JP2024000520A - Method for forming thin films for metasurface devices - Google Patents

Abstract

The present invention provides a metasurface element with antireflection properties and/or protection properties.
A thin film is formed on the surface of a meta-atom of a meta-surface element and has anti-reflection properties and/or properties to protect the meta-atom. The thin film is made of magnesium fluoride and is a single layer film, and when the sum of the numbers of magnesium atoms and fluorine atoms in the thin film made of magnesium fluoride is 100 at%, the content of the magnesium atoms is 30 to 30. The content of the fluorine atoms is 63 to 70 at%.
[Selection diagram] Figure 5A

Description

The present invention relates to a thin film such as an anti-reflection film or a protective film formed on the surface of a metasurface or metamaterial (hereinafter referred to collectively as a metasurface) element, and a method for forming the film. It is related to.

Devices using optical lenses are used in a wide range of fields, and their use is expanding in mobile terminals and automobiles, where miniaturization and power saving are strongly desired. Examples of devices include smartphone cameras and distance measuring sensors such as lidar. A specific example of an optical lens used in a device is an aspherical lens that uses an optical thin film with a multilayer structure on the lens surface.

Aspherical lenses using optical thin films are lenses that take advantage of light refraction and interference effects, and improvements in design and production technology have enabled them to meet the strictest demands of users. However, in order to obtain the desired characteristics, it is necessary to combine multiple lenses. Furthermore, since the obtained signal also contains various noises, it is necessary to analyze it using an integrated circuit. For these reasons, demands for smaller size, lighter weight, and lower power consumption are becoming stricter, and it is becoming more difficult to meet these demands.

Flat lenses using metasurfaces with a negative refractive index are being developed as a new lens to replace aspheric lenses using optical thin films, and have been attracting attention in recent years. On the surface of a flat lens, a large number of structures called meta-atoms with widths and heights of several hundred nanometers are formed at intervals of several tens of nanometers, and light irradiated onto the surface of the meta-atoms is caused to resonate between the meta-atoms, causing a resonance delay. , they succeeded in imparting a negative refractive index by delaying propagation using a meta-atom waveguide. Such microstructures are formed on the lens substrate using techniques such as film deposition, lithography, etching, and nanoimprinting.

With flat lenses using metasurfaces, it is possible to give a single lens the same functionality as a conventional aspherical lens that uses multiple optical thin films, and it is also possible to provide polarization functionality at the same time. It is. Therefore, it becomes possible to reduce the size, weight, and power consumption.

Examples of meta-atom materials used for such metasurfaces include Si, TiO ₂ , GaN, ITO, SiN, SiO ₂ , LiNbO ₃ , and polymers (see, for example, Non-Patent Documents 1 and 2). .

Shane Colburn et al., Optical Material Express, Vol.8, No.8, 2330 (2018) Leshu Liu et al., Nanomaterials Vol.12, 3849 (2022)

However, since metasurfaces use a fine three-dimensional structure (three-dimensional structure), the surface area is much larger than that of an aspherical lens, and the structure is also finer, so their strength is weak and durability remains an issue. Furthermore, since it is necessary to introduce light into the meta-atom, an anti-reflection function is also required.

The problem to be solved by the present invention is to provide metasurface elements with antireflection and/or protective properties.

The present invention solves the above problems by forming a thin film having anti-reflection properties and/or properties that protect the meta-atoms on the surface of the meta-atoms of a meta-surface element using an atomic layer deposition method.

According to the present invention, antireflection properties and/or protection properties can be imparted to a metasurface element.

1 is a graph showing the composition and density of a magnesium fluoride thin film formed by an ALD device and a PVD (vacuum deposition) device according to Example 1 of the present invention. 1 is a graph showing the optical properties of the magnesium fluoride thin film of Example 1. 3 is a graph showing the protective film characteristics of the magnesium fluoride thin film of Example 2 (etching with 0.5% hydrofluoric acid). 3 is a graph showing the protective film characteristics of the magnesium fluoride thin film of Example 2 (etching with 49% concentration hydrofluoric acid). 3 is a SEM photograph showing the coating characteristics of the magnesium fluoride thin film of Example 3. 5A is an SEM photograph showing an enlarged view of the VB section of FIG. 5A. 5A is an SEM photograph showing an enlarged view of the VC section in FIG. 5A. 5A is an SEM photograph showing an enlarged view of the VD section in FIG. 5A. 5A is an SEM photograph showing an enlarged view of the VE section of FIG. 5A.

As a method for forming a thin film such as an anti-reflection film or a protective film on the meta-atom surface of a metasurface element, the present inventors focused on atomic layer deposition (hereinafter also referred to as ALD) and developed ALD. We thought that it would be possible to provide an anti-reflection film or a protective film for the surface of Meta-Atom by using this method, and we conducted extensive research on the optical properties, chemical properties, and coating properties (covering ability) of a three-dimensional structure of the magnesium fluoride thin film formed by ALD. As a result, the present invention was completed. The confirmation results of optical properties including antireflection properties are described in Example 1, the confirmation results of chemical properties including protective properties are described in Example 2, and the confirmation results of coating properties that can exhibit antireflection properties or protective properties are described later in Example 3. do.

The thin film according to the present invention is a thin film formed by ALD and has antireflection properties and/or protective properties for protecting meta-atoms. It is sufficient that the film has at least one of the antireflection properties that can be applied to an antireflection film and the protective properties that can be applied to a protective film, and it is more preferable that it has both of the properties.

The thin film according to the present invention is preferably made of magnesium fluoride (MgF ₂ ), although it is not particularly limited. Further, when the thin film according to the present invention is a thin film made of magnesium fluoride, it is more preferable that the antireflection film is formed of a single layer film, although there is no particular limitation. Furthermore, when the thin film according to the present invention is a thin film made of magnesium fluoride, although not particularly limited, when the sum of the numbers of magnesium atoms and fluorine atoms in the magnesium fluoride thin film is 100 at.%, It is more preferable that the magnesium atom content is in the range of 30 to 37 atom % and the fluorine atom content is in the range of 63 to 70 atom %. In addition, in this specification, "atomic %" refers to atomic percentage, and specifically, the number of magnesium atoms or the number of fluorine atoms when the sum of the number of magnesium atoms and the number of fluorine atoms is 100 atomic %. represent.

When the thin film according to the present invention is a thin film made of magnesium fluoride, the refractive index (n) at a wavelength of 550 nm is 1.39 ± 0.03, and the extinction coefficient (k) is 5 × 10 ⁻ More preferably, it is ⁴ or less. This makes it possible to maintain film properties such that the total value of transmission and reflection (R+T) is both 99.5% or more in the wavelength range of 300 nm to 1200 nm.

When the thin film according to the present invention is a thin film made of magnesium fluoride, although there is no particular limitation, a three-dimensional structure with an aspect ratio of 0.5 to 100 is used as the metaatom of the metasurface element, and the surface of the metaatom is formed by the above-mentioned ALD. When the magnesium fluoride thin film is formed, it is more preferable that the uniformity of the film thickness on the outermost surface, side surface, and bottom surface of the three-dimensional structure is ±20% or less. Note that in this specification, the uniformity of film thickness is defined as (maximum film thickness Max−minimum film thickness Min)/(maximum film thickness Max+minimum film thickness Min).

The wavelengths to which the metasurface element according to the present invention is applied are not particularly limited, but include 200 to 300 nm, 170 to 300 nm (deep ultraviolet region), 193 nm (ArF rays of exposure equipment for semiconductor devices, etc.), and 248 nm (for semiconductor devices, etc.). (KrF rays of an exposure apparatus), 380 to 770 nm (visible region), or 770 nm or more (infrared region).

The thin film according to the present invention is not particularly limited, but preferably has any one of chemical resistance, water vapor barrier resistance, and hydrofluoric acid resistance.

By keeping the composition, optical properties, and coverage (uniformity of film thickness) of magnesium fluoride within the above ranges, it is possible to improve the properties of metasurface elements without losing the optical properties and chemical resistance of magnesium fluoride. can be improved. Specifically, a metasurface element on which the thin film according to the present invention is formed can achieve higher transmittance than a metasurface element without the thin film according to the present invention. Therefore, when applied to smartphone camera lenses, resolution can be increased even in low illuminance, and when applied to distance measurement sensors, it is possible to reduce the power of the light source.

The antireflection film or protective film according to the present invention is not limited to magnesium fluoride, but may be made of calcium fluoride, lithium fluoride, aluminum fluoride, lanthanum fluoride, yttrium fluoride, hafnium fluoride, zirconium fluoride, or fluoride. A wide variety of fluorides can be used, such as gadolinium oxide. Further, oxides can also be used if necessary.

When using a thin film made of fluoride as the antireflection film according to the present invention, the thin film made of magnesium fluoride, calcium fluoride, lithium fluoride, or aluminum fluoride has a low refractive index, It can be composed of a single-layer thin film made of a compound. On the other hand, thin films made of lanthanum fluoride, yttrium fluoride, hafnium fluoride, zirconium fluoride, and gadolinium fluoride have high refractive indexes, so thin films made of any one of these fluorides and other low It can be composed of a multilayer film combining a thin film with a different refractive index.

Meta atoms of the metasurface element according to the present invention are not particularly limited, but include metals, silicon, titanium oxide, gallium nitride, indium tin oxide, silicon nitride, lithium niobate, lithium tantalate, hafnium oxide, zircon oxide, and niobium oxide. , tantalum oxide, a polymer, or a mixture thereof. Although not particularly limited, these meta-atoms are more preferably formed by any one of atomic layer epitaxy, chemical vapor deposition, physical vapor deposition, and nanoimprinting.

As described above, by forming a magnesium fluoride thin film using atomic layer deposition (ALD), an antireflection film or a protective film can be formed on the surface of the meta-atom. Below, the results of confirmation of the optical properties, chemical properties, and coating properties (covering ability) to a three-dimensional structure of the magnesium fluoride thin film formed by ALD will be explained. The confirmation results of optical properties including antireflection properties are shown in Example 1, the confirmation results of chemical properties including protective properties are shown in Example 2, and the confirmation results of coating properties that can exhibit antireflection properties or protective properties are shown in Example 3. explain.

《Example 1》
As Example 1 of the antireflection film according to the present invention, the contents of magnesium and fluorine that can improve optical properties were confirmed, and will be described with reference to FIGS. 1 and 2. FIG. 1 is a graph showing the composition and density of a magnesium fluoride thin film formed using an ALD apparatus and a PVD (vacuum deposition) apparatus according to Example 1 of the present invention, and FIG. It is a graph showing optical characteristics.

In discovering the present invention, the present inventors formed a magnesium fluoride (MgF ₂ ) thin film using an ALD apparatus (Picosun R-200), and The optical properties were confirmed. For comparison, a magnesium fluoride thin film formed by PVD (vacuum deposition) was also evaluated. The composition evaluation shown in FIG. 1 was determined by Rutherford back scattering (Rutherford method scattering analysis, RBS), and the film thickness was measured using a spectroscopic ellipsometer.

As shown in the _MgF2 composition/density evaluation results in FIG. 1, the composition of the magnesium fluoride thin film formed by ALD according to Example 1 of the present invention has a magnesium atom content of 31.8 at % and a fluorine atom content of 31.8 at %. was 68.2%. Furthermore, since the amount of carbon and oxygen contained in the precursor and process gas used in the ALD process was below the detection limit (4 atomic percent), it is presumed that almost no carbon or oxygen is contained within the thin film. be done. Further, the density of the magnesium fluoride thin film formed by ALD was 8.82×10 ²² atoms/cm ³ , 3.03 g/cm ³ , and the film thickness of the magnesium fluoride thin film formed by ALD was 30.9 nm. On the other hand, the composition of the magnesium fluoride thin film formed by PVD (vacuum deposition) had a magnesium atom content of 33 atom % and a fluorine atom content of 67%. Furthermore, the density of the magnesium fluoride thin film formed by PVD (vacuum vapor deposition) is 8.98×10 ²² atoms/cm ³ , 3.10 g/cm ³ , and the film thickness of the magnesium fluoride thin film formed by ALD is 23.3 nm. Ta.

As shown in FIG. 1, the composition of the magnesium fluoride thin film is such that the content of magnesium atoms is in the range of 31.8 to 33% and the content of fluorine is in the range of 67 to 68.2%. Since the lower limit of detection for RBS evaluation is 4%, there is a measurement error of the same degree (±3 to 4%), and the theoretical composition of magnesium fluoride (MgF ₂ ) is based on the content of magnesium atoms. Since the magnesium atom content is 33.3% and the fluorine atom content is 66.7%, the appropriate composition ratio is a magnesium atom content of 30 to 37 atom% and a fluorine atom content of 63 to 70%. It is considered to be within the range.

FIG. 2 shows the optical properties of the magnesium fluoride thin film produced by ALD in Example 1. For the purpose of evaluating optical properties, a thin film was formed on the surface of a flat quartz substrate, and reflection (R), transmission (T), refractive index (n), and extinction coefficient (k) were evaluated. A flat quartz substrate with a total transmission and reflection value (R+T) of 99.7% or more in the wavelength range of 300 nm to 1200 nm was used as the substrate (see Bare quartz in FIG. 2). On this quartz substrate, a magnesium fluoride thin film formed by the ALD described above and a magnesium fluoride thin film formed by PVD (vacuum vapor deposition) for comparison were formed, and the total value of reflection and transmission R+T was measured. As a result, as shown in Fig. 2, in the wavelength range of 300 nm to 1200 nm, both the magnesium fluoride thin film formed by ALD and the magnesium fluoride thin film formed by PVD (vacuum evaporation) were 99.5% or more. The membrane properties were shown.

Magnesium fluoride thin films formed by PVD are widely used as optical thin films. Therefore, since the magnesium fluoride thin film produced by ALD in Example 1 exhibits optical properties equivalent to the magnesium fluoride thin film produced by PVD, which has been widely put into practical use, the magnesium fluoride thin film produced by ALD is also practical as an optical thin film. Confirmed to work. Here, physical quantities indicating optical characteristics include a refractive index (n) and an extinction coefficient (k). The refractive index (n) at a wavelength of 550 nm of a magnesium fluoride thin film produced by ALD is 1.389, and the extinction coefficient (k) is 9.0 × 10 ^-5 . It serves as an indicator. In addition, when forming a film by ALD, the precursor, which is the raw material for ALD film formation, contains carbon, so the process conditions may be lowered to a lower temperature, and depending on the type of precursor, the extinction coefficient (k) will generally increase as the process progresses. Easy to get to. If the extinction coefficient (k) increases, absorption increases and the performance as an optical thin film deteriorates, so it is necessary to suppress the extinction coefficient (k) to below a certain standard. Regardless of the film-forming method, the inventor believes that the numerical value of the extinction coefficient (k) at a wavelength of 550 nm for an optical thin film is preferably 5 × 10 ^-4 or less, and regardless of the film-forming method, It is preferable that the raw materials and process conditions are also optimized to be below these values. In this example, the extinction coefficients (k) of the magnesium fluoride films formed by PVD and ALD at a wavelength of 550 nm are 4.0 × 10 ^-5 and 9.0 × 10 ^-5 , respectively, which are approximately 5. ×10 ⁻⁴ or less is satisfied.

In Example 1, magnesium fluoride was used as the film type, but when the antireflection film according to the present invention is used in an optical element using a metamaterial, the specific film type is not particularly limited. For example, in addition to magnesium fluoride, calcium fluoride (CaF ₂ ), lithium fluoride (LiF), aluminum fluoride (AlF ₃ ), lanthanum fluoride (LaF ₃ ), yttrium fluoride (YF ₃ ), fluoride Other fluorides and oxides used in existing optical thin films, such as hafnium (HfF ₄ ), zirconium fluoride (ZrF ₄ ), and gadolinium fluoride (GdF ₃ ), can also be used.

In addition, in Example 1, a quartz substrate was used as the substrate, but as long as the substrate or base material on which the antireflection film of the present invention is formed is an optical element using a metamaterial, the specific material and type thereof may be is not particularly limited. For example, in addition to being formed on a quartz substrate, the antireflection film according to the present invention can be widely used in camera lenses, ranging sensors, silicon photonics, wafer level optics, and the like.

《Example 2》
As Example 2 of the protective film according to the present invention, the contents of magnesium and fluorine that can improve the chemical properties were confirmed, and will be described with reference to FIGS. 3 and 4. FIG. 3 is a graph showing the etching evaluation using 0.5% hydrofluoric acid of a magnesium fluoride thin film formed by the ALD apparatus according to Example 2 of the present invention, and FIG. It is a graph showing the etching evaluation of a magnesium fluoride thin film formed by the apparatus using hydrofluoric acid at a concentration of 49%. Note that, for comparison with Example 2, the left diagram of FIG. 3 also shows the etching evaluation of a silicon oxide film (SiO ₂ , thermal oxide film) with a thickness of 100 nm using hydrofluoric acid at a concentration of 0.5%.

In Example 2, a magnesium fluoride (MgF ₂ ) thin film was formed on a silicon substrate using an ALD device (Picosun R-200), and the chemical properties (chemical resistance , hydrofluoric acid resistance). As shown in the MgF ₂ composition/density evaluation results in FIG. 1, the composition of the magnesium fluoride thin film formed by ALD according to Example 2 of the present invention has a magnesium atom content of 31.8 at % and a fluorine atom content of 31.8 at %. 68.2% was used. The magnesium fluoride thin film formed on the silicon substrate was immersed in 0.5% hydrofluoric acid and 49% hydrofluoric acid, and changes in film thickness were measured at predetermined intervals.

As shown in the left diagram of FIG. 3, when a silicon oxide film having a thickness of 100 nm was immersed in 0.5% hydrofluoric acid, the film was etched to a thickness of about 60 nm in 15 minutes. On the other hand, as shown in the right diagram of FIG. 3, almost no decrease in film thickness was observed even when the magnesium fluoride thin film produced by ALD was immersed in 0.5% hydrofluoric acid for 60 minutes. From this, it is considered that the magnesium fluoride thin film has superior hydrofluoric acid barrier properties compared to the silicon oxide film. In addition, since a 0.5% concentration hydrofluoric acid solution is 99.5% water, and almost no decrease in film thickness was observed even when immersed in the solution, magnesium fluoride thin film was immersed in water. At the same time, it was shown that the water is not dissolved, and it is expected that it will function as a high water vapor barrier.

Figure 4 shows the change in film thickness when a magnesium fluoride thin film produced by ALD is immersed in 49% hydrofluoric acid. Almost no decrease was observed. From this, the magnesium fluoride thin film produced by ALD exhibits excellent chemical resistance to hydrofluoric acid, and thus functions effectively as a protective film. Since hydrofluoric acid is a chemical with high etching power as an acid, it is expected to be resistant to other chemicals as well.

In Example 2, a silicon substrate was used as the substrate, but the specific material and type of the substrate or base material forming the protective film according to the present invention may be changed as long as it is an optical element using a metamaterial. Not particularly limited. Moreover, although hydrofluoric acid was used as a chemical, the resistance provided by the protective film of the present invention is not particularly limited to the specific chemical when used in an optical element using a metamaterial. For example, the protective film according to the present invention can be widely used in addition to silicon substrates, such as camera lenses, distance measurement sensors, silicon photonics, and wafer level optics. It is also possible to impart resistance to chemicals other than hydrofluoric acid, water vapor, etc.

《Example 3》
As Example 3 of the antireflection film or protective film according to the present invention, it was confirmed that the magnesium fluoride thin film produced by ALD shows a high coverage rate for the three-dimensional structure of the metasurface, so please refer to FIGS. 5A to 5E. and will be explained below. 5A is an SEM photograph showing the coating characteristics of the magnesium fluoride thin film of Example 3, and FIGS. 5B to 5E are SEM photographs showing enlarged portions VB to VE of FIG. 5A, respectively.

Although atomic layer deposition is known to provide high coverage for three-dimensional structures, there are limited examples of ALD for magnesium fluoride thin films. Therefore, as shown in FIG. 5A, a silicon substrate with an uneven structure (aspect ratio of 1:1) with an opening width of 30 μm and a depth of 30 μm is prepared, and an ALD device (R- 200) to form a magnesium fluoride thin film. FIG. 5 shows an SEM photograph of the coating state of the magnesium fluoride thin film using a scanning electron microscope.

From this SEM photograph, the film thickness of the magnesium fluoride thin film is 181.9 nm on the outermost surface (the top surface of the convex part between the concave parts) shown in FIG. 5B, and on the side surface of the concave part shown in FIG. 5C. The thickness was 151.9 nm, 157.5 nm at the bottom of the recess shown in FIG. 5E, and 130.2 nm at the thinnest region at both ends of the bottom shown in FIG. 5D. From this, if the uniformity of film thickness is defined as (maximum film thickness Max - minimum film thickness Min)/(maximum film thickness Max + minimum film thickness Min), then in Example 3 shown in FIG. = 181.9 nm, and the minimum film thickness Min = 130.2 nm, so the uniformity of the film thickness in Example 3 is Max-Min/(Max+Min) = (181.9-130.2)/(181.9+130 .2)=51.7/312.1=±16.5%.

When using physical vapor deposition (PVD), chemical vapor deposition (CVD), or ion beam sputtering (IBS), it is difficult to uniformly coat such a three-dimensional structure with a magnesium fluoride thin film. Have difficulty. Therefore, a magnesium fluoride thin film produced by ALD can exhibit excellent coating properties for three-dimensional structures such as metasurfaces.

Although a silicon substrate was used as the substrate in Example 3, the substrate or base material on which the antireflection film or protective film of the present invention is formed can be made of any specific material as long as it is an optical element using a metamaterial. The type is not particularly limited. For example, the antireflection film or protective film uniformly formed according to the present invention can be used in a wide range of applications other than silicon substrates, such as camera lenses, distance measurement sensors, silicon photonics, and wafer level optics.

The present invention uses an atomic layer growth method to form a single layer or multilayer film on the metaatom surface of a metasurface element, which has antireflection properties and hydrofluoric acid resistance and contains magnesium fluoride. When a three-dimensional structure with an aspect ratio of 0.5 to 100 is used as the meta-atom, when a thin film made of magnesium fluoride is formed on the surface of the meta-atom, each of the outermost surface, side surface and bottom surface of the three-dimensional structure The above problem is solved by forming a thin film whose thickness is uniform within ±20% .

Claims (12)

A thin film formed on the surface of a meta-atom of a meta-surface element and having antireflection properties and/or properties to protect the meta-atom. The thin film according to claim 1, wherein the thin film is made of magnesium fluoride and is a single layer film. When the total number of magnesium atoms and fluorine atoms in the thin film made of magnesium fluoride is 100 at%, the content of magnesium atoms is 30 to 37 at%, and the content of fluorine atoms is 63 to 70 at%. % of the thin film according to claim 2. 3. The thin film according to claim 2, wherein the thin film made of magnesium fluoride has a refractive index n of 1.39±0.03 at a wavelength of 550 nm and an extinction coefficient k of k=5×10 ⁻⁴ or less. When a three-dimensional structure with an aspect ratio of 0.5 to 100 is used as the meta-atom of the meta-surface element, when a thin film made of magnesium fluoride is formed on the surface of the meta-atom, the outermost surface of the three-dimensional structure, 3. The thin film according to claim 2, wherein the film thickness uniformity on each of the side and bottom surfaces is ±20% or less. The thin film according to claim 1, wherein the thin film includes a fluoride or oxide thin film. The thin film contains any one of magnesium fluoride, calcium fluoride, lithium fluoride, aluminum fluoride, lanthanum fluoride, yttrium fluoride, hafnium fluoride, zirconium fluoride, or gadolinium fluoride, and may be a single-layer film or a multilayer film. The thin film according to claim 7, which is a film. The meta-atom of the meta-surface element is metal, silicon, titanium oxide, gallium nitride, indium tin oxide, silicon nitride, lithium niobate, lithium tantalate, hafnium oxide, zircon oxide, niobium oxide, tantalum oxide, polymer, or any of these. The thin film according to claim 1, comprising a mixture. The thin film according to claim 1, wherein the wavelength to which the metasurface element is applied is any one of 200 to 300 nm, 170 to 300 nm, 193 nm, 248 nm, 380 to 770 nm, and 770 nm or more. The thin film according to claim 1, wherein the thin film has any one of chemical resistance, water vapor barrier resistance, and hydrofluoric acid resistance. A film forming method for forming the thin film according to any one of claims 1 to 10 using an atomic layer growth method. 12. The film forming method according to claim 11, wherein the meta-atom of the metasurface element is formed by any one of an atomic layer epitaxy method, a chemical vapor deposition method, a physical vapor deposition method, and a nanoimprint method.