Forming 3D structures using grayscale lithography

Abstract

Forming the three-dimensional structure includes applying a photoresist on the layer and exposing the photoresist using a photolithography system. The photolithography system includes a photomask having a pattern thereon, wherein the pattern provides a varying pattern density on a surface of the photomask and has a pitch that is less than the resolution of the photolithography system. The method includes subsequently developing the photoresist so that the photoresist remaining on the layer has a three-dimensional profile defined by the photomask. An isotropic etchant is used to etch the layer so that the three-dimensional profile of the photoresist is transferred to the layer.

Description

Technical Field

[0001] The present disclosure relates to forming three-dimensional structures using grayscale lithography. Background Art

[0002] Semiconductor manufacturing techniques based on processes developed for silicon integrated circuits are generally suitable for planar structures formed in or on bulk silicon materials. However, there is increasing interest in batch manufacturing techniques that can be used to obtain gradient height profiles (i.e., structures with or varying multiple heights) on semiconductor wafers. Summary of the invention

[0003] The present disclosure describes the use of grayscale lithography to form three-dimensional (3D) structures. For example, using the techniques described in the present disclosure, 3D structures can be formed directly in silicon dioxide (SiO2) or other materials on a substrate such as a semiconductor wafer.

[0004] For example, in one aspect, the present disclosure describes a method for forming a three-dimensional structure, the method comprising applying a photoresist on a layer and exposing the photoresist using a lithography system. The lithography system includes a photomask having a pattern thereon, wherein the pattern provides a varying pattern density on the surface of the photomask and has a pitch less than the resolution of the lithography system. The method includes subsequently developing the photoresist so that the photoresist remaining on the layer has a three-dimensional profile defined by the photomask. An isotropic etchant is used to etch the layer so that the three-dimensional profile of the photoresist is transferred to the layer.

[0005] Some embodiments include one or more of the following features. For example, in some cases, the layer is composed of SiO2. In some embodiments, the pattern density of the photomask increases continuously from a first end of the pattern to an opposite second end of the pattern. The pattern density of the pattern on the photomask can be continuously varied, for example, by using openings of different sizes in the light blocking layer. In some cases, the pattern has a constant pitch size, while in other cases, the pattern has a modulated pitch size. In some cases, the pattern on the photomask includes a border region layout.

[0006] In some embodiments, the isotropic etchant includes fluorine. The isotropic etchant can be selected to provide an etch rate of the photoresist that is approximately the same as an etch rate of SiO2.

[0007] In some embodiments, the three-dimensional profile has a wedge shape and / or a continuously increasing thickness.

[0008] In some embodiments, the layer is disposed above the light sensing device. The method may include forming an optical filter layer on the surface of the layer to which the three-dimensional profile is transferred, or forming a stack of optical filter layers on the surface of the layer to which the three-dimensional profile is transferred. In some cases, the layer to which the three-dimensional profile is transferred is used as a cavity of a Farby-Perot interferometer.

[0009] Thus, the techniques described herein can be used to fabricate three-dimensional (3D) features that can be integrated into various types of devices. For example, the present disclosure describes a device comprising a support and a SiO2 layer disposed on the support, wherein the SiO2 layer has a three-dimensional profile having a continuously increasing thickness.

[0010] The present disclosure also describes a light sensing device comprising a photodiode having a photosensitive region and a back-end 3D (e.g., wedge-shaped) oxide layer (e.g., SiO2) disposed above the photosensitive region. In some embodiments, the light sensing device comprises an optical filter layer or a stack of optical filter layers on a surface of the oxide layer.

[0011] The present disclosure also describes a Fabry-Perot interferometer, which includes one or more photosensitive elements and a wedge-shaped cavity composed of an optical filter material (e.g., SiO 2 ). A first reflector is disposed between a first surface of the optical filter material and the photosensitive element, and a second reflector is disposed above a second surface of the optical filter material, wherein the second surface is on a side of the optical filter material opposite to the first surface.

[0012] Some embodiments provide one or more of the following advantages. For example, by using 3D structuring, the back-end SiO2 optical properties of the photodiode device can be improved. In some cases, the optical responsivity of the device can be improved by using different SiO2 thicknesses, thereby averaging out interference effects (i.e., fluctuations in responsivity). Grayscale lithography and subsequent etching can also be used to fabricate linear variable filters (LVFs). This approach helps reduce complexity and overall manufacturing costs.

[0013] Other aspects, features, and advantages will become apparent from the following detailed description, drawings, and claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 An example of a lithography system is shown.

[0015] Figure 2A A method of achieving grayscale suitable for low mask coverage is shown.

[0016] Figure 2BA method of achieving grayscale suitable for high mask coverage is shown.

[0017] FIG. 3A shows an example of a photomask having different opening area ratios that can be used for grayscale lithography.

[0018] 3B and 3C illustrate portions of the photomask of FIG. 3A.

[0019] Figure 4 An example of a 3D structure formed in a photoresist layer using the photomask of FIG. 3A is shown.

[0020] Figure 5 An example of a light sensing device including a wedge-shaped oxide layer is shown.

[0021] Figure 6 Another example of a light sensing device including a wedge-shaped oxide layer is shown.

[0022] Figure 7 An example of a Fabry-Perot interferometer including a wedge-shaped cavity layer is shown. DETAILED DESCRIPTION

[0023] like Figure 1 As shown, semiconductor lithography system 10 implements a process in which a complex circuit or other pattern 12 on a photomask 14, such as made of a large glass plate, is reduced using an ultra-high performance projection lens 16 and exposed on a silicon or other substrate called a wafer 18. Pattern 12 may be formed using, for example, chrome or other metals. Semiconductor lithography system 10 is operable to align mask 14 and wafer 18, which may be supported on a movable stage 15.

[0024] Silicon wafer 18 can be prepared for photolithography, for example, by coating a silicon nitride layer thereon followed by a silicon dioxide layer, and then applying a photoresist layer. For example, ultraviolet (UV) light 21 from an illuminator 20 is projected through a mask 14, which contains a pattern 12 to be transferred and formed on wafer 18. In some embodiments, the illumination setting can be consistent with the illumination setting for an i-line stepper (i.e., λ=365nm), although other settings may be suitable for some embodiments. Light 22 passing through mask 14 is projected onto the wafer coated with photoresist. In this way, portions of the photoresist are exposed. The exposed photoresist becomes soluble and is subsequently developed away, leaving a reduced image of pattern 12 on wafer 18. Additional circuit elements or other features can be formed on the wafer by using additional photomasks.

[0025] Grayscale lithography can be used to create gradient height structures, and in some cases involves a level lithography process that helps form a gradient height profile in a photoresist mask layer, which is sometimes referred to as a nested mask (i.e., a mask that results in multiple height levels).

[0026] Typically, the fabrication of a gradient height structure (i.e., a structure having multiple heights) in a material layer (e.g., silicon dioxide) comprises a first step in which the shape of the structure is patterned in a photoresist layer by using grayscale lithography, and a second step in which the photoresist layer is used as a nested mask for etching. In the latter step, the structure is transferred into the material layer to a specific depth corresponding to the height of the desired final structure.

[0027] Diffraction is one of the key issues in lithography because it limits the resolution capability of the projection system. Various efforts have been attempted to overcome this limitation. Approaches include lowering the wavelength in the further deep UV, lens systems capable of high numerical aperture (NA) illumination, and the introduction of advanced mask technology, such as phase-shift masks. The present manufacturing method explicitly exploits the diffraction effect to produce a modulated intensity pattern or a modulated exposure dose on the surface of a wafer 18 covered with a photoresist. When the features of the pattern 12 and the spacing between them are sufficiently small (i.e., less than the resolution of the lithography system 10), these features will not be printed in the photoresist. However, the effective light intensity can be adjusted based on the pattern geometry.

[0028] Generally, the intensity of light passing through mask 14 depends on the fill area and the pitch. For example, as shown in FIG2A , if mask 14 is designed with square pixels and a fixed pitch for the pixels, then the intensity depends on the coverage percentage of opaque area 30 (e.g., square pixel size at a fixed pitch). Figure 2B An example is shown where a lower amount of transmission through the mask 14 can be achieved by increasing the coverage of the opaque areas 30. In the latter case, the mask 14 has smaller square openings 34 through which the UV light can pass. Due to mask manufacturing capabilities, Figure 2A The structure in is more suitable for areas with higher transmission. Figure 2B The structure in is more suitable for lower transmission.

[0029] Grayscale lithography can use locally modulated transmission to achieve locally modulated exposure doses to form three-dimensional (3D) structures in photoresists. Different exposure doses result in multiple depths of exposed photoresists on the surface, due to the absorption of UV light energy by the photoactive compounds as the light enters the photoresist. Depending on the exposure dose, after the photoresist is developed, the gradient height photoresist structure corresponding to the 3D structure will remain.

[0030] For grayscale lithography, mask 14 has a modulated pattern density (e.g., percentage of light shielding area such as chrome coverage) on the mask surface. Even though the pattern density varies, in some cases the pitch that remains constant is selected to be below the resolution of projection system 10 so that the distance between pixels remains below the resolution. In some cases, both the pixel size and the pitch are modulated.

[0031] FIG. 3A shows a method for forming a 3D (eg, wedge-shaped) feature 50 (eg, a wafer 18) in a photoresist layer. Figure 4 3A ). In the example of FIG. 3A , the leftmost side 42 of the pattern 12 is completely covered (e.g., covered with chrome) so that a first portion of the photoresist layer is not exposed (i.e., 0% transmission). As one moves to the right of the pattern 12, the pattern provides slowly varying coverage (e.g., by using openings of varying sizes), with the rightmost side 44 of the pattern having little or no coverage to allow full exposure (100% transmission) of a portion of a second portion of the photoresist layer. In the illustrated example, the middle portion 46 of the pattern 12 uses a border region layout to achieve a smooth transition of grayscale by continuously changing the size of the openings as one moves from left to right in FIG. 3A (at a pitch of 400 nm), e.g., from about 235 nm (see FIG. 3B ) to about 340 nanometers (see FIG. 3C ). Because the geometry is below the resolution limit of the lithography system 10, the structures of the border region layout are not printed into the photoresist layer.

[0032] In the illustrated example, the spacing of the different gray levels is constant at approximately 400 nm, although other values ​​may be appropriate for some embodiments. In some cases, structures with dimensions on the order of several μm and a target thickness variation of approximately 0.5 μm can be achieved, resulting in sufficient surface smoothness to allow subsequent deposition of filters or other optical layers. In some cases, even lower thickness variations can be achieved (e.g., for dimensions less than 3 μm). In some embodiments, structures larger than the underlying photosensitive device (e.g., pixels in the case of an imaging sensor) can be used. For larger wedge-shaped structures, each pixel will correspond to a different thickness. In some cases, the intensity of light 22 passing through different portions of the mask 12 slowly transitions from approximately 12% (FIG. 3B) to approximately 55% (FIG. 3C).

[0033] The sub-resolution size of the features in the mask pattern 12 and the slow size variation of the feature size (eg, opening 34 in chrome) result in smooth height transitions in the photoresist layer after exposure and development (see Figure 4). The 3D profile 50 of the developed photoresist layer, which can be performed in a single illumination step, can be transferred to the base layer using a wet or dry etching technique. For example, if the base layer is a SiO2 layer on the semiconductor wafer 18, a fluorine-containing isotropic etchant (e.g., SF6, NF3, CF4O2, or CF4) can be used to transfer the 3D profile 50 of the developed photoresist layer to the SiO2 layer. The SiO2 layer can be a native oxide layer or a separately grown layer. The use of isotropic etching is advantageous for forming a wedge-shaped SiO2 layer with a relatively smooth inclined surface (i.e., having a continuously varying thickness). Preferably, the etchant should be selected so that the etching rate of the photoresist is approximately the same as the etching rate of SiO2 (e.g., a 1:1 ratio) in order to transfer the photoresist profile to the base layer. Other etching rate ratios can be used if the transferred profile / application requires a higher or lower variation than the resist.

[0034] exist Figure 4 In the example of FIG. 3A , it is assumed that the mask is completely transparent around the structure of FIG. 3A . In other embodiments, the mask may be completely opaque around the structure of FIG. 3A in order to block light from the illuminator 20 .

[0035] The following paragraphs describe various examples of devices in which 3D (eg, wedge-shaped) SiO2 layers fabricated according to the foregoing techniques may be incorporated.

[0036] like Figure 5 As shown, the photodiode structure 100 has a tapered rear SiO2 layer 102 on the photosensitive (photoactive) region 101 of the photodiode. For example, after final planarization (e.g., after deposition of the last metal layer 104), the tapered SiO2 layer can be formed on top of the active photodiode region 106. Specifically, the photoresist layer on the SiO2 can be exposed and developed using a mask as described above, and then the SiO2 can be etched so that the wedge-shaped structure in the photoresist is converted to the SiO2 layer. The resulting photodiode structure 100 has a rear SiO2 with a continuously variable thickness. This structure can result in the averaging of ripples caused by light interference (i.e., oscillating reflectivity) because different thicknesses have different phases for the ripples. Therefore, the different thicknesses of the oxide layer 102 can help average out the interference effects, resulting in a smoother photodiode responsivity. Specifically, in some embodiments, the resulting photodiode structure 100 can produce a more consistent device-to-device frequency response than a photodiode with a flat SiO2 layer.

[0037] The relatively smooth upper surface 108 of the tapered SiO2 layer 102 may also facilitate the subsequent deposition of optical quality filter layers. Figure 6As shown, one or more optical filter layers (e.g., color coating layers or dielectric filters) 110 can be formed on the smooth upper surface 108 of the SiO2 layer 102. The above-described combined photolithography and etching can be used to achieve a sufficiently smooth tapered or inclined surface 108 to facilitate the deposition of a dielectric filter layer directly on top of the surface 108.

[0038] The above combined lithography and etching techniques can also be used for single-step fabrication of the cavity of a Fabry-Perot interferometer or etalon to provide filters of different wavelengths. A Fabry-Perot interferometer or etalon can consist of a transparent plate with two partially reflecting surfaces (e.g., partially reflecting mirrors). The transmission spectrum as a function of wavelength exhibits a large transmission peak corresponding to the resonance of the etalon. Figure 7 An example of a structure in which a layer for a Fabry-Perot type bottom filter 120 is deposited on a photodiode array 122 is shown. The optical filter 120 can be optimized for reflectivity in a desired wavelength range and have a specified filter width. A thick layer of filter material (e.g., SiO2) is then deposited on the bottom filter 120, and the combined photolithography and etching techniques described above are used to obtain a wedge-shaped or inclined surface 124. In this case, the photomask can be designed so that a wedge-shaped structure 126 of a specified width remains on the top of the array 122 and serves as a cavity for the Fabry-Perot device. By depositing the last layer 128 of the bottom mirror using the same material as the etalon, the width of the cavity 126 can be optimized for different ranges. Similar results can be achieved using the first layer of the top mirror 130. In addition, the top mirror can be deposited to optimize for the desired reflectivity in a selected range to achieve a specified transmission width. In some cases, another mirror can be deposited on top to block light in a specified wavelength range. For example, this additional mirror can be deposited as part of the same process step as the top Fabry-Perot mirror.

[0039] The combined lithography and etching techniques described above can also be used, for example, to make very small structures (e.g., a few microns in diameter) and designed to act as collimating lenses with a focal length of tens of microns. For example, these structures can be used to improve light collection on small photodiodes to guide light from inactive areas (e.g., metal connections) to active areas, even when the thickness of the back-end oxide is limited (e.g., less than 10um). Examples of such structures include collimating optical elements (e.g., microlenses) or Fresnel lenses for improved light collection. In some cases, anisotropic etching can also be used.

[0040] Although specific embodiments have been described in detail above, various modifications can be made within the spirit of the invention. Accordingly, other implementations are within the scope of the following claims.

Claims

1. A method for forming a three-dimensional structure, the method comprising: applying a photoresist on a layer above a light sensing device, wherein the light sensing device includes a photodiode having a photosensitive region and a back-end tapered oxide layer disposed above the photosensitive region; exposing the photoresist using a photolithography system, wherein the photolithography system includes a photomask having a pattern thereon, the pattern providing a varying pattern density on a surface of the photomask and having a pitch less than a resolution of the photolithography system; subsequently developing the photoresist so that the photoresist remaining on the layer has the three-dimensional contours defined by the photomask; as well as The layer is etched using an isotropic etchant such that the three-dimensional profile of the photoresist is transferred to the layer.

2. The method of claim 1, wherein the layer consists of silicon dioxide. 3 . The method according to claim 1 , wherein the pattern density of the photomask increases continuously from a first end of the pattern to an opposite second end of the pattern.

4. The method according to claim 1 or 2, wherein the pattern density of the pattern on the photomask is continuously varied by using openings of different sizes in a light blocking layer.

5. The method of claim 1 or 2, wherein the pattern has a constant pitch size.

6. The method of claim 1 or 2, wherein the pattern has a modulated pitch size.

7. The method of claim 1 or 2, wherein the isotropic etchant comprises fluorine.

8. The method of claim 1, further comprising forming an optical filter layer on the surface of the layer to which the three-dimensional profile is transferred.

9. The method of claim 1, further comprising forming a stack of optical filter layers on the surface of the layer to which the three-dimensional profile is transferred.

10. The method of claim 1 or 2, the layer to which the three-dimensional profile is transferred being used as a cavity of a Fabry-Perot interferometer.

11. The method of claim 1 or 2, wherein the pattern on the photomask comprises a border region layout.

12. The method of claim 2, wherein the isotropic etchant causes an etch rate of the photoresist to be approximately the same as an etch rate of silicon dioxide.

13. The method of claim 1 or 2, wherein the three-dimensional profile has a wedge shape.

14. The method of claim 1 or 2, wherein the three-dimensional profile has a continuously increasing thickness.

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