Patent Description:
Optical sensors are used in optical sensor devices, such as image sensors, ambient light sensors, proximity sensors, hue sensors, and UV sensors, to convert optical signals into electrical signals, allowing detection of optical signals or image capture. An optical sensor, generally, includes one or more sensor elements and one or more optical filters disposed over the one or more sensor elements.

For example, a color image sensor includes a plurality of color filters disposed in an array, i.e., a color filter array (CFA). The CFA includes different types of color filters having different color passbands, e.g., red, green, and blue (RGB) filters.

Conventionally, absorption filters formed using dyes are used as color filters. Unfortunately, such dye-based color filters have relatively broad color passbands, resulting in less brilliant colors. Alternatively, dichroic filters, i.e., interference filters, formed of stacked dielectric layers may be used as color filters. Such all-dielectric color filters have higher transmission levels and narrower color passbands, resulting in brighter and more brilliant colors. However, the color passbands of all-dielectric color filters undergo relatively large center-wavelength shifts with changes in incidence angle, resulting in undesirable shifts in color.

Furthermore, all-dielectric color filters, typically, include a large number of stacked dielectric layers and are relatively thick. Consequently, all-dielectric color filters are expensive and difficult to manufacture. In particular, all-dielectric color filters are difficult to etch chemically. Lift-off processes are, therefore, preferred for patterning. Examples of lift-off processes for patterning all-dielectric color filters in CFAs are disclosed in <CIT>, in <CIT>, in <CIT>, in <CIT>, and in <CIT>. However, lift-off processes are, generally, limited to a filter spacing of about twice the filter height, which makes it difficult to achieve all-dielectric CFAs suitable for smaller color image sensors.

In addition to transmitting visible light in color passbands, both dye-based and all-dielectric color filters also transmit infrared (IR) light, which contributes to noise. Therefore, a color image sensor, typically, also includes an IR-blocking filter disposed over the CFA. IR-blocking filters are also used in other optical sensor devices operating in the visible spectral range. Conventionally, absorption filters formed of colored glass or dichroic filters formed of stacked dielectric layers are used as IR-blocking filters. Alternatively, induced transmission filters formed of stacked metal and dielectric layers may be used as IR-blocking filters. Examples of metal-dielectric IR-blocking filters are disclosed in <CIT>, and in <CIT>.

To avoid the use of an IR-blocking filter, induced transmission filters formed of stacked metal and dielectric layers may be used as color filters. Metal-dielectric optical filters, such as metal-dielectric color filters, are inherently IR-blocking. Typically, metal-dielectric color filters have relatively narrow color passbands that do not shift significantly in wavelength with changes in incidence angle. Furthermore, metal-dielectric color filters are, generally, much thinner than all-dielectric color filters. Examples of metal-dielectric color filters are disclosed in <CIT>, in <CIT>, in <CIT>, in <CIT>, and in <CIT>.

<CIT>, <CIT>, <CIT>, <CIT>, <CIT> and <CIT> are also useful in understanding the present invention.

Typically, the metal layers in metal-dielectric optical filters, such as metal-dielectric color filters, are silver or aluminum layers, which are environmentally unstable and which deteriorate when exposed to even small amounts of water or sulfur. Chemically etching the silver layers exposes the edges of the silver layers to the environment, allowing deterioration. Therefore, in most instances, metal-dielectric color filters in CFAs are patterned by adjusting the thicknesses of only the dielectric layers to select different color passbands for the metal-dielectric color filters. In other words, different types of metal-dielectric color filters having different color passbands are required to have the same number of silver layers as one another and the same thicknesses of the silver layers as one another. Unfortunately, these requirements severely limit the possible optical designs for the metal-dielectric color filters.

The present invention provides metal-dielectric optical filters that are not subject to these requirements, which are particularly suitable for use in image sensors and other sensor devices, such as ambient light sensors, proximity sensors, hue sensors, and UV sensors.

Accordingly, the present invention relates to a method of fabricating an optical filter defined in claim <NUM>,.

The present invention can provide a sensor device comprising: one or more sensor elements; and one or more optical filters disposed over the one or more sensor elements, wherein each of the one or more optical filters are defined herein. The present invention also discloses a method of fabricating an optical filter, the method comprising: providing a substrate; applying a photoresist layer onto the substrate; patterning the photoresist layer to uncover a filter region of the substrate, whereby an overhang is formed in the patterned photoresist layer surrounding the filter region; depositing a multilayer stack, including one or more metal layers stacked in alternation with one or more dielectric layers, onto the patterned photoresist layer and the filter region of the substrate; removing the patterned photoresist layer and a portion of the multilayer stack on the patterned photoresist layer so that a portion of the multilayer stack remaining on the filter region of the substrate forms the optical filter, wherein each of the one or more metal layers in the optical filter has a tapered edge, extending along an entire periphery of the metal layer at a periphery of the optical filter, that is protectively covered along the entire periphery of the metal layer by at least one of the one or more dielectric layers.

The present invention will be described in greater detail with reference to the accompanying drawings wherein:.

The present invention provides a metal-dielectric optical filter having protected metal layers, which is particularly suitable for use in a sensor device, such as an image sensor, an ambient light sensor, a proximity sensor, a hue sensor, or an ultraviolet (UV) sensor. The optical filter includes one or more dielectric layers and one or more metal layers stacked in alternation. The metal layers are intrinsically protected by the dielectric layers. In particular, the metal layers have tapered edges that are protectively covered by one or more of the dielectric layers. Accordingly, the metal layers have increased resistance to environmental degradation, resulting in a more environmentally durable optical filter.

In some embodiments, the one or more dielectric layers and the one or more metal layers are stacked without any intervening layers. With reference to <FIG>, a first embodiment of the optical filter <NUM>, disposed on a substrate <NUM>, includes three dielectric layers <NUM> and two metal layers <NUM> stacked in alternation. The metal layers <NUM> are each disposed between and adjacent to two dielectric layers <NUM> and are, thereby, protected from the environment. The dielectric layers <NUM> and the metal layers <NUM> are continuous layers that do not have any microstructures formed therein.

The metal layers <NUM> have tapered edges <NUM> at a periphery <NUM> of the optical filter <NUM>. In other words, the metal layers <NUM> are substantially uniform in thickness throughout a central portion <NUM> of the optical filter <NUM>, but taper off in thickness at the periphery <NUM> of the optical filter <NUM>. The tapered edges <NUM> extend along the entire peripheries of the metal layers <NUM> at the periphery <NUM> of the optical filter <NUM>. Likewise, the dielectric layers <NUM> are substantially uniform in thickness throughout the central portion <NUM> of the optical filter <NUM>, but taper off in thickness at the periphery <NUM> of the optical filter <NUM>. Accordingly, the central portion <NUM> of the optical filter <NUM> is substantially uniform in height, whereas the periphery <NUM> of the optical filter <NUM> is sloped. In other words, the optical filter <NUM> has a substantially flat top and sloped sides. Typically, the sides of the optical filter <NUM> are sloped at an angle of less than about <NUM>° from horizontal. Preferably, the sides of the optical filter <NUM> are sloped at an angle of less than about <NUM>° from horizontal, more preferably, at an angle of less than about <NUM>° from horizontal.

Advantageously, the tapered edges <NUM> of the metal layers <NUM> are not exposed to the environment. Rather, the tapered edges <NUM> of the metal layers <NUM> are protectively covered by one or more of the dielectric layers <NUM> along the entire peripheries of the metal layers <NUM>. The one or more dielectric layers <NUM> suppress environmental degradation, e.g., corrosion, of the metal layers <NUM>, e.g., by inhibiting the diffusion of sulfur and water into the metal layers <NUM>. Preferably, the metal layers <NUM> are substantially encapsulated by the dielectric layers <NUM>. More preferably, the tapered edges <NUM> of the metal layers <NUM> are protectively covered by adjacent dielectric layers <NUM>, and the metal layers <NUM> are substantially encapsulated by adjacent dielectric layers <NUM>. In some instances, a top dielectric layer <NUM>, i.e., a dielectric layer <NUM> at the top of the optical filter <NUM>, protectively covers the tapered edges <NUM> of all of the metal layers <NUM> below.

With reference to <FIG>, the first embodiment of the optical filter <NUM> is fabricated by a lift-off process. With particular reference to <FIG>, in a first step, the substrate <NUM> is provided. With particular reference to <FIG>, in a second step, a photoresist layer <NUM> is applied onto the substrate <NUM>. Typically, the photoresist layer <NUM> is applied by spin coating or spray coating.

With particular reference to <FIG>, in a third step, the photoresist layer <NUM> is patterned to uncover a region of the substrate <NUM> where the optical filter <NUM> is to be disposed, i.e., a filter region. Other regions of the substrate <NUM> remain covered by the patterned photoresist layer <NUM>. Typically, the photoresist layer <NUM> is patterned by first exposing a region of the photoresist layer <NUM> covering the filter region of the substrate <NUM> to UV light through a mask, and then developing, i.e., etching, the exposed region of the photoresist layer <NUM> by using a suitable developer or solvent.

The photoresist layer <NUM> is patterned in such a manner that an overhang <NUM>, i.e., an undercut, is formed in the patterned photoresist layer <NUM> surrounding the filter region. In an example not falling under the scope of claim <NUM>, the overhang <NUM> is formed by chemically modifying, e.g., by using a suitable solvent, a top portion of the photoresist layer <NUM>, so that the top portion develops more slowly than a bottom portion of the photoresist layer <NUM>. Alternatively, the overhang <NUM> may be formed by applying a dual-layer photoresist layer <NUM>, consisting of a top layer that develops more slowly and a bottom layer that develops more quickly, to the substrate <NUM>.

The overhang <NUM> should be large enough to ensure that the coating, i.e., the multilayer stack <NUM>, subsequently deposited on the patterned photoresist layer <NUM> and the substrate <NUM> is not continuous from the substrate <NUM> to the patterned photoresist layer <NUM>, as shown in <FIG>. The overhang <NUM> is, typically, greater than <NUM>, preferably, greater than <NUM>. In general, the coating should not cover the sides of the patterned photoresist layer <NUM>.

With reference to <FIG>, when the coating <NUM> is continuous over the substrate <NUM> and the patterned photoresist layer <NUM>, during subsequent lift-off of the photoresist layer <NUM> and the portion of the coating <NUM> thereon, the coating <NUM> is broken at the bottom edges of the patterned photoresist layer <NUM>, exposing the edges of the optical filter formed from the coating <NUM>, in particular, the edges of the metal layers of the optical filter, to the environment. Unfortunately, the exposed edges are susceptible to environmental attack, e.g., when exposed to high humidity and temperature, leading to corrosion, as shown in <FIG> for a silver-containing optical filter <NUM>.

With reference to <FIG>, in an embodiment according to the invention that provides a non-continuous coating <NUM>, the photoresist layer has a bilayer structure, and includes a top layer <NUM> and a bottom layer <NUM>. The top layer <NUM> is photosensitive and is patternable by selective exposure to UV light. The bottom layer <NUM> is, generally, not photosensitive and acts as a release layer. Suitable examples of resists include AZ Electronic Materials nLOF <NUM> for the top photosensitive layer <NUM> and Microchem Corp. LOR <NUM> B for the bottom release layer <NUM>.

When the photoresist layer is developed, the extent of the overhang <NUM> is controlled by the development time. In <FIG>, the development time was selected to provide an overhang <NUM> of about <NUM>. Preferably, the thickness of the bottom release layer <NUM> is greater than about <NUM>, and the overhang <NUM> is greater than about <NUM> To ensure clean lift-off, i.e., lift-off without breakage of the deposited coating <NUM>, the thickness of the coating <NUM> should, generally, be less than about <NUM> % of the thickness of the bottom release layer <NUM>. In <FIG>, the thickness of the bottom release layer <NUM> is about <NUM>, the thickness of the top photosensitive layer <NUM> is about <NUM>, and the thickness of the coating is about <NUM>. The sides of the optical filter <NUM> under the overhang <NUM> are sloped at an angle of about <NUM>°.

With reference to <FIG>, in some instances, a thicker bottom release layer <NUM> is used, and a larger overhang <NUM> is produced by using a longer development time, e.g., about <NUM> to about <NUM> for some processes. These features improve edge durability by decreasing the slope of the sides of the optical filter <NUM> and increasing the thickness of the top dielectric layer <NUM> at the periphery of the optical filter <NUM>. In <FIG>, the development time was selected to provide an overhang <NUM> of about <NUM>. Preferably, the thickness of the bottom release layer <NUM> is greater than about <NUM>, and the overhang <NUM> is greater than about <NUM>. The thickness of the coating layer <NUM> should, generally, be less than about <NUM> % of the thickness of the bottom release layer <NUM>. In <FIG>, the thickness of the bottom release layer <NUM> is about <NUM>, the thickness of the top photosensitive layer <NUM> is about <NUM>, and the thickness of the coating <NUM> is about <NUM>. The sides of the optical filter <NUM> under the overhang <NUM> are sloped at an angle of about <NUM>°.

With particular reference to <FIG>, in a fourth step, a multilayer stack <NUM> is deposited as a non-continuous coating onto the patterned photoresist layer <NUM> and the filter region of the substrate <NUM>. A portion of the multilayer stack <NUM> disposed on the filter region of the substrate <NUM> forms the optical filter <NUM>. The layers of the multilayer stack <NUM>, which correspond to the layers of the optical filter <NUM>, may be deposited by using a variety of deposition techniques, such as: evaporation, e.g., thermal evaporation, electron-beam evaporation, plasma-assisted evaporation, or reactive-ion evaporation; sputtering, e.g., magnetron sputtering, reactive sputtering, alternating-current (AC) sputtering, direct-current (DC) sputtering, pulsed DC sputtering, or ion-beam sputtering; chemical vapor deposition, e.g., plasma-enhanced chemical vapor deposition; and atomic layer deposition. Moreover, different layers may be deposited by using different deposition techniques. For example, the metal layers <NUM> may be deposited by sputtering of a metal target, and the dielectric layers <NUM> may be deposited by reactive sputtering of a metal target in the presence of oxygen.

Because the overhang <NUM> shadows a periphery of the filter region of the substrate <NUM>, the deposited layers taper off in thickness towards the periphery <NUM> of the optical filter <NUM>. The overhang <NUM> generates a soft roll-off of the coating towards the periphery <NUM> of the optical filter <NUM>. When a dielectric layer <NUM> is deposited onto a metal layer <NUM>, the dielectric layer <NUM> covers not only the top surface of the metal layer <NUM>, but also the tapered edges <NUM> of the metal layer <NUM>, thereby, protecting the metal layer <NUM> from the environment. Moreover, the top dielectric layer <NUM>, generally, serves as a protective layer for the metal layers <NUM> below. For example, in the embodiment of <FIG>, a top dielectric layer <NUM> having a thickness of about <NUM> extends over and protectively covers the less durable metal layers below, in particular, the tapered edges of the metal layers, as shown in <FIG>.

With particular reference to <FIG>, in a fifth step, a portion of the multilayer stack <NUM> on the patterned photoresist layer <NUM> is removed, i.e., lifted off, together with the photoresist layer <NUM>. Typically, the photoresist layer <NUM> is stripped by using a suitable stripper or solvent. A portion of the multilayer stack <NUM> remaining on the filter region of the substrate <NUM> forms the optical filter <NUM>. The substrate <NUM> may, for example, be a conventional sensor element.

It should be noted that the lift-off process of <FIG> may also be used to simultaneously form a plurality of optical filters <NUM> of the same type, i.e., having the same optical design, on the substrate <NUM>. Moreover, the lift-off process may be repeated to subsequently form one or more optical filters of a different type, i.e., having a different optical design, on the same substrate <NUM>. In some instances, one or more optical filters that are more environmentally durable may be subsequently formed on the substrate <NUM> so that they partially overlap with one or more optical filters <NUM> that are less environmentally durable, as explained in further detail hereafter, by using a lift-off process or, in some instances, by using a dry or wet etching process. Thereby, an optical filter array may be formed on the substrate <NUM>. The substrate <NUM> may, for example, be a conventional sensor array.

With particular reference to <FIG>, in an optional sixth step, an additional protective coating <NUM> is deposited onto the optical filter <NUM>. The protective coating <NUM> may be deposited by using one of the deposition techniques mentioned heretofore. The protective coating <NUM> covers both the central portion <NUM> and the periphery <NUM> of the optical filter <NUM>, i.e., all exposed portions of the optical filter <NUM>, thereby, protecting the optical filter <NUM> from the environment.

In other embodiments, the optical filter includes a plurality of corrosion-suppressing layers, disposed between the dielectric layers and the metal layers, which further protect the metal layers. With reference to <FIG>, a second embodiment of the optical filter <NUM>, disposed on a substrate <NUM>, is similar to the first embodiment of the optical filter <NUM>, but further includes four corrosion-suppressing layers <NUM> inserted between the three dielectric layers <NUM> and the two metal layers <NUM>.

The metal layers <NUM> are each disposed between and adjacent to two corrosion-suppressing layers <NUM> and are, thereby, further protected from the environment. The corrosion-suppressing layers <NUM> suppress corrosion of the metal layers <NUM>, principally during the deposition process. In particular, the corrosion-suppressing layers <NUM> protect portions of the metal layers <NUM> in the optical path, inhibiting degradation of the optical properties of the metal layers <NUM>. Preferably, tapered edges <NUM> of the metal layers <NUM> are protectively covered by adjacent corrosion-suppressing layers <NUM>, as well as by nearest dielectric layers <NUM>. Thus, the metal layers <NUM> are, preferably, substantially encapsulated by adjacent corrosion-suppressing layers <NUM>, as well as by nearest dielectric layers <NUM>.

The second embodiment of the optical filter <NUM> may be fabricated by a lift-off process similar to that used to fabricate the first embodiment of the optical filter <NUM>. However, the layers of the multilayer stack deposited in the fourth step correspond to the layers of the optical filter <NUM>. In particular, corrosion-suppressing layers <NUM> are deposited before and after each metal layer <NUM>. Advantageously, the corrosion-suppressing layers <NUM> suppress corrosion, i.e., oxidation, of the metal layers <NUM> during deposition of the dielectric layers <NUM>. The corrosion-suppressing layers <NUM> are particularly useful when the metal layers <NUM> contain silver or aluminum. In such embodiments, the corrosion-suppressing layers <NUM> suppress the reaction between silver or aluminum from the metal layers <NUM> and oxygen from the dielectric layers <NUM> to form silver oxide or aluminum oxide.

The corrosion-suppressing layers <NUM> may be deposited as metal compound, e.g., metal nitride or metal oxide, layers by using one of the deposition techniques mentioned heretofore, e.g., reactive sputtering. Alternatively, the corrosion-suppressing layers <NUM> may be formed by first depositing suitable metal layers, by using one of the deposition techniques mentioned heretofore, and subsequently oxidizing the metal layers. Preferably, the corrosion-suppressing layers <NUM> on top of the metal layers <NUM> are each formed by first depositing a suitable metal layer, oxidizing the metal layer, and then depositing a metal oxide layer. For example, these corrosion-suppressing layers <NUM> may be formed by sputtering of a suitable metal target followed by oxidation, followed by reactive sputtering of a suitable metal target in the presence of oxygen. Further details of methods of forming corrosion-suppressing layers are provided hereafter, and are disclosed in <CIT>.

The optical filter of the present invention may have a variety of optical designs. The optical designs of exemplary optical filters will be described in further detail hereafter. In general, the optical design of the optical filter is optimized for a particular passband by selecting suitable layer numbers, materials, and/or thicknesses.

The optical filter includes at least one metal layer, and at least one dielectric layer. Often, the optical filter includes a plurality of metal layers and a plurality of dielectric layers. Typically, the optical filter includes <NUM> to <NUM> metal layers, <NUM> to <NUM> dielectric layers, and, optionally, <NUM> to <NUM> corrosion-suppressing layers. In general, increasing the number of metal layers provides a passband with steeper edges, but with a lower in-band transmittance.

The first or bottom layer in the optical design, i.e., the first layer deposited on the substrate, may be a metal layer or a dielectric layer. The last or top layer in the optical design, i.e., the last layer deposited on the substrate, is usually a dielectric layer. When the bottom layer is a metal layer, the optical filter may consist of n metal layers (M) and n dielectric layers (D) stacked in a sequence of (M/D)n, where n ≥ <NUM>. Alternatively, the optical filter may consist of n metal layers (M), n dielectric layers (D), and 2n corrosion-suppressing layers (C), stacked in a sequence of (C/M/C/D)n, where n ≥ <NUM>. When the bottom layer is a dielectric layer, the optical filter may consist of n metal layers (M) and n + <NUM> dielectric layers (D) stacked in a sequence of D(M/D)n, where n ≥ <NUM>. Alternatively, the optical filter may consist of n metal layers (M), n + <NUM> dielectric layers (D), and 2n corrosion-suppressing layers (C), stacked in a sequence of D(C/M/C/D)n, where n ≥ <NUM>.

The metal layers are each composed of a metal or alloy. In some embodiments, the metal layers are each composed of silver. Alternatively, the metal layers may each be composed of a silver alloy. For example, a silver alloy consisting essentially of about <NUM> wt% gold, about <NUM> wt% tin, and a balance of silver may provide improved corrosion resistance. In other embodiments, the metal layers are each composed of aluminum. The choice of metal or alloy depends on the application. Silver is usually preferred for optical filters having a passband in the visible spectral region, and aluminum is usually preferred for optical filters having a passband in the UV spectral region, although silver may sometimes be used when the passband is centered at a wavelength greater than about <NUM>.

Generally, but not necessarily, the metal layers are composed of the same metal or alloy, but have different thicknesses. Typically, the metal layers each have a physical thickness between about <NUM> and about <NUM>, preferably, between about <NUM> and about <NUM>.

The dielectric layers are each composed of a dielectric material that is transparent in the passband of the optical filter.

For optical filters with a passband in the visible spectral region, the dielectric layers are, typically, each composed of a high-index dielectric material having a refractive index greater than about <NUM> at <NUM> that is transparent in the visible spectral region. Suitable examples of high-index dielectric materials for such filters include titanium dioxide (TiO<NUM>), zirconium dioxide (ZrO<NUM>), hafnium dioxide (HfO2), niobium pentoxide (Nb<NUM>O<NUM>), tantalum pentoxide (Ta<NUM>O<NUM>), and mixtures thereof. Preferably, the high-index dielectric material for such filters is also UV-absorbing, i.e., absorbing in the near-UV spectral region. For example, a high-index dielectric material including or consisting of TiO<NUM> and/or Nb<NUM>O<NUM> may provide enhanced UV blocking, i.e., lower out-of-band transmittance in the near-UV spectral region. Preferably, the high-index dielectric material has a refractive index greater than about <NUM> at <NUM>, more preferably, greater than about <NUM> at <NUM>. A higher refractive index is usually desirable. However, the transparent high-index dielectric materials that are currently available, generally, have refractive indices less than about <NUM> at <NUM>.

For filters with a passband in the UV spectral region, the dielectric layers are, typically, each composed of an intermediate-index dielectric material having a refractive index between about <NUM> and <NUM> at <NUM> or, preferably, of a high-index dielectric material having a refractive index greater than about <NUM> at <NUM>, more preferably, greater than about <NUM> at <NUM> that is transparent in the UV spectral region. Suitable examples of intermediate-index and high-index dielectric materials for filters with a passband in the UV spectral region include Ta<NUM>O<NUM>, hafnium dioxide (HfO<NUM>), aluminum trioxide (Al<NUM>O<NUM>), silicon dioxide (SiO<NUM>), scandium trioxide Sc<NUM>O<NUM>, yttrium trioxide (Y<NUM>O<NUM>), ZrO<NUM>, magnesium dioxide (MgO2), magnesium difluoride (MgF<NUM>), other fluorides, and mixtures thereof. For example, Ta<NUM>O<NUM> may be used as a high-index dielectric material for passbands centered at wavelengths above about <NUM>, and HfO<NUM> may be used as a high-index dielectric material for passbands centered at wavelengths below about <NUM>.

Generally, but not necessarily, the dielectric layers are composed of the same dielectric material, but have different thicknesses. Typically, the dielectric layers each have a physical thickness between about <NUM> and about <NUM>. Preferably, the top dielectric layer has a physical thickness of greater than about <NUM>, more preferably, greater than about <NUM>, to enable the top dielectric layer to serve as a protective layer for the metal layers below. The physical thickness of each dielectric layer is selected to correspond with a quarter wave optical thickness (QWOT) required by an optical design. The QWOT is defined as <NUM>nt, where n is the refractive index of the dielectric material and t is the physical thickness. Typically, the dielectric layers each have a QWOT between about <NUM> and about <NUM>.

The optional corrosion-suppressing layers are each composed of a corrosion-suppressing material. Typically, the corrosion-suppressing layers are composed of a corrosion-suppressing dielectric material. Examples of suitable corrosion-suppressing dielectric materials include silicon nitride (Si<NUM>N<NUM>), TiO<NUM>, Nb<NUM>O<NUM>, zinc oxide (ZnO), and mixtures thereof. Preferably, the corrosion-suppressing dielectric material is a compound, e.g., a nitride or an oxide, of a metal having a higher galvanic potential than the metal or alloy of the metal layers.

In some instances, the corrosion-suppressing layers below the metal layers are composed of ZnO, whereas the corrosion-suppressing layers above the metal layers include a very thin layer, e.g., having a thickness of less than <NUM>, composed of zinc, and a thin layer composed of ZnO. The zinc layers are deposited on the metal layers, and then post-oxidized to prevent optical absorption. The ZnO layers below and above the metal layers are, typically, deposited by reactive sputtering. Advantageously, depositing the zinc layers on the metal layers before depositing the ZnO layers prevents the metal layers from being exposed to the activated, ionized oxygen species that are produced during reactive sputtering. The zinc layers preferentially absorb oxygen, suppressing the oxidation of the metal layers.

The corrosion-suppressing layers are, generally, suitably thin to substantially avoid contributing to the optical design of the optical filter, especially when they are absorbing in the visible spectral region. Typically, the corrosion-suppressing layers each have a physical thickness between about <NUM> and about <NUM>, preferably, between about <NUM> and about <NUM>. Further details of suitable corrosion-suppressing layers are disclosed in <CIT>.

The optional protective coating is, typically, composed of a dielectric material. The protective coating may be composed of the same dielectric materials and may have the same range of thicknesses as the dielectric layers. Often, the protective coating is composed of the same dielectric material as the top dielectric layer and has a thickness that is a portion of the design thickness, i.e., the thickness required by the optical design, of the top dielectric layer. In other words, the top dielectric layer of the optical design is split between a dielectric layer and a dielectric protective coating. Alternatively, the protective coating may be composed of an organic material, e.g., epoxy.

With reference to <FIG>, the optical filter <NUM>, typically, has a filter height h, i.e., a height of the central portion of the optical filter <NUM> from the substrate <NUM>, of less than <NUM>, preferably, of less than <NUM>. It should be noted that the filter height, generally, corresponds to the thickness of the deposited coating referred to heretofore. When used in a image sensor, the optical filter <NUM>, typically, has a filter width w, i.e., a width of the central portion of the optical filter <NUM>, of less than <NUM>, preferably, of less than <NUM>. Advantageously, the relatively small filter height allows a smaller filter spacing when a plurality of optical filters <NUM> are formed by a lift-off process. Typically, the optical filters <NUM> in an image sensor have a filter spacing d, i.e., a spacing between the central portions of nearest optical filters <NUM>, of less than <NUM>, preferably, of less than <NUM>. When used in other sensor devices with larger pixel sizes, the filter width may be from about <NUM> to about <NUM>.

The optical filter is a metal-dielectric bandpass filter, i.e., an induced transmission filter, having a high in-band transmittance and a low out-of-band transmittance. In some embodiments, the optical filter is a color filter having a relatively narrow color passband in the visible spectral region. For example, the optical filter may be a red, green, blue, cyan, yellow, or magenta filter. In other embodiments, the optical filter is a photopic filter having a photopic passband, i.e., a passband matching the photopic luminosity efficiency function that mimics the spectral response of the human eye to relatively bright light, in the visible spectral region. In yet other embodiments, the optical filter is an IR-blocking filter having a relatively broad passband in the visible spectral region.

In such embodiments, the optical filter, typically, has a maximum in-band transmittance of greater than about <NUM> %, an average out-of-band transmittance of less than about <NUM> % between about <NUM> and about <NUM>, i.e., in the near-UV spectral region, and an average out-of-band transmittance of less than about <NUM> % between about <NUM> and about <NUM>, i.e., in the infrared (IR) spectral region. In contrast, conventional all-dielectric color and photopic filters are not, typically, inherently IR-blocking. Generally, in such embodiments, the optical filter also has a low angle shift, i.e., center-wavelength shift with change in incidence angle from <NUM>°. Typically, the optical filter has an angle shift at an incidence angle of <NUM>° of less than about <NUM>% or about <NUM> in magnitude for an optical filter centered at <NUM>. In contrast, conventional all-dielectric color and photopic filters are, typically, very angle-sensitive.

Optical designs, i.e., layer numbers, materials, and thicknesses, for exemplary red, green, and blue filters, i.e., an exemplary RGB filter set, are tabulated in <FIG>, <FIG>, and <FIG>, respectively. An optical design for an exemplary photopic filter is tabulated in <FIG>. The layers of each optical design are numbered starting from the first or bottom layer deposited on the substrate.

The metal layers are each composed of silver, and have physical thicknesses between about <NUM> and about <NUM>. The dielectric layers are each composed of a high-index dielectric material (H), and have QWOTs between about <NUM> and about <NUM>. For example, the high-index dielectric material may be a mixture of Nb<NUM>O<NUM> and TiO<NUM> having a refractive index of about <NUM> at <NUM>. The corrosion-suppressing layers are each composed of ZnO and each have a physical thickness of about <NUM>.

When the high-index dielectric material has a refractive index of about <NUM> at <NUM>, the filter height of the red filter is <NUM>, that of the green filter is <NUM>, that of the blue filter is <NUM>, and that of the photopic filter is <NUM>. These filter heights are considerably smaller than those of conventional all-dielectric color and photopic filters.

Transmission spectra <NUM>, <NUM>, and <NUM> for the exemplary red, green, and blue filters, respectively, are plotted in <FIG>. The transmission spectrum <NUM> for the exemplary red filter includes a red passband centered at about <NUM>, the transmission spectrum <NUM> for the exemplary green filter includes a green passband centered at about <NUM>, and the transmission spectrum <NUM> for the exemplary blue filter includes a blue passband centered at about <NUM>.

Transmission spectra <NUM> (<NUM>°) and <NUM> (<NUM>°) for the exemplary photopic filter at incidence angles of <NUM>° to <NUM>° are plotted in <FIG>. The transmission spectrum <NUM> for the exemplary photopic filter at an incidence angle of <NUM>° includes a photopic passband centered at about <NUM>. In the transmission spectrum <NUM> for the exemplary photopic filter at an incidence angle of <NUM>°, the photopic passband is centered at about <NUM>. In other words, the angle shift of the exemplary photopic filter at an incidence angle of <NUM>° is about -<NUM>. Advantageously, the angle shift of the exemplary photopic filter is considerably smaller than the angle shift of a conventional all-dielectric photopic filter.

The exemplary color and photopic filters each have a maximum in-band transmittance of greater than about <NUM> %. Advantageously, the exemplary color and photopic filters provide improved IR blocking relative to conventional dye-based and all-dielectric color and photopic filters, reducing noise caused by IR leaking. Specifically, the exemplary color and photopic filters each have an average out-of-band transmittance of less than about <NUM> % between about <NUM> and about <NUM>, i.e., in the IR spectral region. The exemplary color and photopic filters, particularly the exemplary red filter, also provide improved UV blocking relative to some conventional metal-dielectric color filters, reducing noise caused by UV leaking. Specifically, the exemplary color and photopic filters each have an average out-of-band transmittance of less than about <NUM> % between about <NUM> and about <NUM>, i.e., in the near-UV spectral region.

A color gamut <NUM> for the exemplary RGB filter set is plotted on a CIE xy chromaticity diagram in <FIG>, along with a color gamut <NUM> for a conventional dye-based RGB filter set for comparison. Advantageously, the color gamut <NUM> of the exemplary RGB filter set is considerably larger than the color gamut <NUM> of the conventional dye-based RGB filter set.

A color trajectory <NUM> for the exemplary red filter at incidence angles of <NUM>° to <NUM>° is plotted on a CIE xy chromaticity diagram in <FIG>, along with a color trajectory <NUM> for a conventional all-dielectric red filter at incidence angles of <NUM>° to <NUM>°. A color trajectory <NUM> for the exemplary photopic filter at incidence angles of <NUM>° to <NUM>° is plotted on a CIE xy chromaticity diagram in <FIG>. Advantageously, the angle shift of the exemplary red and photopic filters is considerably smaller than the angle shift of conventional all-dielectric red and photopic filters.

In some embodiments, the optical filter is a UV filter having a relatively narrow passband in the UV spectral region, e.g., between about <NUM> and about <NUM>. For example, the optical filter may be an ultraviolet-A (UVA) or ultraviolet-B (UVB) filter. In such embodiments, the optical filter typically, has a maximum in-band transmittance of greater than about <NUM> %, preferably, greater than about <NUM> %, and an average out-of-band transmittance of less than about <NUM> % between about <NUM> and about <NUM>, i.e., in the visible and IR spectral regions. In contrast, conventional all-dielectric UV filters are not, typically, inherently IR-blocking. Generally, in such embodiments, the optical filter also has a low angle shift, i.e., center-wavelength shift with change in incidence angle from <NUM>°. Typically, the optical filter has an angle shift at an incidence angle of <NUM>° of less than about <NUM>% or about <NUM> in magnitude for an optical filter centered at <NUM>. In contrast, conventional all-dielectric UV filters are, typically, very angle-sensitive.

Optical designs, i.e., layer numbers, materials, and thicknesses, for exemplary UVA, UVB, and <NUM>-nm-centered filters are summarized in <FIG>. The metal layers are each composed of aluminum, and have physical thicknesses between about <NUM> and about <NUM>. The dielectric layers are each composed of a high-index dielectric material, namely, Ta<NUM>O<NUM> for the UVA filter, and HfO<NUM> for the UVB and <NUM>-nm-centered filters, and have physical thicknesses between about <NUM> and about <NUM>. The exemplary UV filters do not include corrosion-suppressing layers, as the additional protection they provide is not usually necessary when the metal layers are composed of aluminum.

The filter height of the UVA filter is <NUM>, that of the UVB filter is <NUM>, and that of the <NUM>-nm-centered filter is <NUM>. These filter heights are considerably smaller than those of conventional all-dielectric UV filters.

Transmission spectra <NUM> (<NUM>°) and <NUM> (<NUM>°) for the exemplary UVA filter at incidence angles of <NUM>° to <NUM>° are plotted in <FIG>, transmission spectra <NUM> (<NUM>°) and <NUM> (<NUM>°) for the exemplary UVB filter at incidence angles of <NUM>° to <NUM>° are plotted in <FIG>, and transmission spectra <NUM> (<NUM>°) and <NUM> (<NUM>°) for the exemplary <NUM>-nm-centered filter at incidence angles of <NUM>° to <NUM>° are plotted in <FIG>. The transmission spectrum <NUM> for the exemplary UVA filter at an incidence angle of <NUM>° includes a UVA passband centered at about <NUM>, the transmission spectrum <NUM> for the exemplary UVB filter at an incidence angle of <NUM>° includes a UVB passband centered at about <NUM>, and the transmission spectrum <NUM> for the <NUM>-nm-centered filter at an incidence angle of <NUM>° includes a passband centered at about <NUM>. The angle shift of the exemplary UV filters at an incidence angle of <NUM>° is less than about <NUM> in magnitude. Advantageously, the angle shift of the exemplary UV filters is considerably smaller than the angle shift of conventional all-dielectric UV filters.

The exemplary UV filters each have a maximum in-band transmittance of greater than about <NUM> %. In particular, the UVA and UVB filters each have a maximum in-band transmittance of greater than about <NUM> %. Advantageously, the exemplary UV filters provide improved IR blocking relative to conventional all-dielectric UV filters, reducing noise caused by IR leaking. Specifically, the exemplary UV filters each have an average out-of-band transmittance of less than about <NUM> % between about <NUM> and about <NUM>, i.e., in the visible and IR spectral regions.

The optical filter of the present invention is particularly useful when included as part of a sensor device or other active device. The sensor device may be any type of sensor device including one or more sensor elements, in addition to one or more optical filters according to the present invention. In some instances, the sensor device may also include one or more conventional optical filters. For example, the sensor device may be an image sensor, an ambient light sensor, a proximity sensor, an hue sensor, a UV sensor, or a combination thereof. The one or more sensor elements may be any type of conventional sensor elements. Typically, the one or more sensor elements are photodetectors, such as photodiodes, charge-coupled device (CCD) sensor elements, complementary metal-oxide semiconductor (CMOS) sensor elements, silicon detectors, or special UV-sensitive detectors. The one or more sensor elements may be front- or back-illuminated. The sensor elements may be formed of any typical sensor material, such as silicon, indium gallium arsenide (In<NUM>-xGaxAs), gallium arsenide (GaAs), germanium, lead sulfide (PbS), or gallium nitride (GaN).

The one or more optical filters are disposed over the one or more sensor elements, so that the one or more optical filters filter light provided to the one or more sensor elements. Typically, each optical filter is disposed over one sensor element. In other words, each pixel of the sensor device, typically, includes one optical filter and one sensor element. Preferably, the one or more optical filters are disposed directly on the one or more sensor elements, e.g., on a passivation layer of the one or more sensor elements. For example, the one or more optical filters may be formed on the one or more sensor elements by a lift-off process. However, in some instances, there may be one or more coatings disposed between the one or more optical filters and the one or more sensor elements. In some instances, the one or more optical filters may be integrated with the one or more sensor elements.

In some examples not falling under the scope of protection, the sensor device includes a single sensor element and a single optical filter according to the present invention disposed over the sensor element. With reference to <FIG>, a first example of the sensor device <NUM> includes a sensor element <NUM> and an optical filter <NUM> disposed on the sensor element <NUM>. For example, the sensor device <NUM> may be an ambient light sensor, the sensor element <NUM> may be a photodiode, and the optical filter <NUM> may be a photopic filter, such as the exemplary photopic filter of <FIG>, or an IR-blocking filter. For another example, the sensor device <NUM> may be a UV sensor, the sensor element <NUM> may be a photodiode, and the optical filter <NUM> may be a UV filter, such as the exemplary UVA, UVB, or <NUM>-nm-centred filter of <FIG>.

In an example of an ambient light sensor, a photopic filter according to the present invention is integrated with a photodiode. The photopic filter is disposed on the photodiode, typically, on a planarized passivation layer, e.g., composed of Si<NUM>N<NUM>, of the photodiode. An optional protective coating or encapsulation layer, e.g., composed of epoxy, may be disposed over the photopic filter and the photodiode. The optical design of the photopic filter is optimized by taking the passivation layer and, when present, the encapsulation layer into account.

Transmission spectra <NUM> (<NUM>°) and <NUM> (<NUM>°) for an exemplary photopic filter optimized for integration with a photodiode at incidence angles of <NUM>° to <NUM>° are plotted in <FIG>, along with a normalized photopic response curve <NUM>. The transmission spectra <NUM> and <NUM> are matched to a Si<NUM>N<NUM> passivation layer and an epoxy encapsulation layer. The transmission spectrum <NUM> for the exemplary photopic filter at an incidence angle of <NUM>° includes a photopic passband centered at about <NUM>. The transmission spectra <NUM> for the exemplary photopic filter follow the normalized photopic response curve <NUM> reasonably well at incidence angles of <NUM>° to <NUM>°. Moreover, the exemplary photopic filter blocks both UV and IR light at incidence angles of <NUM>° to <NUM>°, and has a low angle shift. Advantageously, the exemplary photopic filter is also environmentally durable, e.g., at a temperature of <NUM> and a relative humidity of <NUM> % for <NUM> hours.

In other examples, the sensor device includes a plurality of sensor elements, and a plurality of optical filters according to the present invention disposed over the plurality of sensor elements. Typically, the sensor elements are disposed in an array. In other words, the sensor elements form a sensor array, such as a photodiode array, a CCD array, a CMOS array, or any other type of conventional sensor array. Also typically, the optical filters are disposed in an array. In other words, the optical filters form an optical filter array, such as a color filter array (CFA). Preferably, the sensor array and the optical filter array are corresponding two-dimensional arrays, i.e., mosaics. For example, the arrays may be rectangular arrays having rows and columns.

Often, in such examples, the optical filters are substantially separate from one another. In other words, the peripheries of the optical filters are not usually in contact with one another. However, in some instances, the dielectric layers of the optical filters may unintentionally touch, while the metal layers, particularly, the tapered edges, remain separate from one another.

Typically, the plurality of optical filters includes different types of optical filters having different passbands from one another. For example, the plurality of optical filters may include color filters, such as red, green, blue, cyan, yellow, and/or magenta filters, photopic filters, IR-blocking filters, UV filters, or a combination thereof. In some embodiments, the plurality of optical filters includes different types of color filters, forming a CFA. For example, the plurality of optical filters may include red, green, and blue filters, such as the exemplary red, green, and blue filters of <FIG>, forming an RGB filter array, such as a Bayer filter array. For another example, the plurality of optical filters may include cyan, magenta, and yellow filters, forming a CMY filter array.

Advantageously, the different types of optical filters may have different numbers of metal layers and/or different thicknesses of the metal layers from one another. In some embodiments, at least two of the different types of optical filters include different numbers of metal layers from one another. In the same or other embodiments, at least two of the different types of optical filters have different metal-layer thicknesses from one another. For example, the exemplary blue filter of <FIG> has a different number of metal layers from the exemplary red and green filters of <FIG> and <FIG>. Moreover, all of the exemplary red, green, and blue filters of <FIG> have different metal-layer thicknesses from one another.

With reference to <FIG>, a second example of the sensor device <NUM> includes a plurality of sensor elements <NUM> and a plurality of optical filters <NUM> and <NUM> disposed on the plurality of sensor elements <NUM>. The plurality of optical filters <NUM> and <NUM> includes a first type of optical filter <NUM> having a first passband, and a second type of optical filter <NUM> having a second passband, different from the first passband. For example, the sensor device <NUM> may be an image sensor, the plurality of sensor elements <NUM> may form a CCD array, and the plurality of optical filters <NUM> and <NUM> may form a Bayer filter array, of which only a portion of one row is illustrated. The first type of optical filter <NUM> may be a green filter, such as the exemplary green filter of <FIG>, and the second type of optical filter <NUM> may be a red filter, such as the exemplary red filter of <FIG>, or a blue filter, such as the exemplary blue filter of <FIG>.

Any of the examples of the sensor device described heretofore may be combined with one or more additional optical filters that are more environmentally durable and one or more additional sensor elements.

Accordingly, in some examples, the sensor device includes one or more second optical filters disposed over one or more second sensor elements, in addition to one or more first optical filters according to the present invention disposed over one or more first sensor elements. The one or more second optical filters are more environmentally durable than the one or more first optical filters. For example, the one or more first optical filters may be silver-dielectric optical filters according to the present invention, in which the metal layers are composed of silver or a silver alloy. The second one or more second optical filters may be an aluminum-dielectric optical filters according to the present invention, in which the metal layers are composed of aluminum. Alternatively, the one or more second optical filters may be conventional optical filters, such as all-dielectric, silicon-dielectric, or hydrogenated-silicon-dielectric optical filters.

In such examples, the one or more second optical filters partially overlap with the one or more first optical filters, such that the one or more second optical filters that are more environmentally durable protectively cover the peripheries of the one or more first optical filters that are less environmentally durable. Advantageously, this overlapping layout provides the one or more first optical filters, particularly, the tapered edges of the metal layers, with additional protection from environmental degradation such as corrosion. Owing to the small slope of the filter sides and the small filter height of the one or more first optical filters, the one or more second optical filters conform when deposited on the sloped sides at the peripheries of the one or more first optical filters and the substrate, providing continuous layers in the one or more second optical filters.

The one or more second optical filters extend over the sloped sides at the peripheries of the one or more first optical filters, including the tapered edges of the metal layers, preferably, along the entire peripheries of the one or more first optical filters. Preferably, the one or more second optical filters completely cover the sloped sides at the peripheries of the one or more first optical filters. However, the one or more second optical filters do not cover or obstruct the one or more first sensor elements.

Typically, the one or more first optical filters and the one or more second optical filters have different passbands from one another. For example, the one or more first optical filters may be color filters, such as red, green, blue, cyan, yellow, or magenta filters, photopic filters, IR-blocking filters, or a combination thereof. In particular, the one or more first optical filters may be silver-dielectric color filters, such as the exemplary red, green, and/or blue filters of <FIG>, silver-dielectric photopic filters, such as the exemplary photopic filter of <FIG>, or silver-dielectric IR-blocking filters.

The one or more second optical filters may, for example, be UV filters or near-IR filters, or a combination thereof. In particular, the one or more second optical filters may be aluminum-dielectric UV filters, such as the exemplary UVA, UVB, and/or <NUM>-nm-centered filters of <FIG>, or all-dielectric UV filters. Alternatively, the one or more second optical filters may be silicon-dielectric or hydrogenated-silicon-dielectric near-IR filters, such as the optical filters described in <CIT>.

Typically, the sensor device, in such examples, is multifunctional and combines different types of optical sensors having different functions, mainly determined by the passbands of the one or more first optical filters and the one or more second optical filters. The one or more first optical filters and the one or more first sensor elements form a first type of optical sensor, and the one or more second optical filters and the one or more second sensor elements form a second type of optical sensor. For example, the first type of optical sensor may be an ambient light sensor including a photopic filter or an IR-blocking filter, a hue sensor including one or more different types of color filters, or an image sensor including a plurality of different types of color filters. The second type of optical sensor may, for example, be a UV sensor including a UV filter, or a proximity sensor including a near-IR filter.

With reference to <FIG>, a third example of a sensor device <NUM> includes a first sensor element <NUM> and a first optical filter <NUM> according to the present invention disposed on the first sensor element <NUM>, forming a first type of optical sensor. The sensor device <NUM> further includes a second sensor element <NUM> and a second optical filter <NUM> that is more environmentally durable disposed on the second sensor element <NUM>, forming a second type of optical sensor.

For example, the first type of optical sensor may be an ambient light sensor, and the first optical filter <NUM> may be a silver-dielectric photopic filter, such as the exemplary photopic filter of <FIG>, or a silver-dielectric IR-blocking filter. The second type of optical sensor may, for example, be a UV sensor, and the second optical filter <NUM> may be an aluminum-dielectric UV filter, such as the exemplary UVA, UVB, or <NUM>-nm-centered filter of <FIG>, or an all-dielectric UV filter. Alternatively, the second type of optical sensor may be a proximity sensor, and the second optical filter <NUM> may be a near-IR filter, such as an all-dielectric, silicon-dielectric, or hydrogenated-silicon-dielectric near-IR filter. The first sensor element <NUM> and the second sensor element <NUM> may be photodiodes.

With particular reference to <FIG>, the second optical filter <NUM> extends over the sloped sides of the first optical filter <NUM> along the entire periphery of the first optical filter <NUM>. Thereby, the second optical filter <NUM> protectively covers the periphery of the first optical filter <NUM>, including the tapered edges of the metal layers.

With particular reference to <FIG> and <FIG>, the first optical filter <NUM> covers and filters light provided to the first sensor element <NUM>. The second optical filter <NUM> covers and filters light provided to the second sensor element <NUM>, and surrounds, but does not cover, the first sensor element <NUM>. In the layout illustrated in <FIG>, the first sensor element <NUM> and the second sensor element <NUM> are disposed in a row between rows of bond pads <NUM>. In an alternative layout illustrated in <FIG>, the second sensor element <NUM> is annular and surrounds the first sensor element <NUM>.

With reference to <FIG>, a fourth example of a sensor device <NUM> includes a plurality of first sensor elements <NUM> and a plurality of first optical filters <NUM>, <NUM>, and <NUM> according to the present invention disposed on the plurality of first sensor elements <NUM>, forming a first type of optical sensor. The sensor device <NUM> further includes a second sensor element <NUM> and a second optical filter <NUM> disposed over the second sensor element <NUM>, forming a second type of optical sensor.

Claim 1:
A method of fabricating an optical filter (<NUM>) disposed on a substrate (<NUM>), the optical filter comprising:
a multilayer stack (<NUM>) of a plurality of dielectric layers (<NUM>) and a plurality of metal layers (<NUM>), stacked in alternation with the dielectric layers on the substrate and surrounded by the dielectric layers, and
wherein each metal layer has a tapered edge (<NUM>) extending along an entire periphery (<NUM>) of the metal layer that is protectively covered along the entire periphery of the metal layer by at least one of the one or more dielectric layers, and
wherein the one or more metal layers are not adjacent, with respect to a length of the substrate, to any other metal layer;
wherein the optical filter is formed by a lift-off process comprising the steps of:
providing the substrate (<NUM>);
applying a photoresist layer (<NUM>) having a bilayer structure including a top layer (<NUM>) and a bottom release layer (<NUM>) onto the substrate (<NUM>);
patterning the photoresist layer (<NUM>) to uncover a region of the substrate where the optical filter is to be disposed in such a manner that an overhang (<NUM>) is formed in the patterned photoresist layer (<NUM>) surrounding the filter region;
forming the optical filter (<NUM>) in the filter region with a non-continuous coating (<NUM>) on the top layer (<NUM>) of the photoresist layer (<NUM>) having a thickness that is less than <NUM>% of the thickness of the bottom release layer (<NUM>) of the photoresist layer (<NUM>); and
lifting the photoresist layer (<NUM>) with a clean lift-off to provide the multilayer stack (<NUM>) coating without breakage .