patterned mask disposed over the waveguide layer, the patterned mask exposing segments of the waveguide layer, a hardmask layer disposed over the patterned mask, a grey-tone resist layer disposed over the hardmask layer, and a top anti-reflective coating (TARC) layer disposed over the grey-tone resist layer; forming a thickness profile in the grey-tone resist layer, the forming the thickness profile comprising: exposing the waveguide layer to light of a varying intensity via a lithography process, the varying intensity corresponding to the thickness profile to be formed, removing the TARC layer, and developing the grey-tone resist layer to form the thickness profile in the grey-tone resist layer; forming the thickness profile in the hardmask layer via a transfer etch process, the transfer etch process removing the grey-tone resist layer; and performing an etch process, the etch process removing the hardmask layer and exposed segments of the waveguide layer exposed by the patterned mask, the removing the exposed segments forming a waveguide with waveguide structures, each of the waveguide structures having a depth corresponding to the thickness profile.

2. The method of claim 1 , wherein the etch process is a reactive ion (RIT) etch process.

3. The method of claim 1 , wherein the transfer etch process has an etch selectivity ratio from 1 :1 to 10:1.

4. The method of claim 1 , wherein the waveguide structures forming an input coupling grating, an output coupling grating, and a pupil expansion grating.

5. The method of claim 4, wherein the waveguide layer comprises a device layer disposed on a substrate, and the waveguide structures are formed in the device layer.

6. The method of claim 4, wherein the waveguide layer comprises a substrate and the waveguide structures are formed in the substrate of the waveguide.

7. The method of claim 6, wherein the substrate includes silicon, silicon dioxide, germanium, silicon germanium, indium phosphide, gallium arsenide, gallium nitride, fused silica, quartz, sapphire, or glass.

8. A method, comprising: disposing a film stack over a waveguide layer, the film stack comprising: a patterned mask disposed over the waveguide layer, the patterned mask exposing segments of the waveguide layer, a hardmask layer disposed over the patterned mask, and a grey-tone resist layer disposed over the hardmask layer; forming a thickness profile in the grey-tone resist layer, the forming the thickness profile comprising: exposing the waveguide layer to light of a varying intensity via a lithography process, the varying intensity corresponding to the thickness profile to be formed, and developing the grey-tone resist layer to form the thickness profile in the grey-tone resist layer; forming the thickness profile in the hardmask layer via a transfer etch process, the transfer etch process removing the grey-tone resist layer; and performing a etch process, the etch process removing the hardmask layer and exposed segments of the waveguide layer exposed by the patterned mask, the removing the exposed segments forming a waveguide with waveguide structures, each of the waveguide structures having a depth corresponding to the thickness profile.

9. The method of claim 8, wherein the etch process is a reactive ion (RIT) etch process.

10. The method of claim 8, wherein the transfer etch process has an etch selectivity ratio from 1 :1 to 10:1.

11. The method of claim 8, wherein the waveguide structures forming an input coupling grating, an output coupling grating, and a pupil expansion grating.

12. The method of claim 11 , wherein the waveguide layer comprises a device layer disposed on a substrate, and the waveguide structures are formed in the device layer.

13. The method of claim 11 , wherein the waveguide layer comprises a substrate and the waveguide structures are formed in the substrate of the waveguide.

14. The method of claim 13, wherein the substrate includes silicon, silicon dioxide, germanium, silicon germanium, indium phosphide, gallium arsenide, gallium nitride, fused silica, quartz, sapphire, or glass.

15. A method, comprising: disposing a film stack over a waveguide layer, the film stack comprising: a patterned mask disposed over the waveguide layer, the patterned mask exposing segments of the waveguide layer, a hardmask layer disposed over the patterned mask, a grey-tone resist layer disposed over the hardmask layer, and a top anti-reflective coating (TARC) layer disposed over the grey-tone resist layer; forming a thickness profile in the grey-tone resist layer, the forming the thickness profile comprising: exposing the waveguide layer to light of a varying intensity via a lithography process, the varying intensity corresponding to the thickness profile to be formed, removing the TARC layer, and developing the grey-tone resist layer to form the thickness profile in the grey-tone resist layer; forming the thickness profile in the hardmask layer via a transfer etch process, the transfer etch process removing the grey-tone resist layer; and performing a reactive ion (RIT) etch process, the RIT etch process removing the hardmask layer and exposed segments of the waveguide layer exposed by the patterned mask, the removing the exposed segments forming a waveguide with waveguide structures, each of the waveguide structures having a depth corresponding to the thickness profile.

16. The method of claim 15, wherein the transfer etch process has an etch selectivity ratio from 1 :1 to 10:1.

17. The method of claim 15, wherein the waveguide structures forming an input coupling grating, an output coupling grating, and a pupil expansion grating.

18. The method of claim 17, wherein the waveguide layer comprises a device layer disposed on a substrate, and the waveguide structures are formed in the device layer.

19. The method of claim 17, wherein the waveguide layer comprises a substrate and the waveguide structures are formed in the substrate of the waveguide.

20. The method of claim 19, wherein the substrate includes silicon, silicon dioxide, germanium, silicon germanium, indium phosphide, gallium arsenide, gallium nitride, fused silica, quartz, sapphire, or glass.