Terahertz absorber and method of forming the same

Abstract

Various embodiments may relate to a terahertz absorber. The terahertz absorber may include a reflector layer. The terahertz absorber may also include a first metasurface structure including a first metal layer and a dielectric layer on the first metal layer. The terahertz absorber may also include a spacer layer between the reflector layer and the first metasurface structure. The metasurface absorber may additionally include a second metasurface structure including a second metal layer such that the first metasurface structure and the second metasurface structure are spaced by a gap. The metasurface absorber may also include a metal connection in contact with the reflector layer and the second metasurface structure.

Description

TERAHERTZ ABSORBER AND METHOD OF FORMING THE SAME CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority of Singapore application No. 10202500217S filed January 23, 2025, the contents of it being hereby incorporated by reference in its entirety for all purposes.TECHNICAL FIELD

[0002] Various embodiments of this disclosure may relate to a terahertz absorber. Various embodiments of this disclosure may relate to a method of forming a terahertz absorber.BACKGROUND

[0003] Multispectral terahertz (THz) imagers are vital for water and chemical sensing in space, agricultural and pharmaceutical applications. Higher resolution in the imager is critical for high clarity images of the objects that are being taken as well as to resolve fine features, which ensures that closely spaced objects can be distinguished. The resolution of the imager is determined by the size of the pixel. The smaller the pixel size, higher is the resolution of the imager since more pixels can be packed within the same given area of the imager.

[0004] However, one of the major challenges of achieving high resolution in a THz imager is the long wavelength of THz waves, which inadvertently limits the pixel size. For instance, consider the frequency of 0.3 THz, the wavelength (X) is 1000 pm. Generally, the size of a unit cell of a typical planar absorber, the key component of a pixel, is around X / 4 (= 250 pm) by A / 4 (= 250 pm), which leads to the single pixel area of 62,500 pm2.SUMMARY

[0005] Various embodiments may relate to a terahertz absorber. The terahertz absorber may include a reflector layer. The terahertz absorber may also include a first metasurface structure including a first metal layer and a dielectric layer on the first metal layer. The terahertz absorber may also include a spacer layer between the reflector layer and the first metasurface structure. The metasurface absorber may additionally include a second metasurface structure including a second metal layer such that the first metasurface structure and the second metasurface structure are spaced by a gap. The metasurface absorber may also include a metal connection in contact with the reflector layer and the second metasurface structure.

[0006] Various embodiments may relate to a method of forming a terahertz absorber. The method may include forming a reflector layer. The method may also include forming a first metasurface structure including a first metal layer and a dielectric layer on the first metal layer. The method may further include forming a spacer layer between the reflector layer and the first metasurface structure. The method may additionally include forming a second metasurface structure including a second metal layer such that the first metasurface structure and the second metasurface structure are spaced by a gap. The method may also include forming a metal connection in contact with the reflector layer and the second metasurface structure.BRIEF DESCRIPTION OF THE DRAWINGS

[0007] In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily drawn to scale, emphasis instead generally being placed upon illustrating the principles of various embodiments. In the following description, various embodiments of the invention are described with reference to the following drawings.FIG. 1 shows a general illustration of a terahertz absorber according to various embodiments.FIG. 2 shows a general illustration of a method of forming a terahertz absorber according to various embodiments.FIG. 3A shows a schematic of a terahertz absorber according to various embodiments.FIG. 3B shows (below) a schematic of a portion of the terahertz absorber according to various embodiments as shown in FIG. 3A, with (above) a conventional planar terahertz absorber. FIG. 4A shows a plot of absorption (in percent or %) as a function of frequency (in terahertz or THz) illustrating simulated absorption of the absorber according to various embodiments, with the inset illustrating the surface current traveling through the metal connections, thereby enabling the absorption.FIG. 4B shows (above) parameters of the absorber according to various embodiments; and (below) dimension values of the parameters used in the simulation of the absorber according to various embodiments for the results in FIG. 4A.FIG. 5A shows a plot of absorption (in percent or %) as a function of frequency (in terahertz or THz) illustrating simulated absorption of the conventional planar absorber.FIG. 5B shows (above) parameters of the conventional planar absorber; and (below) dimension values of the parameters used in the simulation of the conventional planar absorber as shown in FIG. 5A.FIG. 5C shows a table comparing the dimensional values of the terahertz absorber according to various embodiments and the conventional planar absorber with similar absorption performances.FIG. 6A shows a table illustrating the dimension values for parameters of the absorber according to various embodiments including a spacer layer of silicon, and the dimension values for parameters of an absorber with air spacer.FIG. 6B shows a plot of absorption (in percent or %) as a function of frequency (in terahertz or THz) illustrating simulated absorption of the absorber according to various embodiments including a spacer layer of silicon and the absorber with air spacer.FIG. 7A shows a table illustrating that as the thickness of the spacer layer of a terahertz absorber according to various embodiments increases from 20 pm to 40 pm, the pixel size can be reduced from 3500 pm2to 1382 pm2, representing a scaling factor relative to the conventional planar absorber from 9 x to 23 x.FIG. 7B shows a plot of absorption (in percent or %) as a function of frequency (in terahertz or THz) illustrating simulated absorption of the absorbers with different thicknesses of spacer layers according to various embodiments.FIG. 7C shows dimension values of the parameters used in the simulation of the terahertz absorber according to various embodiments for the results in FIGS. 7 A - B.FIG. 8A shows a plot of absorption (in percent or %) as a function of frequency (in terahertz or THz) illustrating simulated absorption of a terahertz absorber according to various embodiments at different air gap values between the second metasurface structure and the alumina layer of the first metasurface structure.FIG. 8B shows dimension values of the parameters used in the simulation of the terahertz absorber according to various embodiments for the results in FIG. 8A.FIG. 8C shows a table comparing the tunability of the terahertz absorber according to various embodiments and the tunability of the conventional planar absorber.FIG. 8D shows a schematic of the conventional planar absorber. The absorber may show no degradation of tunable performance.FIG. 9A shows a schematic of the terahertz absorber according to various embodiments. FIG. 9B shows plots of absorption (in percent or %) as a function of frequency (in terahertz or THz) illustrating simulated absorption of a terahertz absorber according to variousembodiments at different air gap values (air gap 2) between the further second metasurface structure and the further first metasurface structure, with the air gap value (air gap 1) between the second metasurface structure and the first metasurface structure fixed at 0 pm.FIG. 9C shows plots of absorption (in percent or %) as a function of frequency (in terahertz or THz) illustrating simulated absorption of the terahertz absorber according to various embodiments at different air gap values (air gap 2) between the further second metasurface structure and the further first metasurface structure, with the air gap value (air gap 1) between the second metasurface structure and the first metasurface structure fixed at 0.3 pm.FIG. 9D shows plots of absorption (in percent or %) as a function of frequency (in terahertz or THz) illustrating simulated absorption of the terahertz absorber according to various embodiments at different air gap values (air gap 2) between the further second metasurface structure and the further first metasurface structure, with the air gap value (air gap 1) between the second metasurface structure and the first metasurface structure fixed at 0.5 pm.FIG. 10A shows a schematic of the terahertz absorber according to various embodiments. FIG. 10B shows a plot of absorption (in percent or %) as a function of frequency (in terahertz or THz) illustrating simulated absorption of a terahertz absorber according to various embodiments at different backside allowances between the two portions of the reflector. FIG. 10C shows dimension values of the parameters used in the simulation of the terahertz absorber according to various embodiments for the results in FIG. 10B.FIG. 11 shows a schematic illustrating a method of forming a terahertz absorber according to various embodiments.FIG. 12 shows a table comparing the terahertz absorber according to various embodiments with conventional solutions.FIG. 13 shows a schematic illustrating a method of forming a terahertz absorber according to various embodiments.DESCRIPTION

[0008] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized, and structural, logical, and electrical changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.

[0009] Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments. Features that are described in the context of an embodiment may correspondingly be applicable to the other embodiments, even if not explicitly described in these other embodiments. Furthermore, additions and / or combinations and / or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.

[0010] In the context of various embodiments, the articles “a”, “an” and “the” as used with regard to a feature or element include a reference to one or more of the features or elements.

[0011] In the context of various embodiments, the terms “about” or “approximately” as applied to a numeric value encompasses the exact value and a reasonable variance, e.g. within 10% of the specified value.

[0012] As used herein, the term “and / or” includes any and all combinations of one or more of the associated listed items.

[0013] By “comprising” it is meant including, but not limited to, whatever follows the word “comprising”. Thus, use of the term “comprising” indicates that the listed elements are required or mandatory, but that other elements are optional and may or may not be present.

[0014] By “consisting of’ is meant including, and limited to, whatever follows the phrase “consisting of’. Thus, the phrase “consisting of’ indicates that the listed elements are required or mandatory, and that no other elements may be present.

[0015] Various embodiments may seek to allow for shrinking of the device in the horizontal direction, thereby achieving a smaller device footprint.

[0016] FIG. 1 shows a general illustration of a terahertz absorber according to various embodiments. The terahertz absorber may include a reflector layer 102 (alternatively referred to as reflector). The terahertz absorber may also include a first metasurface structure 104 including a first metal layer 104a and a dielectric layer 104b on the first metal layer 104a. The terahertz absorber may also include a spacer layer 106 (alternatively referred to as spacer) between the reflector layer 102 and the first metasurface structure 104. The metasurface absorber may additionally include a second metasurface structure 108 including a second metal layer 108a such that the first metasurface structure 104 and the second metasurface structure 108 are spaced by a gap (e.g., an air gap). The metasurface absorber may also include a metal connection 110 in contact with the reflector layer 102 and the second metasurface structure 108.

[0017] In other words, various embodiments may relate to a terahertz absorber having a reflector layer 102, a spacer layer 106 over the reflector layer 102, a first metasurface structure 104 over the spacer layer 106, and a second metasurface structure 108 over the first metasurface structure 104. A gap (e.g., air gap) between the first metasurface structure 104 and the second metasurface structure 108 maybe changed or varied.

[0018] For avoidance of doubt, FIG. 1 serves to provide illustrate certain features of a terahertz absorber, and is not intended to limit, for instance, the dimensions, sizes, orientation etc. of the various features.

[0019] The terahertz absorber may be referred to as a three-dimensional (3D) metasurface absorber, a metamaterial absorber or a 3D metamaterial absorber. The effective size of theterahertz absorber may be determined vertically, allowing shrinkage in the horizonal directions, thereby leading to smaller device footprint. For instance, a lateral length of the absorber may be of any suitable value, e.g., less than 200 pm, e.g., equal or less than 180 pm, e.g., equal or less than 70 pm, e.g., equal or less than 50 pm, and a lateral width of the absorber may also be of any suitable value, e.g., less than 200 pm, e.g., equal or less than 180 pm, e.g., equal or less than 70 pm, e.g., equal or less than 50 pm. Also, there may be higher interaction vertically between the metasurface structures 104, 108 and the spacer layer 106, thereby reducing metasurface size and device area.

[0020] In various embodiments, the metal connection 110 may connect the reflector layer 102 and the second metasurface 108 such that a resonance is excited in response to terahertz waves incident onto the terahertz absorber. The metal connection 110 may extend through an entire thickness of the spacer layer 106 to connect or contact the reflector layer 102 and the second metasurface structure 108.

[0021] In various embodiments, the reflector layer 102 may be patterned such that the reflector layer 102 includes a plurality of pixels, each of the plurality pixels being electrically isolated from other pixels of the plurality of pixels.

[0022] In various embodiments, the reflector 102 may allow for pixelated readout. In various embodiments, the spacer layer 106 may include any suitable dielectric material(s), e.g., silicon, germanium, polyimide, silicon dioxide, aluminum oxide, hafnium oxide, barium titanate (BTO) or any combination thereof. For instance, the spacer layer 106 may include a 10 pm thick silicon sub-layer, a 2 pm thick silicon dioxide sub-layer and a 50 nm thick aluminum oxide sub-layer. The spacer layer 106 may alternatively be referred to as spacer.

[0023] In various embodiments, the reflector layer 102 may include aluminum (Al), molybdenum (Mo) or tungsten (W).

[0024] In various embodiments, the first metal layer 104a may include aluminum (Al), molybdenum (Mo) or tungsten (W).

[0025] In various embodiments, the second metal layer 108a may include aluminum (Al), molybdenum (Mo) or tungsten (W).

[0026] In various embodiments, the dielectric layer 104b may provide electrical isolation between the first metasurface structure 104 and the second metasurface structure 108 when the (air) gap between the first metasurface structure 104 and second metasurface structure 108 is zero. Without the dielectric layer 104b for electrical isolation, the two metasurface structures 104, 108 (i.e., first metal layer 104a of the first metasurface structure 104 and second metal layer 108a of the second metasurface structure 108) may be electrically shorted and a huge surge of current may happen. This may break the device due to high temperatures melting the metasurface structures 104, 108 and welding them together. The dielectric layer 104b may include any suitable material(s), e.g., alumina, silicon nitride, aluminum nitride, silicon dioxide, titanium dioxide, or any combination thereof.

[0027] The metal connection 110 in contact with the reflector layer 102 and the second metasurface structure 108 (i.e., the top metasuface structure) may help to generate resonance of the terahertz waves while the first metasurface structure 104 is used with the second metasurface structure 108 for actuation, thereby increasing the absorption of the terahertz waves and allowing a device which occupies a smaller area / volume to be used without compromising absorption performance. The resonance may be excited by the reflector layer 102 and the second metasurface 108 arranged vertically with respect to each other, and connected by the metal connection 110 extending vertically between the reflector layer 102 and the second metasurface 108. Accordingly, the resonance may be referred to as a vertical resonance or a three-dimensional (3D) resonance.

[0028] In various embodiments, the metal connection 110 may include aluminum or copper. In various embodiments, the metal connection 110 may be a through silicon via structure.

[0029] In various embodiments, the terahertz absorber may be configured such that the (air) gap between the first metasurface structure 104 and the second metasurface structure 108 is controlled for spectral tunability by a stimulus (e.g., a voltage bias applied between the first metal layer 104a of the first metasurface structure 104 and the second metal layer 108a of the second metasurface structure 108, or a current bias). Generally speaking, an external source or equipment such as a power source, a circuit arrangement or a controller may be used to provide the stimulus. In various embodiments, the terahertz absorber may include a controller or circuit arrangement coupled to the first metasurface structure 104 and the second metasurface structure 108, the controller or circuit arrangement configured to adjust the (air) gap by applying the stimulus. For instance, the second metasurface structure 108 may include an actuator (e.g., including a piezoelectric material) such that the second metasurface structure 108 may deflect to change the (air) gap in response to a change in the voltage bias or current bias. Alternatively, the second metasurface structure 108 may be deflected via electrostatic actuation.

[0030] In various embodiments, the terahertz absorber may include a further first metasurface structure including a further first metal layer and a further dielectric layer on the further first metal layer such that the spacer layer 106 is between the reflector layer 102 and the further first metasurface structure. The terahertz absorber may also include a further second metasurface structure including a further second metal layer such that the further first metasurface structure and the further second metasurface structure are spaced by a further (air) gap. The terahertz absorber may additionally include a further metal connection in contact with the reflector layer 102 and the further second metasurface structure. The further metal connection may extend through an entire thickness of the spacer layer 106 to connect the reflector layer 102 and the further second metasurface structure. The second metasurfacestructure and the further second metasurface structure may be spaced or separated by a lateral gap.

[0031] To achieve independent tuning of the (air) gap and the further (air) gap, different voltages can be applied between first metasurface structure 104 and the second metasurface structure 108, and between the further first metasurface structure and the further second metasurface structure.

[0032] One common voltage may be applied to the second metasurface structure 108 and the further second metasurface structure since they are electrically connected via the reflector 102. Different voltages can then be applied to the first metasurface structure 104 and the further first metasurface structure.

[0033] FIG. 2 shows a general illustration of a method of forming a terahertz absorber according to various embodiments. The method may include, in 202, forming a reflector layer. The method may also include, in 204, forming a first metasurface structure including a first metal layer and a dielectric layer on the first metal layer. The method may further include, in 206, forming a spacer layer between the reflector layer and the first metasurface structure. The method may additionally include, in 208, forming a second metasurface structure including a second metal layer such that the first metasurface structure and the second metasurface structure are spaced by a gap, e.g., an air gap. The method may also include, in 210, forming a metal connection in contact with the reflector layer and the second metasurface structure.

[0034] In other words, the method may relate to a method of forming a terahertz absorber including a reflector layer, a first metasurface structure, a spacer layer, a second metasurface structure and a metal connection.

[0035] For avoidance of doubt, FIG. 2 is intended to provide a general illustration of steps of forming a terahertz absorber according to various embodiments, and is not intended to limit the sequence of the various steps.

[0036] In various embodiments, the reflector layer may be formed on a handle substrate. The spacer layer may be formed on the reflector layer after forming the reflector layer.

[0037] In various embodiments, the first metasurface structure may be formed on the spacer layer after forming the spacer layer. Patterning may be carried out to form the first metasurface structure such that a portion of the underlying spacer layer is exposed. The method may further include forming a sacrificial layer including a sacrificial material in contact with the first metasurface layer and the exposed portion of the underlying spacer layer.

[0038] In various embodiments, forming the metal connection may include etching through the sacrificial layer and the spacer layer to form a via hole. Forming the metal connection may also include depositing a metal into the via hole and on or over the sacrificial layer. Forming the metal connection may additionally include removing the metal deposited on the sacrificial layer using etching or grinding to expose the sacrificial layer. The metal deposited into the via hole may form the metal connection.

[0039] In various embodiments, the second metasurface structure may be formed on the sacrificial layer and in contact with the metal connection. The sacrificial layer may be removed after forming the second metasurface structure.

[0040] In various embodiments, the metal connection may connect the reflector layer and the second metasurface such that a resonance is excited in response to terahertz waves incident onto the terahertz absorber. In various embodiments, the metal connection may be a through silicon via structure formed via a through silicon via process.

[0041] In various embodiments, the terahertz absorber may be configured such that the (air) gap between the first metasurface structure and the second metasurface structure is controlled for spectral tunability by a stimulus. The method may include coupling a controller or circuit arrangement to the first metasurface structure and the second metasurface structure, the controller or circuit arrangement configured to adjust the (air) gap by applying the stimulus.

[0042] In various embodiments, the reflector layer may be patterned such that the reflector layer includes a plurality of pixels, each of the plurality pixels being electrically isolated from other pixels of the plurality of pixels.

[0043] Various embodiments may relate to a terahertz absorber including first metasurface structures and second metasurface structures separated from the first metasurface structures by a gap (i.e., air gap). Second metasurface structures (also referred to as top metasurface structures) may each be or include a metal plate suspended over the first metasurface structures (also referred to as bottom metasurface structures), which may include an alumina-coated metal layer patterned and fixed to a spacer layer. The second metasurface structures may be electrically connected to a bottom reflector layer via metal connections.

[0044] By connecting the second metasurface structures with the reflector using metal connections, a vertical metasurface absorber may be realized. The effective size of the absorber may now be determined vertically, hence allowing shrinkage in horizontal directions, leading to smaller device footprint. In addition, the high interaction vertically between the three-dimensional metasurface and the dielectric spacer may also reduce the metasurface size and hence device area.

[0045] FIG. 3A shows a schematic of a terahertz absorber according to various embodiments. The terahertz absorber may include a reflector layer 302. The terahertz absorber may also include a first metasurface structure 304 including a first metal layer 304a and a dielectric layer 304b on the first metal layer 304a. The terahertz absorber may also include a spacer layer 306 between the reflector layer 302 and the first metasurface structure 304. The terahertz absorber may additionally include a second metasurface structure 308 including a second metal layer 308a such that the first metasurface structure 304 and the second metasurface structure 308 are spaced by a gap (e.g., an air gap). The terahertz absorber may also include a metal connection 310 in contact with the reflector layer 302 and the secondmetasurface structure 308. The terahertz absorber may further include a further first metasurface structure 304’ including a further first metal layer 304a’ and a further dielectric layer 304b’ on the further first metal layer 304a’ such that the spacer layer 306 is between the reflector layer 302 and the further first metasurface structure 304’. The terahertz absorber may additionally include a further second metasurface structure 308’ including a further second metal layer 308a’ such that the further first metasurface structure 304’ and the further second metasurface structure 308’ are spaced by a further gap (e.g., a further air gap). The terahertz absorber may also include a further metal connection 310’ in contact with the reflector layer 302 and the further second metasurface structure 308’. The second metasurface structure 308 and the further second metasurface structure 308’ may be spaced or separated by a lateral gap. Likewise, the first metasurface structure 304 and the further first metasurface structure 304’ may be spaced or separated by the lateral gap.

[0046] Terahertz waves may be incident at the top of the absorber with the metasurface structures 308, 308’, which may be designed to prevent reflection. On the other hand, the reflector layer 302 may prevent or reduce transmission. By optimizing the thickness of the spacer layer 306, the terahertz waves reflected from the reflector layer 302 may destructively interfere at the metasurface structures 308, 308’. The absorption mode may be manifested as a surface current which circulates between the metasurface structures 308, 308’, the metal connections 310, 310’ and the reflector layer 302.

[0047] FIG. 3B shows (below) a schematic of a portion of the terahertz absorber according to various embodiments as shown in FIG. 3A, with (above) a conventional planar terahertz absorber. As mentioned earlier, the terahertz absorber according to various embodiments may include a reflector layer 302, a first metasurface structure including a first metal layer 304a and a dielectric (e.g., alumina) layer 304b on the first metal layer 304a. The terahertz absorber may also include a spacer layer 306 between the reflector layer 302 and the first metasurfacestructure. The terahertz absorber may additionally include a second metasurface structure 308 including a second metal layer such that the first metasurface structure and the second metasurface structure 308 are spaced by a gap (e.g., an air gap). The terahertz absorber may also include a metal connection 310 in contact with the reflector layer 302 and the second metasurface structure 308. The reflector layer 302 may be on or over a handle substrate 312.

[0048] The terahertz absorber according to various embodiments may have a reduced lateral size as compared to the conventional planar terahertz absorber.

[0049] The surface current is shown in the inset of FIG. 4A. FIG. 4A shows a plot of absorption (in percent or %) as a function of frequency (in terahertz or THz) illustrating simulated absorption of the absorber according to various embodiments, with the inset illustrating the surface current traveling through the metal connections, thereby enabling the absorption. FIG. 4B shows (above) parameters of the absorber according to various embodiments; and (below) dimension values of the parameters used in the simulation of the absorber according to various embodiments for the results in FIG. 4A.

[0050] Simulations - Comparison with Conventional Solutions

[0051] To compare the size reduction of the absorber according to various embodiments with a conventional planar absorber, the conventional planar absorber with similar performance may be simulated. FIG. 5A shows a plot of absorption (in percent or %) as a function of frequency (in terahertz or THz) illustrating simulated absorption of the conventional planar absorber. FIG. 5B shows (above) parameters of the conventional planar absorber; and (below) dimension values of the parameters used in the simulation of the conventional planar absorber as shown in FIG. 5A. FIG. 5C shows a table comparing the dimensional values of the terahertz absorber according to various embodiments and the conventional planar absorber with similar absorption performances. FIG. 5C shows that the footprint of the conventional planar absorber is 9.3x that of the absorber according to various embodiments.

[0052] As mentioned above, the surface current induced by the absorption may run through the dielectric spacer layer, and may hence be influenced by the dielectric constant of the spacer layer. Simulations may be carried out using silicon (cr= 11.9) and air (cr= 1) for the spacer layer. FIG. 6A shows a table illustrating the dimension values for parameters of the absorber according to various embodiments including a spacer layer of silicon, and the dimension values for parameters of an absorber with air spacer. FIG. 6B shows a plot of absorption (in percent or %) as a function of frequency (in terahertz or THz) illustrating simulated absorption of the absorber according to various embodiments including a spacer layer of silicon and the absorber with air spacer.

[0053] Simulations - Thickness of Spacer Layer

[0054] As mentioned above, the effective size of the absorber may be determined vertically. Various embodiments may enable the further shrinkage of the pixel size by increasing the thickness of the spacer layer. FIG. 7A shows a table illustrating that as the thickness of the spacer layer of a terahertz absorber according to various embodiments increases from 20 pm to 40 pm, the pixel size can be reduced from 3500 pm2to 1382 pm2, representing a scaling factor relative to the conventional planar absorber from 9 x to 23 x. FIG. 7B shows a plot of absorption (in percent or %) as a function of frequency (in terahertz or THz) illustrating simulated absorption of the absorbers with different thicknesses of spacer layers according to various embodiments. FIG. 7C shows dimension values of the parameters used in the simulation of the terahertz absorber according to various embodiments for the results in FIGS. 7 A - B. The values for Px, Py and spacer thickness may be varied.

[0055] Simulations - Spectral Tunability

[0056] The air gap between the second metasurface structure and the alumina layer of the first metasurface structure may be varied. Using tuning mechanisms / methods such as microelectromechanical systems (MEMS)-based mechanisms / methods, the air gap may be varied,e.g., by changing voltage bias applied between the first metasurface structure and the second metasurface structure. FIG. 8A shows a plot of absorption (in percent or %) as a function of frequency (in terahertz or THz) illustrating simulated absorption of a terahertz absorber according to various embodiments at different air gap values between the second metasurface structure and the alumina layer of the first metasurface structure. FIG. 8B shows dimension values of the parameters used in the simulation of the terahertz absorber according to various embodiments for the results in FIG. 8A. The values for air gap may be varied. As shown in FIG. 8A, when the air gap value is changed from 0 to 0.5 pm, the absorption frequency moves towards higher frequencies. At the same time, there is a slight decrease in the peak absorption strength. The tunability range may be refined as the change in frequency where the absorption is > 85% . Using such a definition, the terahertz absorber according to various embodiments may be found to display a tunability of 0.0891 THz or 28.74% of the initial absorption frequency. The absorber may show little or no degradation of tunable performance. In comparison, a conventional planar absorber shows a tunability of 0.0738 THz or 24.52% of the initial absorption frequency. FIG. 8C shows a table comparing the tunability of the terahertz absorber according to various embodiments and the tunability of the conventional planar absorber. FIG. 8D shows a schematic of the conventional planar absorber.

[0057] In various embodiments, the two air gaps of the terahertz absorber may be independently tuned. FIG. 9A shows a schematic of the terahertz absorber according to various embodiments. To avoid clutter, not all features of the terahertz absorber are labelled in FIG.9A. The terahertz absorber according to various embodiments may have two air gaps, with the first air gap (alternatively referred to as air gap or air gap 1) between the second metasurface structure 908 and the alumina layer of the first metasurface structure 904, and the second air gap (alternatively referred to as further air gap or air gap 2) between the further secondmetasurface structure 908’ and the further alumina layer of the further first metasurface structure 904’. Accordingly, a high tuning resolution may be achieved.

[0058] FIG. 9B shows plots of absorption (in percent or %) as a function of frequency (in terahertz or THz) illustrating simulated absorption of a terahertz absorber according to various embodiments at different air gap values (air gap 2) between the further second metasurface structure and the further first metasurface structure, with the air gap value (air gap 1) between the second metasurface structure and the first metasurface structure fixed at 0 pm. FIG. 9C shows plots of absorption (in percent or %) as a function of frequency (in terahertz or THz) illustrating simulated absorption of the terahertz absorber according to various embodiments at different air gap values (air gap 2) between the further second metasurface structure and the further first metasurface structure, with the air gap value (air gap 1) between the second metasurface structure and the first metasurface structure fixed at 0.3 pm. FIG. 9D shows plots of absorption (in percent or %) as a function of frequency (in terahertz or THz) illustrating simulated absorption of the terahertz absorber according to various embodiments at different air gap values (air gap 2) between the further second metasurface structure and the further first metasurface structure, with the air gap value (air gap 1) between the second metasurface structure and the first metasurface structure fixed at 0.5 pm.

[0059] Sub-pixel, pixel or sub-array level control and readout

[0060] Each unit cell of the terahertz absorber may be isolated to enable pixelized control and readout. FIG. 10A shows a schematic of the terahertz absorber according to various embodiments. To avoid clutter, not all features of the terahertz absorber are labelled in FIG.10A. The reflector layer 1002 may be patterned, thereby allowing independent access to each unit cell. The reflector layer may include a first portion 1002a, a second portion 1002b and an edge portion 1002c surrounding portions 1002a, 1002b. One unit cell may include the first portion 1002a of the reflector layer 1002, the metal connection 1010, the second metasurfacestructure 1008, and the first metasurface structure 1004, as well as the second portion 1002b of the reflector layer 1002, the further metal connection 1010’, the further second metasurface structure 1008’, and the further first metasurface structure 1004’. The edge portion 1002c may electrically isolate the unit cell from neighboring unit cells. The edge portion 1002c may include any suitable material, such as silicon oxide (SiCh), alumina, or high resistivity (HR) silicon. The edge portion 1002c may have a width of a predetermined value (denoted as “Backside Allowance”) along the width of the reflector layer 1002 and a width of half the predetermined value (denoted as “0.5*Backside Allowance”) along the length of the reflector layer 1002. In various embodiments, the first portion 1002a and the second portion 1002b may be electrically isolated from each other. In various other embodiments, the first portion 1002a and the second portion 1002b may not be electrically isolated from each other. In such a case, in order to implement individual control, a common voltage may be applied to the reflector layer 1002, and different voltages may be applied to the first metasurface structure 1004 and the further first metasurface structure 1004’.

[0061] FIG. 10B shows a plot of absorption (in percent or %) as a function of frequency (in terahertz or THz) illustrating simulated absorption of a terahertz absorber according to various embodiments at different backside allowances between the two portions of the reflector. FIG.10C shows dimension values of the parameters used in the simulation of the terahertz absorber according to various embodiments for the results in FIG. 10B.

[0062] FIG. 11 shows a schematic illustrating a method of forming a terahertz absorber according to various embodiments. To avoid clutter, only one instance of the same element is labelled. A handle substrate 1112 may be bonded with a metal layer 1102, which acts as the reflector layer. The handle substrate 112 may be, for instance, a silicon, glass or sapphire wafer. The metal layer 1102 may include any suitable metal such as aluminum (Al), molybdenum (Mo) or tungsten (W). A spacer layer 1106 may be formed on the reflector. The spacer layer1106 may include any suitable dielectric material, such as high-resistivity (HR) silicon (Si), germanium (Ge), barium titanate (BTO) or hafnium oxide (HfCh). A metal layer 1104a’ may be formed on the spacer layer 1106, and an alumina layer 1104b’ may be formed on the metal layer 1104a’. The metal layer 1104a’ and the alumina layer 1104b’ may be patterned to form the first metal layer 1104a and the dielectric layer 1104b, respectively, of the first metasurface structure. A sacrificial material (e.g., silicon oxide (SiCh)) may be deposited over the first metal layer 1104a, the dielectric layer 1104b, and the spacer layer 1106, and planarized to form a sacrificial layer 1114. A via hole may be formed extending through the sacrificial layer 1114 and the spacer layer 1106 to expose a portion of the reflector layer 1102. The via hole may be filled by a metal (e.g., aluminum (Al) or copper (Cu)) to form a metal connection 1110. A metal (e.g., aluminum (Al), molybdenum (Mo) or tungsten (W)) may be deposited to form the second metasurface structure 1108. The sacrificial layer 1114 may then be removed via selective etching.

[0063] FIG. 12 shows a table comparing the terahertz absorber according to various embodiments with conventional solutions.

[0064] FIG. 13 shows a schematic illustrating a method of forming a terahertz absorber according to various embodiments. To avoid clutter, not all instances of the same element have been labelled. In (i), a dielectric layer 1316 (including a dielectric material such as silicon oxide (SiCh)) may be formed on a spacer substrate 1306’. In (ii), a portion of the dielectric layer 1316 and a portion of the spacer substrate 1306’ (underlying the portion of the dielectric layer 1316) may be etched to form a through silicon via (TSV) hole 1310’. In (iii), a suitable liner oxide may be deposited to form a liner oxide layer 1318 in the TSV hole 1310’. The liner oxide layer 1318 may cover sidewalls and a bottom surface of the TSV hole 1310’. In (iv), a metal structure 1310” including a metal such as copper may be formed via electroplating such that a first portion of the metal structure 1310” is within the TSV hole 1310’ and a second portion of themetal structure 1310” is on the dielectric layer 1316. In (v), chemical mechanical polishing (CMP) may be carried out such that the second portion of the metal structure 1310” on the dielectric layer 1316 is removed to form through silicon via (TSV) structure 1310 within TSV hole 1310’. The dielectric layer 1316 may act as a stop layer. In (vi), a reflector layer 1302 may be formed in contact with the TSV structure 1310 and the dielectric layer 1316, and a bonding stack 1318 may be formed in contact with the reflector layer 1302. In (vii), the bonding stack 1318 may be used as an interface to bond to a handle substrate 1312. The handle substrate 1312 may include a suitable material such as glass or silicon (Si). In (viii), the spacer substrate 1306’ may undergo backgrinding to form thinned spacer substrate 1306”. The backgrinding may be stopped before the TSV structure 1310 is exposed, to prevent or reduce damage to the TSV structure 1310. In (ix), a dry isotropic etch may be used to further etch the thinned spacer substrate 1306” to form spacer layer 1306 and expose the TSV structure 1310. The portion of the liner oxide layer 1318 covering the bottom surface of the initial TSV hole 1310’ may also be removed. In (x), a first metasurface structure 1304 including a first metal layer and a dielectric layer on the first metal layer may be formed on the spacer layer 1306. Metal may first be deposited on the spacer layer 1306, followed by depositing a dielectric material (e.g., alumina) on the deposited metal layer, before etching the deposited metal and dielectric material to form the first metal layer and the dielectric layer. In (xi), a sacrificial material may be deposited over the first metasurface structure 1304 and the spacer layer 1306 and polished to form sacrificial layer 1314. Polishing may help to planarize the surface of the sacrificial layer 1314 and expose a surface of the TSV structure 1310. In (xii), metal may be deposited on the sacrificial layer 1314 and exposed surface of the TSV structure 1310, and etched to form a second metasurface structure 1308. The second metasurface structure 1308 maybe in electrical connection with the TSV structure 1310. In (xiii), the sacrificial layer 1314 may be removed via selective etching and the second metasuface structure 1308 may be released.

[0065] Various embodiments may relate to a terahertz absorber with reduced form factor. The introduction of metal connections between the top metasurface structure and the reflector layer may introduce verticality in the definition of the size of the 3D structure. The traditional (i.e., planar) approach limits the scaling of the metasurface in the horizontal direction, resulting in a large lateral footprint / surface area. In contrast, various embodiments may result in reduced lateral dimensions.

[0066] By using high electric constant material (e.g., silicon) for the spacer layer, the lateral area of the terahertz absorber may further be reduced. For instance, an absorber including a silicon spacer layer may occupy 7.3 x smaller layer compared to an absorber using air as spacer.

[0067] Tunability within each unit cell may be individually controlled for multispectral terahertz absorption.

Claims

1. A terahertz absorber comprising:a reflector layer;a first metasurface structure comprising a first metal layer and a dielectric layer on the first metal layer;a spacer layer between the reflector layer and the first metasurface structure; a second metasurface structure comprising a second metal layer such that the first metasurface structure and the second metasurface structure are spaced by a gap; anda metal connection in contact with the reflector layer and the second metasurface structure.

2. The terahertz absorber according to claim 1,wherein the metal connection connects the reflector layer and the second metasurface such that a resonance is excited in response to terahertz waves incident onto the terahertz absorber.

3. The terahertz absorber according to claim 1,wherein the terahertz absorber is configured such that the gap between the first metasurface structure and the second metasurface structure is controlled for spectral tunability by a stimulus.

4. The terahertz absorber according to claim 3, further comprising:a controller or circuit arrangement coupled to the first metasurface structure and the second metasurface structure, the controller or circuit arrangement configured to adjust the gap by applying the stimulus.

5. The terahertz absorber according to claim 1,wherein the reflector layer is patterned such that the reflector layer comprises a plurality of pixels, each of the plurality pixels being electrically isolated from other pixels of the plurality of pixels.

6. The terahertz absorber according to claim 1,wherein the spacer layer comprises silicon, germanium, polyimide, silicon dioxide, aluminum oxide, hafnium oxide, barium titanate (BTO) or any combination thereof.

7. The terahertz absorber according to claim 1,wherein the metal connection comprises aluminum or copper.

8. The terahertz absorber according to claim 1,wherein the reflector layer comprises aluminum, molybdenum or tungsten.

9. The terahertz absorber according to claim 1,wherein the first metal layer comprises aluminum, molybdenum or tungsten; and wherein the second metal layer comprises aluminum, molybdenum or tungsten.

10. The terahertz absorber according to claim 1, further comprising:a further first metasurface structure comprising a further first metal layer and a further dielectric layer on the further first metal layer such that the spacer layer is between the reflector layer and the further first metasurface structure;a further second metasurface structure comprising a further second metal layer such that the further first metasurface structure and the further second metasurface structure are spaced by a further gap; anda further metal connection in contact with the reflector layer and the further second metasurface structure.

11. A method of forming a terahertz absorber, the method comprising:forming a reflector layer;forming a first metasurface structure comprising a first metal layer and a dielectric layer on the first metal layer;forming a spacer layer between the reflector layer and the first metasurface structure;forming a second metasurface structure comprising a second metal layer such that the first metasurface structure and the second metasurface structure are spaced by a gap; andforming a metal connection in contact with the reflector layer and the second metasurface structure.

12. The method according to claim 11,wherein the reflector layer is formed on a handle substrate; andwherein the spacer layer is formed on the reflector layer after forming the reflector layer.

13. The method according to claim 12,wherein the first metasurface structure is formed on the spacer layer after forming the spacer layer;wherein patterning is carried out to form the first metasurface structure such that a portion of the underlying spacer layer is exposed; andwherein the method further comprises forming a sacrificial layer including a sacrificial material in contact with the first metasurface layer and the exposed portion of the underlying spacer layer.

14. The method according to claim 13,wherein forming the metal connection comprises:etching through the sacrificial layer and the spacer layer to form a via hole;depositing a metal into the via hole and on the sacrificial layer; and removing the metal deposited on the sacrificial layer using etching or grinding to expose the sacrificial layer, the metal deposited into the via hole forming the metal connection.

15. The method according to claim 14,wherein the second metasurface structure is formed on the sacrificial layer and in contact with the metal connection.

16. The method according to claim 15,wherein the sacrificial layer is removed after forming the second metasurface structure.

17. The method according to claim 11,wherein the metal connection connects the reflector layer and the second metasurface such that a resonance is excited in response to terahertz waves incident onto the terahertz absorber.

18. The method according to claim 11,wherein the terahertz absorber is configured such that the gap between the first metasurface structure and the second metasurface structure is controlled for spectral tunability by a stimulus.

19. The method according to claim 18, further comprising:coupling a controller or circuit arrangement to the first metasurface structure and the second metasurface structure, the controller or circuit arrangement configured to adjust the gap by applying the stimulus.

20. The method according to claim 11,wherein the metal connection is a through silicon via structure formed via a through silicon via process.

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