NL2016081B1 - Photonic Crystal Mirrors on Tethered Membrane Resonator. - Google Patents

Abstract

The present invention is in the field of photonic crystal mirror systems, which combines a tethered membrane and photonic crystal arrays of pillars in one device. Photonic crystal mirror systems focus on the interaction between light and mechanical motion on low energy scales. Photonic crystal (optomechanical) mirror systems may be used in several applications, such as for testing macroscopic quantum physics, high precision sensors, e.g. chemical sensors, or accelerometers.

Description

Title: Photonic Crystal Mirrors on Tethered Membrane Resonator

FIELD OF THE INVENTION

The present invention is in the field of photonic crystal mirror systems, which combine a tethered membrane and photonic crystal array in one device.

BACKGROUND OF THE INVENTION

Photonic crystal mirror systems are considered to focus on the interaction between light and mechanical motion on low energy scales. In order to have enhanced sensing capabilities and to have mechanical resonators which may be find application in optomechanical experiments, one needs a high reflectivity, a low noise, a low dissipation and a small resonator mass. Achieving all of these in one device has been a fundamental challenge to the physics and sensing community up to now. A measure for the performance of mechanical properties is the mechanical quality factor. The mechanical quality factor is typically defined as the energy stored in the membrane related to the energy lost, per cycle. In other words, the mechanical quality factor is a measure of the strength against the damping of its oscillators. Since the mechanical quality factor is defined as a ratio of energy gained to energy lost, it is a dimensionless parameter and it is typically in order of 106 for prior art devices. An alternative method to measure the quality factor of a resonator is to measure its so-called ring-down time.

Various investigations have been done on optomechanical systems. An example thereof is a publication by Kleckner et al. in "Optomechanical trampoline resonators", Opt. Express 19, 19708-19716 (2011), which describe a trampoline resonator with multi-layer stacks of dielectrics to form Bragg grating mirrors. Despite their high reflectivities, large mechanical systems are still required to suspend the large masses of the resonators. Another problem with Bragg grating mirrors on trampoline structures is their high mechanical dissipation.

Another recent research on optomechanical systems is a publication by Makles et al. in "2D photonic-crystal optomechanical nano resonator" Opt. Lett.40, 174-177 (2015), which describes photonic crystal mirrors. Therein photonic crystal have also been fabricated on fully clamped square membranes which are not tethered to the substrate. While these are highly reflective (exceeding 99%), they do not exhibit the same mechanical isolation observed in trampoline structures.

It is noted that clamping can reduce the mechanical quality factor.

Another recent research focuses on an optomechanical system combining a tethered trampoline membrane and photonic crystal arrays of holes. Norte et al. in "Mechanical resonators for quantum optomechanics experiments at room temperature" describes that despite having a high reflectivity, the mentioned optomechanical system, with photonic crystal arrays of holes does not exhibit a high mechanical dissipation.

In addition to the above problems, most of optomechanical systems (can only) operate at cryogenic temperatures, imposing severe technical challenges and fundamental constraints.

Photonic crystal (optomechanical) mirror systems may be used in several applications, such as for testing macroscopic quantum physics, high precision sensors, e.g. chemical sensors, or accelerometers.

The present invention therefore relates to an improved optomechanical system, which solves one or more of the above problems and drawbacks of the prior art, providing reliable results, without jeopardizing functionality and advantages.

SUMMARY OF THE INVENTION

It is an object of the invention to overcome one or more limitations of the devices of the prior art and at the very least to provide an alternative thereto.

In a first aspect, the invention relates to a photonic crystal mirror assembly according to claim 1. The present invention relates to an optomechanical system with low mass, low dissipation, low noise and high mechanical quality factor. The quality factor of a mechanical resonator is considered to be a parameter of prime importance for different applications e.g. sensing application and in the present application is in the order of 108. Typical values obtained are from 1.1-5.0 10s, such as 2-3.5 108. A photonic crystal mirror assembly (see fig. 1; 100) comprising a tethered 2-500 nm thin film membrane resonator 12 comprising a tether material with a tensile strength > 0.5 GPa. The tethers are connected to a substrate 17 and to the membrane typically comprising in a central location thereof the mirror. The mirror is a patterned photonic crystal array having an array with reflective pillars 14. A typical membrane window cross-sectional dimension is 50 pm to 5mm. The surface area 5 of the pillars and height 6 of the pillars are adapted for reflecting light, which acts as a high reflectivity mirror for impinging light, e.g. a free space laser. The array of pillars is preferably periodical in at least one dimension, preferably in two dimensions, i.e. forming a regular row or column and preferably regular rows and columns. The tethers and membrane are relatively thin and preferably as thin as possible, having a thickness of 3-250 nm, preferably 4-100 nm, more preferably 5-50 nm, such as 10-20 nm. The tethers and membrane are thus typically much thinner than the pillars are tall.

The system comprises a tethered thin film membrane resonator comprising a tethered material with a high tensile strength >0.5 GPa. A tethered structure is used to reduce a clamping area to the substrate. A high tensile strength material is found to improve the mechanical quality factor.

Typical dimensions of tethers are a length of 10-2000 pm, preferably 20-1000 pm, more preferably 50-500 pm, such as 100-250 pm, a width of 0.2-200 pm, preferably 1-100 pm, more preferably 2-50 pm, such as 10-25 pm, and a thickness of 3-250 nm, preferably 4-100 nm, more preferably 5-50 nm, such as 10-20 nm.

The system comprises a patterned photonic crystal array functioning as a mirror, which is typically in a central location of the mirror. Such a photonic crystal array is made of pillars together forming a combined mirror. The pillars preferably have a circular i.e. cylindrical shape to have minimized scattering and losses, but may also be square, rectangular, or multigonal, such as hexagonal. The present pillars preferably also have a contracting characteristics, meaning that they relax in such a way that they contract (diminish in size) slightly and thereby increase the stress.

The membrane in the present invention could be as thin as 2-500 nm, but typically is 2-100 nm in thickness, such as 50 nm. It is considered that the thinner membranes gets, the thicker a substrate may be required; from a practical point of view this is achievable.

In order to have a high reflectiveness the pillars are adapted, typically in both their height and top surface area (per pillar). By optimally adapting the pillars it has been found that every photon can be reflected. A second aspect of the invention relates to a method of making a photonic crystal mirror assembly according to claim 14, comprising various steps, e.g. of providing a substrate, such as Si, and glass. The Si substrate thickness is in range of 100 μιη-20 mm, such as 200 ym-l mm. Thereafter a step of depositing a high stress layer, such as a SiN layer. The stress layer may have a thickness of 100-400 nm. The high stress layer is thereafter patterned to form pillars and tethers, such as by using an electron beam resist for the pillars. The Electron beam resist used in the present invention may be a standard E-beam resist, i.e. ZEP. The resist is patterned using electron beam lithography to define various elements of the present system, such as a window. A system used may be EBPG 5000 or EBPG 5200 (of Raith). Electron beam lithography is also used for fabrication of the present thin tethers. It has been found that by using a smaller wavelength of electrons, compared to the wavelength of light used in photo-lithography, a higher resolution compared to photo-lithography is obtained. Photo-lithography could also be used, but such would imply a need of more time and additional calibration. After patterning the resist is developed and rinsed. Then directional plasma etching is performed to form the array of pillars, e.g. by using CHF3, or C4F8/SF6. In view of time needed for etching it is noted that an etching rate of a material such as Si3N4 is around 1 nm/sec. For releasing the membrane and tethers a wet etch process may be used. A temperature used for wet etching is found to be not very critical and could therefore be from room temperature up to 80 °C, e.g. 75 °C. Wet etching is used in this step due to higher aspect ratio achieved by wet etching compared to dry etching.

By using this release step it has been found that the stress at the clamps can be engineered, i.e. the stress is increased up to values well above 1 GPa.

The present invention provides a solution to one or more of the above mentioned problems and overcomes drawbacks of the prior art.

Advantages of the present description are detailed throughout the description.

DETAILED DESCRIPTION OF THE INVENTION

In an exemplary embodiment of the present photonic mirror assembly, for a given wavelength or range of wavelengths the pillars have a height 6 which is 0.10-0.50 * wavelength or 0.10-0.50 * weighted mean of said range of wavelengths, respectively. It has been found that the best results in terms of reflectance (%) is when the radius or cross-sectional length of the pillars is about 0.25-0.5* of a wavelength to be reflected, preferably 0.3-0.45, more preferably 0.33-0.40 of said wavelength. For instance for a wavelength of 1550 nm the height is at least 150 nm, preferably 200 nm or more.

Likewise, if a wavelength range is provided, such as from 360-600 nm with a weighted mean of 480 nm the height is preferably 160 nm. It is noted that a height may be smaller than indicated. For instance in the above example for a wavelength of 1550 nm a height of 100 nm may be selected; however the reflectance than drops to about 90%; even smaller pillars, such as 50 nm high, result in even lower reflectance (e.g. 80%). For some applications such lower reflectance's could still be acceptable.

Suitable mirrors can be made for optical wavelength in a range of 210-3000 nm. For each wavelength a suitable reflective material can be selected.

In an exemplary embodiment of the present photonic mirror assembly for a given wavelength or range of wavelengths the pillars have a cross-sectional length 7 which is 0.25-0.50 * wavelength or 0.25-0.50 * weighted mean of said range of wavelengths, respectively. A similar calculation as above for the height may be applied.

The space area 8 between the pillars (i.e. total mirror area minus the sum of pillar areas) is in a range of 35-97% of a surface area 13 of the mirror, preferably 50-80%, such as 60-75%, and the remainder of the surface area of the mirror is thus being formed by a top surface of the pillars. In a similar approach a lattice constant may be used as a reference, the lattice constant being a distance between two adjacent or periodical pillars for one dimension. The radius of the pillars is preferably selected such that it falls in the range of 0.1-0.45 * lattice constant, preferably 0.12-0.4, more preferably 0.15-0.35, such as 0.2-0.3 * lattice constant.

To reduce losses and scattering and to have a high reflectivity, the height and width of the preferably circular pillars are substantially the same and tuned to be around 0.25-0.50 * wavelength, typically 0.33-0.4*wavelength. The said pillars may be ordered periodically. The photonic crystal mirrors made by using pillars are able to achieve a reflectivity exceeding 99.9% without the need to fabricate expensive and complicated multilayer stacks of dielectrics.

The said photonic crystal mirrors can be easily incorporated in the nanofabrication process for trampoline membranes without a requirement to any additional step.

In an exemplary embodiment of the present photonic mirror assembly the resonator is attached to a support by at least two, preferably at least four, tethers, such as in a "trampoline" structure. The tethers are preferably attached to the support and membrane in a regular manner, e.g. in every corner of a rectangular support. In case of four tethers, the overhang may be minimized further due to the nature of a rectangular lattice etching of <100> Silicon.

In an exemplary embodiment of the present photonic mirror assembly the resonator is a MEMS and/or NEMS. Micro electro mechanical systems (MEMS) is a technology for very small devices. The present invention, having a MEMS structure can be patterned using techniques from the semiconductor industry. Likewise, Nano electromechanical systems (NEMS) relates to combining electrical and mechanical systems on the nanoscale. Nowadays technologies for producing MEMS and NEMS at least partly overlap.

In an exemplary embodiment of the present photonic mirror assembly the assembly is an on-chip assembly. The photonic mirror assembly may be an integrated on-chip device, which may make it easy to integrate to an experimental and sensing setup. As a result, with only some fabrication effort, the present system has improved characteristics, i.e. a good reflectivity (> 99.99%), a low mass, a low noise, and a low dissipation, compared to the prior art. In general the mechanical characteristics are reflected in the mechanical quality factor.

In an exemplary embodiment of the present photonic mirror assembly the resonator is a mechanical resonator. Mechanical resonators can be used in circuits to generate signals of a precise frequency. One example of a mechanical resonator is piezoelectric resonator made from quartz. Among different metrics in mechanical resonators, frequency and quality factors are considered the most important ones.

In an exemplary embodiment of the present photonic mirror assembly the membrane is formed of a material selected from S13N4, S1O2, SiC, InGaP, Si, and combinations thereof. It is found that by depositing a layer, such as S13N4, by low pressure chemical vapour deposition (LPCVD) with high (silicon) substrate temperatures (300 - 700 °C) and low deposition rates (0.5-5 nm/min.) increased condensation reactions during film growth and high stress film are obtained.

In an exemplary embodiment of the present photonic mirror assembly the tensile stress is >1 GPa, preferably > 2 GPa. Maximum values that were obtained are in the order of 4-5 GPa, i.e. close to rupture of the layers. Such high values are obtained by not relaxing the layers deposited and optionally also further increasing the stress, especially of the tethers. It has been found that in high stress membranes mechanical frequencies are achieved which are independent of thickness and are stress dominated. It is noted that an ultimate yield strength of a S13N4 thin films is about 6 GPa, which may be a result of the present processing.

In an exemplary embodiment of the present photonic mirror assembly a surface area of the crystal array is in a range 0.01 mm2-100 mm2, hence relatively small to mesoscopic sizes. The surface area of the crystal array may depend on a final application and an amount of power and frequency that is needed to be generated, for a given resonance. For instance to be used as a chemical sensor, a higher surface area for the photonic mirror assembly could be used.

In an exemplary embodiment of the present photonic mirror assembly at least one tether has a varying width 3, wherein the width varies from a larger width at the ends of the tether to a smaller width in a middle section of the tether, preferably wherein the width at the end is >2* width at the middle section. The varying width of the tethers is found to contribute further to the mechanical quality factor, such as an increase of 10-25% relative.

In a further aspect the present invention relates to a product comprising the present mirror assembly, such as a

Quantum transducer, a sensor, such as a chemical sensor, an accelerometer, preferably comprising an optical read out system, wherein the read out system is capable of detecting or measuring at least one of a change in frequency, a change in amplitude, and a change in damping.

The present invention can be used in high precision measurements, ultra-high resolution force and mass sensors, and optical sensors. Optical sensors can be used in very sensitive conditions.

In an exemplary embodiment the present method comprises a step of reducing of the stress at the central part of the mirror.

In an exemplary embodiment, method of producing a photonic mirror comprises the steps of providing a substrate, such as Si, depositing a high stress layer, and forming the array of pillars.

The high stress layer could be S13N4 deposited by low pressure chemical vapour deposition (LPCVD) on <100> Silicon wafer substrates, or other high stress layers such as S1O2,

SiC, InGaP and Si. Precursors used to deposit Si3N4 are dichlorosilane (SiH2Cl2) (DCS) and ammonia (NH3) . It is found that S13N4 layers deposited by LPCVD at 300-700 °C have few impurities (impurities are annealed at high temperature) and low optical absorptions, which is found crucial for minimizing optical losses. The higher the deposition temperature of S13N4, the higher stress is obtained (100 MPa-1.5 GPa). The deposition rate is around 1 nm/min, and a typical thickness of S13N4 layers in this invention is 15 nm. The surface area roughness of the S13N4 layers is better than 2 nm as can be measured by light scattering (e.g. Semilab SRP-2100); hence a very smooth surface is obtained.

The invention will hereafter be further elucidated through the following examples which are exemplary and explanatory of nature and are not intended to be considered limiting of the invention. To the person skilled in the art it may be clear that many variants, being obvious or not, may be conceivable falling within the scope of protection, defined by the present claims.

SUMMARY OF THE FIGURES

Figures 1-6 show certain aspects of the present assembly.

Figures 7a-m show certain aspects of an exemplary method of producing the present photonic mirror.

DETAILED DESCRIPTION OF FIGURES

Figure 1 shows a fabricated device 100. Therein a Si substrate 17 is partly etched in a central part thereof. Four tethers 11 are attached to a central membrane resonator 12. A cross-sectional dimension of the resonator is less than 100 pm. A central part of the assembly indicated with a dotted square is detailed in figure 2.

Figure 2 shows a schematic layout of the resonator 12. Four inner sections of tethers 11 are shown attached to the resonator 12. In a central part a mirror 13 is provided with a regular two-dimensional array pillars 14. Each pillar has a surface area 5. Typically the surface areas (and likewise heights) of the pillars are substantially the same or equal with respect to one and another (90-110% relative, preferably 99-101% relative, with respect to a mean surface area/height). A dotted line A-A' is indicated; figure 3 represents a cross-sectional view over line A-A'.

Figure 3 shows a schematic representation of a cross-section of the present mirror. A membrane resonator 12 having pillars 14 with a height 6 are shown. The height of the pillars it typically much larger than a thickness of the membrane.

Figure 4 shows a pillar 14 with a cross-sectional length 7, which for a circular pillar is typically the same as the diameter of the pillar.

Figure 5 shows a schematic layout of the resonator of figure 2. The mirror 13 and surface area 8 in between pillars are indicated.

Figure 6 shows a tether 11 with a width 3 that varies from the ends of the tether (left and right} to a smaller width in a central portion of the tether.

Figure 7a shows a substrate, typically a Si wafer, on which on both sides 250 nm Si3N4 is grown using an LPCVD technique with SiH4, NH4+ at about 10 kPa at a growth rate of 50 nm in 13 min. In figure 7b in a subsequent step a resist is spun on both sides on the silicon nitride. In figure 7c an optical lithographic step is used to define a window on a top part of the product. In figure 7d the window is developed and rinsed thereby defining a frame. In figure 7e the exposed silicon nitride is removed by a C4F8/SF6 plasma etch step. On the bottom side, as is shown in figure 7f, tethers and a central membrane part, forming a trampoline like structure, are defined and the resist is removed. In figure 7g the silicon nitride is removed by C4F8/SF6 plasma etch. Thereafter the remaining resist on the top and bottom side are removed, such as by a wet etch, as is shown in figure 7h. On the trampoline side a second resist is spun and a mask is used to define pillars, as is shown in figure 7i. Everything except the pillars are exposed in a subsequent lithographic step, typically using an E-beam (EBPG6-5000 system) and "ZEP" E-beam resist as is shown in figure 7j. In figure 7k it is shown that the trampoline structure, the frame and parts of the membrane are dry etched using C4Fg/SF5, thereby thinning down the structure, and leaving pillars behind. The height of the pillars is clearly equal to a thickness of material (Si3N4) thinned down. Thereafter the resist is removed (figure 71). In a next step the substrate (Si) is etched through using 30% KOH or TMAH, at an etch rate of about 1 pm/min, leaving behind the suspended trampoline structure, a frame, and pillars together forming a mirror.

Claims (14)

A photonic crystal mirror assembly (100) comprising a fastened (11) 2-500 nm thin film membrane resonator (12) comprising a tumbling material with a tensile strength> 0.5 GPa, the tufts being connected to a substrate (17) and with the membrane (12) comprising the mirror (13), wherein the mirror is a patterned photonic crystal arrangement with an arrangement with reflecting columns (14), wherein a surface of the pillars (5) and height (6) of the pillars are adapted for reflecting light. The photonic mirror according to claim 1, wherein for a given wavelength or wavelength range, the pillars have a height (6) of 0.10-0.50 * wavelength or 0.10-0.50 * weighted average of said wavelength range, respectively. A photonic mirror according to any one of the preceding claims, wherein for a given wavelength or wavelength range, the pillars have a cross-sectional length (7) of 0.25-0.50 * wavelength or 0.25-0.50 * weighted average, respectively of the series of wavelengths, e / or where a space region (8) between the pillars is in a range of 35-97% of a surface of the mirror (13), the remainder of the surface of the mirror being formed by the upper surfaces of the pillars. A photonic mirror according to any one of the preceding claims, wherein the resonator is attached to a support at at least two, preferably at least four, tines (11), such as in a "trampoline" structure. A photonic mirror according to any one of the preceding claims, wherein the resonator is a MEMS and / or NEMS. A photonic mirror according to any one of the preceding claims, wherein the assembly is an on-chip assembly. A photonic mirror according to any one of the preceding claims, wherein the resonator is a mechanical resonator. A photonic mirror according to any one of the preceding claims, wherein the membrane is formed from a material selected from Si 3 N 4, SiO 2, SiC, InGaP, Si, and combinations thereof. A photonic mirror according to any one of the preceding claims, wherein the tensile stress is> 1 GPa, preferably> 2 GPa. The photonic mirror of any one of the preceding claims, wherein a surface of the crystal arrangement is in the range 0.01 mm 2 - 100 mm 2. A photonic mirror according to any one of the preceding claims, wherein at least one pouch has a varying width (3), wherein the width varies from a larger width at the ends of the pouch to a smaller width in a middle part of the pouch, preferably wherein the width at the end is> 2 * the width of the middle part. 12. Product comprising a mirror assembly according to any one of the preceding claims, such as a Quantum transducer, a sensor, such as a chemical sensor, an accelerometer, preferably comprising an optical read-out system, wherein the read-out system is capable of detecting or measuring at least one of a change in frequency, a change in amplitude, and a change in attenuation. A method for manufacturing a photonic crystal mirror according to any one of the preceding claims, comprising a step of reducing the tensile strength at the central part. A method for manufacturing a photonic mirror according to any of claims 1-13, comprising the steps of providing a substrate such as Si, applying a high tensile layer, and forming the arrangement of columns.