Micro-electromechanical system Fabry-Perot filter mirrors

[0001] This is a continuation-in-part application of application Ser. No. 11/447,779, entitled “MICRO-ELECTROMECHANICAL SYSTEM FABRY-PEROT FILTER CAVITY” and filed on Jun. 6, 2006. The entire contents of that application are incorporated herein by reference.

[0002] Devices may sense the presence (or absence) of particular molecules. For example, a miniature or hand-held spectrometer might be used to detect biological, chemical, and/or gas molecules. Such devices might be useful, for example, in the medical, pharmaceutical, and/or security fields. By way of example, a hand-held device might be provided to detect the presence of explosive materials at an airport.

[0003] In some sensing devices, light reflected from a sample of molecules is analyzed to determine whether or not a particular molecule is present. For example, the amount of light reflected at various wavelengths might be measured and compared to a known “signature” of values associated with that molecule. When the reflected light matches the signature, it can be determined that the sample includes that molecule.

[0004] In some sensing devices, a Fabry-Perot filter, or etalon, such as the one illustrated in FIG. 1 is used to analyze light reflected from a sample of molecules. The filter 100 includes a first partially reflecting mirror 110 and a second partially reflecting mirror 120 that define a resonant cavity C. Broadband light enters the filter 100, and some photons reflect off of the first mirror 110 while others pass through the mirror 110 and enter the cavity C. While in the cavity C, the photons bounce between the first and second mirrors 110, 120, and eventually some of the photons pass through the second mirror 120 and exit the filter 100.

[0005] As the photons bounce within the cavity C, interference occurs and an interference pattern is produced in light exiting the filter 100. As a result, light having a specific wavelength may exit the filter 100. Note that the interference occurring within the cavity C is associated with the distance d between the two mirrors 110, 120. Thus, the filter 100 may be “tuned” to output a particular wavelength of light by varying the distance d between the mirrors 110, 120 (e.g., by moving at least one of the mirrors 110, 120). By measuring light reflected from a sample using various distances d (i.e., at various wavelengths), and comparing the results with a known signature of values, a spectrometer associated with the Fabry-Perot filter 100 may determine whether or not a particular molecule is present in a sample.

[0006] One figure of merit associated with the Fabry-Perot filter's transmission bandwidth is “finesse,” which can be defined as FSR/FWHM where FSR represents the free spectral range of the Fabry-Perot filter 100 and FWHM is the width of the transmission peak at its 50% point.

[0007] The finesse of the Fabry-Perot filter 100 may be a function of the reflectivity (R) of the mirrors 110, 120. For an ideal etalon or interferometer, finesses may be associated with π(R)^1/2/(1−R). Note that imperfections in the optical surfaces of the mirrors 110, 120 errors in parallelism between the mirrors 110, 120, and inclusions and imperfections in any material between the reflective surfaces of the mirrors 110, 120 may reduce the finesse of the Fabry-Perot filter 100.

[0008] To achieve proper performance for some applications, the filter 100 may need to have a relatively large finesse (e.g., to detect trace amounts of a substance). For example, to achieve a resolution of less that 5 centimeters (cm)−1 and a tuning range of more than 100 nanometers (nm) for a spectrometer, the finesse of the filter 100 may need to be relatively large (e.g., in excess of 100). Moreover, relatively small distances d may increase the need for high finesses and highly reflective mirrors 110, 120.

[0009] Because finesse is associated with π(R)^1/2/(1−R), one way of increasing it's value to increase the reflectivity R of the mirrors 110, 120. For example, mirrors 110, 120 that are very reflective (e.g., 99%) may provide a relatively large finesse. In some cases, however, the mirrors 110, 120 may be formed of silicon, which has a relatively poor reflectance of approximately 25%. The mirrors 110, 120 could be coated with gold or silver to improve performance these materials might only improve the reflectance of the mirrors 110, 120 to approximately 98%—which might not be sufficient for some applications.

[0010] According to some embodiments, first and second mirrors define a cavity. Moreover, a photonic band-gap structure is formed on a surface of at least one of the first or second mirrors. The cavity might be associated with, for example, Fabry-Perot filter.

[0011] Some embodiments comprise: means for forming a first vertical mirror in a wafer; means for forming a second vertical mirror in the wafer, first and second vertical mirrors defining a Fabry-Perot cavity; means for growing a layer of a relatively low index substance for the first vertical mirror; and means for growing a layer of a relatively high index substance for the first vertical mirror.

[0012] In other embodiments, first and second mirrors define a first cavity. A third mirror is also provide such that a second cavity is defined between the second and third mirrors. These cavities might be associated with, for example, Fabry-Perot filter.

[0013] Still other embodiments comprise: means for reflecting light off of an analyte sample into a first Fabry-Perot filter formed in a silicon wafer; means for transmitting light from the first Fabry-Perot filter into a second Fabry-Perot filer formed in the silicon wafer; and means for analyzing light output from the second Fabry-Perot filter to determine information about the analyte sample.

[0014] Yet other embodiments may be associated with a spectrometer having a laser source and an analyte sample to reflect light from the laser source. A Fabry-Perot filter cavity to receive the reflected light may include: (i) an actuator within a plane, (ii) at least one movable mirror oriented substantially normal to the plane, wherein the actuator is to move the movable mirror, (ii) a fixed mirror oriented substantially normal to the plane and substantially parallel to the movable mirror, and (iii) a one-dimensional photonic band-gap structure on a surface of at least one of the movable or fixed mirrors. A detector may detect photons exiting the Fabry-Perot filter cavity over time as the movable mirror is moved by the actuator. A decision unit may also be provided to determine if the analyte sample is associated with at least one type of molecule based on the sensed photons.

[0027] FIG. 2 is a is a side view of a Fabry-Perot filter 200 in accordance with an exemplary embodiment of the invention. The filter 200 includes two mirrors 210, 220 that define a cavity C. Moreover, a coating 212 is provided on one of the mirrors 210 to improve reflectivity.

[0028] According to some embodiments, the coating 212 is a “photonic band-gap structure” on a surface of the mirror 210. As used herein, the phrase “photonic band-gap structure” may refer to, for example, any material associated with a band-gap that inhibits the propagation of photons. The coating 212 might comprise, for example, a one-dimensional photonic band-gap structure associated with a “Bragg mirror.” As used herein, the phrase “Bragg mirror” may refer to, for example, a Bragg reflector or a quarter-wave mirror formed by creating the coating 212 with alternating sequences of optical materials (e.g., high and low refractive index materials). Moreover, each optical layer thickness may be associated with one quarter of the wavelength for which the mirror 210 is designed. Interfaces between the two materials can contribute a Fresnel reflection, and, as a result, a group delay dispersion of the mirror 210 may be reduced at the design wavelength (e.g., which could improve the reflectivity of the mirror 210 to substantially 99%).

[0029] FIG. 3 is a side view of a Fabry-Perot filter 300 in accordance with an exemplary embodiment of the invention. The filter 300 includes a first partially reflecting mirror 310 and a second partially reflecting mirror 320 that define a resonant cavity C. The first mirror 310 acts a movable mirror while the second mirror 320 is fixed. Note that the movable mirror 310 may be substantially parallel to the fixed mirror 320. According to this embodiment, both mirrors 320 have coatings 312, 322 associated with photonic band-gap structures to improve the finesse of the filter 300.

[0030] The filter 300 further includes an actuator 330 within a plane, such as a plane defined by a surface of a silicon wafer. Note that the movable and/or fixed mirrors 310, 320 may be oriented substantially normal to that plane (e.g., vertically within the wafer).

[0031] According to some embodiments, the actuator 330 is coupled to the movable mirror 310 via an attachment portion 340. Moreover, the actuator 330 may move or “scan” the movable mirror 310 left and right in FIG. 3 to vary distance d over time.

[0032] As the movable mirror 310 is scanned, broadband light may enter the filter 300 (e.g., via fiber optic cable introducing the light through the fixed mirror 320) and some photons may reflect off of the fixed mirror 310 while others pass through the mirror 320 and coating 322 and enter the cavity C. While in the cavity C, the photons may reflect between the coatings 312, 322 of the fixed and movable mirrors 310, 320, and eventually some of the photons may pass through the coating 312 of the movable mirror 320 and exit the filter 300. Note that because the coatings 312, 322 are associated with one-dimensional photonic band-gap structures the reflectivity of the mirrors 310, 320 may be increased (and thus, the finesse of the filter 300 may be improved).

[0033] In this way, the filter 300 may act as a narrow-band optical filter and the wavelength of light that exits the filter may vary over time (as d is varied). That is, the wavelength of light output from the filter 300 will scan back and forth across a range of the optical spectrum over time. By measuring the intensity of the light at various times (and, therefore, various distances d and wavelengths), information about the light entering the filter can be determined. Moreover, the presence of the coatings 312, 322 may provide highly reflective mirrors 310, 320 (e.g., 99%) and the filter 300 might be associated with, for example, a finesse of approximately 100.

[0034] Although a single pair of mirrors 310, 320 are illustrated in FIG. 3, additional mirrors may be provided (e.g., to define multiple cavities as described with respect to FIG. 8). Moreover, although flat, rectangular mirrors 310, 330 are illustrated in FIG. 3 other configurations may be provided. For example, one or both of the mirrors 310, 320 might be curved. Similarly, one or both of the mirrors 310, 320 might be U-shaped or I-shaped.

[0035] The actuator 330 may be any element capable of moving the movable mirror 310. Note that the actuator 330 may be provided separate from the movable mirror 310. That is, the activation may be decoupled from the optics (e.g., the mirrors do not act as electrodes or movable membranes). As a result, the tunability of the filter 300 may be improved. In addition, the filter 300 may be scanned over longer distances and spatial (and therefore spectral) resolution may be increased. Also note that having the light enter the Fabry-Perot filter via the fixed mirror 320 (as opposed to the movable mirror 310) may reduce stiction issues and prevent fluctuations in any gap between a fiber optic cable and the filter 300.

[0036] According to some embodiments, the actuator 330 may be a bi-stable structure. In this case, the actuator 330 may be snapped between the two stable positions to scan the filter 300. The actuator 330 might be associated with, for example, a thermal device, an electrostatic device, and/or a magnetic device. According to some embodiments, a spring may be coupled to the movable mirror 310 and/or actuator 330 to improve control.

[0037] The Fabry-Perot filter 300 may be associated with, for example, a spectrometer. FIG. 4 illustrates a spectrometer 400 that might be associated with, for example, a Raman device, an infra-red absorption device, and/or a fluorescence spectroscopy device.

[0038] According to this embodiment, the spectrometer 400 includes a light source 410 (e.g., a laser associated with λ_L) that provides a beam of light to an analyte sample 420. Photons are reflected off of the analyte sample 420 and pass through the Fabry-Perot filter 300 as described, for example, with respect to FIG. 3. According to some embodiments, another filter 430 may also be provided (e.g., a Rayleigh filter to remove λ_L).

[0039] Because the Fabry-Perot filter 300 is scanning d_i over time, a detector 440 may measure light having varying wavelengths λ_L over time. These values may be provided to a decision unit 450 that compares the values with a signature of a known molecule (or sets of molecules) signatures. Based on the comparison, the decision unit 450 may output a result (e.g., indicating whether or not any of the signatures were detected).

[0040] The photonic band-gap structure (or structures) provided in the Fabry-Perot filter 300 may result in cavity mirrors having a reflectance of 99% and the filter 300 itself might have a finesse of more than 300. Such a spectrometer 400 might be associated with a resolution of less than 5 cm−1 and a tuning range of more than 100 nm.

[0041] FIG. 5 illustrates a method according to some embodiments. At Step 502, a first vertical mirror is formed in a wafer. At Step 504, a second vertical mirror is formed in the wafer, and the first and second vertical mirrors define a Fabry-Perot cavity. The wafer might be associated with, for example, a silicon substrate.

[0042] At Step 506, a layer of a relatively low index substance is grown for the first vertical mirror. At Step 508, a layer of a relatively high index substance is grown for the first vertical mirror (e.g., on top of the low index substance). As a result a Bragg mirror may be formed of these layers of relatively high and relatively low refractive index materials.

[0043] According to some embodiments, the relatively low index material is silicon oxide (with n being approximately 1.46) and the relatively high index material is silicon (with n being approximately 3.62). According to another embodiment, the relatively low index material is air (with n being 1) and the relatively high index material is silicon.

[0044] Steps 506 and 508 may be repeated, according to some embodiments. For example, FIG. 6 illustrates multiple layers 612 of coating on a mirror 610. In this case, each layer includes a relatively low index material 614 and a relatively high index material 616. Note that the order of these materials 614, 616 might be reversed.

[0045] By way of example only, assume that the relatively low index material 614 is silicon oxide and the relatively high index material 616 is silicon. In this case, the reflectance of the mirror 600 might be approximately 68% when two layers 612 are present and 99% when four layers 612 are present. As another example, assume that the relatively low index material 614 is air and the relatively high index material 616 is silicon. In this case, the reflectance of the mirror 600 might be approximately 92% when a two layers 612 is present and 99% when three layers 612 are present.

[0046] Consider a Bragg mirror designed for light having a quarter wave optical thickness (λ/4) of 980 nm/4, or 245 nm. For this value, the actual thickness might be (λ/4)*n. Since n is approximately 3.62 for silicon, the physical thickness of a silicon layer 612 might be associated with approximately 886.9 nm.

[0047] According to some embodiments, a movable or fixed mirror may be associated with a crystallographic plane of silicon and a Fabry-Perot filter may be associated with a Micro-electromechanical System (MEMS) device. For example, FIG. 7 is a perspective view of a wafer 700 that may be associated with a Fabry-Perot filter in accordance with an exemplary embodiment of the invention. In this case, portions of the wafer 700 may be etched away resulting in a pair of vertical mirrors 710, 720. Moreover, an actuation portion 730 may be etched onto the surface of the wafer 700 to move the movable mirror 710. Note that the vertical orientation of the mirrors 710, 720 might provide for taller, more stable structures as compared to horizontal ones. For example, a cavity 3 microns wide might be associated with mirrors having a height of 250 microns. According to some embodiments, areas are etched away and released by dry etching to avoid stiction issues that might result from drying wet surfaces. Note that an optical coating 722 might be provided one or both mirrors 710, 720 to adjust reflection (and thereby increase transmission).

[0048] Instead of, or in addition to, using a coating 722 to improve the performance of a Fabry-Perot filter, according to some embodiments multiple Fabry-Perot filters and/or filter cavities may be provided. For example, FIG. 8 is a side view of a Fabry-Perot filter 800 having a first mirror 810 and a second mirror 820 that define a first cavity C1. According to this embodiment, a third mirror 830 is also provided to define a second cavity C2 between the second mirror 820 and the third mirror 830 to form a second cavity C2. The optical interference in the cavities may depend in part on the distances d1, d2 between the mirrors 810, 820, 830.

[0049] In this case, the cavities C1, C2 may act as a pair of Fabry-Perot filters. Note that more than two Fabry-Perot filters may be provided. For example, FIG. 9 illustrates a system 900 wherein photons pass through a first Fabry-Perot filter 910 and a second Fabry-Perot filter 920 and so on exiting a last Fabry-Perot 930.

[0050] Referring again to FIG. 8, note that the first mirror 810 may be formed substantially thicker as compared to the other mirrors 820, 830 resulting in an “unbalanced” mirror design. For example, a movable mirror 810 may be substantially thicker than fixed mirrors 820, 830. Such a design might, for example, improve the stability of the movable mirror 810 (e.g., reducing the problems caused by vibration of the Fabry-Perot filter 800) and/or increase an amount of light that is transmitted from the Fabry-Perot filter 800. Moreover, the movable mirror 810 might be less likely to flex or otherwise deform as it is scanned.

[0051] FIG. 10 illustrates a method associated with Fabry-Perot filter cavities according to some embodiments. At Step 1002, light is reflected from an analyte sample into a first Fabry-Perot filter formed in a silicon wafer. At Step 1004, light is transmitted from the first Fabry-Perot filter into a second Fabry-Perot filer formed in the silicon wafer. At step 1006, light output from the second Fabry-Perot filter is analyzed to determine information about the analyte sample. According to some embodiments, the method may further include moving a mirror associated with the first or second Fabry-Perot filter such that the light output from the second Fabry-Perot filter may be analyzed across an optical spectral range as described with respect to FIG. 12.

[0052] FIG. 11 is a side view of Fabry-Perot filter cavities C1, C2 formed by three partially reflecting mirrors 1110, 1120, 1130. In this case, one of the mirrors 1130 has a coating 1132 of a one-dimensional photonic band-gap structure. The coating 1132 might be associated with any of the embodiments described herein (e.g., with respect to FIGS. 1 through 7). According to some embodiments, one or more of the other mirrors 1110, 1120 may also be coated with a one-dimensional photonic band-gap structure. Moreover, the second mirror 1120 might have coatings on both surfaces.

[0053] FIG. 12 is a side view of Fabry-Perot filters formed by three mirrors 1210, 1220, 1230 and at least one motion portion 1240 in accordance with an exemplary embodiment of the invention. In this case, the first mirror 1210 may be a movable mirror oriented substantially normal to a plane defined by a surface of a silicon wafer. The second mirror 1220 may also be oriented substantially normal to the plane defined by the surface of the silicon wafer. Moreover, the third mirror 1230 is also oriented substantially normal to the plane defined by the surface of the silicon wafer, and the second and third mirrors 1220, 1230 are substantially parallel to the movable mirror 1210. The moveable portion might comprise, for example, a comb drive (within the plane defined by the surface of the silicon wafer) to move the movable mirror 1210.

[0054] The following illustrates various additional embodiments of the invention. These do not constitute a definition of all possible embodiments, and those skilled in the art will understand that the present invention is applicable to many other embodiments. Further, although the following embodiments are briefly described for clarity, those skilled in the art will understand how to make any changes, if necessary, to the above-described apparatus and methods to accommodate these and other embodiments and applications.

[0055] Although a single movable mirror has been provided in some embodiments described herein, note that both mirrors associated with a Fabry-Perot cavity might be movable (and each mirror might be simultaneously moved with respect to the other mirror).

[0056] Further, although particular coatings, layouts and manufacturing techniques have been described herein, embodiments may be associated with other coatings, layouts and/or manufacturing techniques. For example, cap wafers with optical and/or electrical ports may be provided for any of the embodiments described herein. Such wafers may, for example, be used to interface with an Application Specific Integrated Circuit (ASIC) device.

[0057] Moreover, although Fabry-Perot filter designs have been described with respect to spectrometers, note that such filters may be used with any other types of devices, including telecommunication devices, meteorology devices, and/or pressure sensors.

[0058] The present invention has been described in terms of several embodiments solely for the purpose of illustration. Persons skilled in the art will recognize from this description that the invention is not limited to the embodiments described, but may be practiced with modifications and alterations limited only by the spirit and scope of the appended claims.