Method for fabricating a sensor, a sensor, and a method for sensing

A method for fabricating a sensor, a sensor so fabricated, and a method for sensing a stimulus are provided. The method includes providing an elongated open channel, such as, a V-groove, in a substrate, the open channel providing a first surface; removing at least some material from at least a portion of the open channel to provide a second surface displaced from the first surface; positioning a diaphragm on the second surface; and positioning an elongated wave-guide having a beveled end in the elongated open channel wherein the beveled end is positioned over the diaphragm to define an interferometric cavity between the diaphragm and the outer surface of the wave-guide. The sensor so fabricated can provide an effective sensor for detecting acoustic emission waves, among other pressure waves.

BACKGROUND

1. Technical Field

The present invention generally relates to sensors and methods for fabricating sensors. More particularly, the present invention relates to sensors, for example, interferometric sensors, fabricated by photolithographic methods having improved reliability and sensitivity.

2. Background of the Invention

Acoustic emission (AE) monitoring has been proven as a suitable nondestructive technology for structure integrity monitoring, diagnostics, and prognostics, among other things. For example, elastic strain waves generated by rapid release of energy produce AE waves during dislocations in materials. These dislocations can be produced, for example, by fatigue cracks (and their growth), impact, inter-surface slippage, twinning, phase transformations, plastic deformation, and corrosion fatigue.

In order to detect AE activity, AE sensors are typically integrated into the target structure to detect and monitor characteristic signals. Typically, the detected (and, typically, recorded) signals are compared to the theoretical and standard sample signal waveforms. The comparison of the waveforms can be used to determine whether the AE activity detected is from material damage or environmental noise. When a sensor array is used, the location of the AE source can be determined. Typical applications of AE detection include their use on pressure vessels, storage tanks, heat exchangers, piping, reactors, aerial lift devices, and nuclear power plants and equipment, among many other types of structures that can be monitored.

Aircraft fatigue monitoring is a prime example of the use of AE monitoring. For example, critical structures within an aircraft, such as, the connecting lugs between the wings and the main fuselage, can be monitored with an AE sensor to detect fatigue cracking. In such applications, an AE sensor should impose the least impact to the structure's weight, surface shape, and mechanical/chemical properties. Therefore, it is preferred that an AE sensor be compact, lightweight, reliable, sensitive, and have low power consumption.

AE sensing can also be used in partial discharge (PD) acoustic detection in high voltage transformers in the power industry. In this application, the principal considerations for selection of AE sensors are immunity to electromagnetic interference and immunity to chemical erosion.

Typical prior art sensors that are used for AE detection include piezoelectric sensors, Fiber-Bragg-Grating (FBG) sensors, and Fabry-Perot (F-P) optical fiber sensors. Of these types of AE sensors, piezoelectric sensors are most widely used because of their high sensitivity, low cost, and ease of use. However, piezoelectric sensors are characterized by the following disadvantages:1. Most traditional piezoelectric AE sensing systems are bulky. The piezoelectric disk is typically so brittle that special packaging to prevent breakage is typically required. In addition, since electrical signals cannot be transmitted far away without electrical amplifiers, piezoelectric sensors require electrical connections and associated electrical devices, which greatly increase the size and the difficulty of mounting piezoelectric sensors. Furthermore, the complexity of piezoelectric systems typically decreases the reliability of systems employing these sensors.2. Piezoelectric AE sensors normally have large contact surfaces. Typically, such sensors are 6.35 mm or larger in diameter. As a consequence, the output signal from a piezoelectric AE sensor comprises the integration of all points within the contact area. This inherently decreases the accuracy of the piezoelectric sensor.3. Piezoelectric AE sensors are electrical devices and, as such, are also sensitive to electromagnetic noise. Therefore, piezoelectric AE sensors require special signal processing methods to minimize their sensitivity to noise. Moreover, piezoelectric AE sensors are not suitable in some environments, such as, to monitor nuclear power equipment.4. In addition, piezoelectric AE sensors are limited by the electronic device and the Curie temperature of the piezoelectric components. Piezoelectric AE sensors are not suitable for applications where the environment temperature is over 573 K.

Optical AE sensors have shown high resolution and accuracy using an interferometric detection technique, such as, in Fabry-Perot (F-P) cavity or Fiber-Bragg-Grating (FBG) AE sensors. The small size and geometrical flexibility of such optical AE sensors make them easy to be mounted in positions close to critical locations, for example, where cracking and damage are expected to initiate, while optical AE sensors typically do not influence the mechanical properties and performance of target structure. Optical connections and non-conducting sensors make the system immune to electromagnetic interference, insensitive to thermal variation, and inert to chemical erosion. Optical AE sensors can transmit a signal faster and farther than electrical devices. Another outstanding advantage of optical AE sensors is their capability of survival in high-pressure and high-temperature cure environments that are common during structure fabrication, system integration, and daily use.

However, FBG-type AE sensors and high finesse F-P-type AE sensors are typically sensitive to the noise from the environment. The spectrum of these optical sensors is so sharp that small deviations of the laser wavelength or small changes in the environment can shift the spectrum greatly. FBG sensors and intrinsic F-P interferometric (IFPI) sensors may drift greatly due to the uncertain polarization state, refractive index variation with temperature, and unreliable bonding points. Currently, the most common prior art solution is to lock the laser wavelength to the center of the optimized modulation position in the reflection spectrum. However, locking the laser wavelength increases the complexity and cost of the optical AE sensor system. This problem becomes intolerable when multiple sensors are used to establish a network, and each of optical AE sensors needs an independent monitoring and tuning system. Another solution is to use a short FBG or a short-cavity-length F-B sensor. Also, the sensing area of an optical AE sensor, such as, the length of the FBG, should be less than the wavelength of the acoustic wave detected. Otherwise, the output from the FBG-type AE sensor will be distorted by the averaging effect on the change of the grating pitch or FBG cavity. However, this typically will decrease the sensitivity of the FBG AE sensor and increase the fabrication difficulty of F-P AE sensor.

Diaphragm-based, extrinsic F-P interferometric (EFPI) optical sensors can avoid the disadvantages mentioned above optical AE sensors. EFPI AE sensors are small and compact in size while maintaining the advantages of the optical fiber sensors at the same time. According to aspects of the present invention, as will be discussed below, a diaphragm of an F-P sensor can be fabricated by MEMS technology, which has high potential for providing low cost, good repeatability, and high yield.

As is known in the art, because acoustic waves from AE are typically from 100 k Hz to 1 MHz, a spectrum demodulation method is typically not fast enough for EFPI AE sensors and an intensity demodulation method is normally used. However, as will be discussed below, aspects of the present invention overcome or minimize this disadvantage of EFPI AE sensors.

Moreover, accurate cavity length control is very important for EFPI AE sensor fabrication and high quality thin diaphragm fabrication for EFPI AE sensors are difficult to achieve with current design and fabrication techniques. For example, although cavity length control of 3 nanometers (nm) precision has been reported, the diaphragm thickness used was about 5 μm is too thick to achieve the high sensitivity desired for AE detection. This undesirable diaphragm thickness limitation and poor repeatability was a result of the fabrication method used.

Though photolithographic methods have been used in the prior art to fabricate diaphragms, the uniformity of the cavity length in the F-P cavity is difficult to control and the yields are poor.

In addition, prior art methods of mounting optical fibers, whose end face serves as one of the reflection surfaces of F-P cavity, are typically bonded by epoxy glues. The use of such glues introduces problems for F-P-type AE sensors, such as, reduced reliability and spectrum shift caused by temperature variation.

U.S. Pat. No. 5,381,231 of Tu; U.S. Pat. No. 5,087,124 of Smith, et al.; and U.S. Patent Publication 2007/000663 of Zerwekh, et al. all disclose interferometric sensors having optical fibers. However, none of these references provide the teachings or advantages of aspects of the present invention.

The prior art methods of fabricating optical AE sensors cannot meet the requirements of high performance, high yield, and low cost at the same time because of the difficulty in controlling cavity length and diaphragm thickness. Prior art methods of fabrication are complex and costly fabrication process. Accordingly, there is a need in the art for method of fabricating an optical AE sensor that provides precise cavity length control, high sensitivity, good thermal stability and repeatability, simple fabrication and packaging process, and high-volume production. Moreover, there is a need in the art for accurate optical AE sensors having high sensitivity, good thermal stability, and good repeatability. Aspects of the present invention address these shortcomings and disadvantages of the prior art.

SUMMARY OF THE INVENTION

Aspects of the present invention overcome the disadvantages of the prior art by providing sensors, methods of fabricating sensors, and methods of sensing that employ precise dimensional control of critical sensing parameters and enhanced sensitivity that is not found in the prior art. For example, aspects of the invention provide improved interferometric cavity length tolerance, thus increasing fabrication accuracy and repeatability. Aspects may also allow minimization of diaphragm thickness, thus increasing sensor sensitivity.

Aspects of the present invention are based upon the external Fabry-Perot interferometer (EFPI) principle, that is, the illumination of a target diaphragm with a source of electromagnetic radiation, typically, a laser, and the detection of the variation of the interference patterns from the radiation reflected from the diaphragm due to deflection of the diaphragm. Aspects of the present invent may employ standard etching and photolithography processes that are capable of providing high yield at low cost.

Although conventional etching processes are insufficient to provide the rigorous uniformity in cavity length desired, standard, well-controlling etching methods can provide high volume production. Photolithography and other MEMS-related processes are the most important processes commonly used in semiconductor fabrication. Such processes can be used to fabricate ultra precise patterns with ultra-high repeatability and at low cost. In conventional photolithographic methods, patterns are used as masks for other fabrication processes, such as, etching. In aspects of the present invention, cavity length of an EFPI-type AE sensor is established and controlled by photolithography, for example, by photolithography alone.

One aspect of the invention is a method for fabricating a sensor, the method comprising or including providing an elongated open channel in a top surface of a substrate, the open channel providing a first surface and a direction of elongation; removing at least some material from at least a portion of the first surface of the open channel to provide a second surface in the open channel displaced from the first surface; positioning a diaphragm on the second surface, the diaphragm having a top surface and a bottom surface; removing at least some material from the substrate beneath the diaphragm; and positioning an elongated wave-guide having a beveled end in the elongated open channel wherein an outer surface of the wave-guide contacts the first surface and wherein the beveled end is positioned over the diaphragm to define an interferometric cavity between the diaphragm and the outer surface of the wave-guide. For example, the interferometric cavity length may be defined as the length from the top surface of the diaphragm, or the bottom surface of the diaphragm, or both the top and bottom surfaces of the diaphragm to the outer surface of the wave-guide. In one aspect of the invention, multiple diaphragm sensors may be fabricated, for example, on a single substrate. The multiple sensors may be tested after fabrication and, depending upon performance, characterized or selected for use, for example, selected for packaging or storage.

In another aspect of the invention the top surface of the diaphragm or the bottom surface of the diaphragm and/or the wave-guide may be isolated from the surrounding media or mediums. For example, the top surface and the wave-guide may be isolated by some form of sealed enclosure, for example, an enclosure or coating containing or encapsulating a medium having a desired or known refractive index, such as, a gas, for example, air, or a liquid, for example, an oil, or anther compressible material. In another aspect, the bottom surface of the diaphragm may also be isolated, for example, by means of sealing the aperture to provide an aperture having a medium having a desired or known refractive index, such as, a gas, for example, air, or a liquid, for example, an oil, or another compressible material. The bottom surface of the diaphragm may also be exposed to a depression or blind aperture or hole containing the desired medium, such as, air. In one aspect, the isolated or sealed surface of the diaphragm, for example, the aperture side, may contain the reference medium, such as, air, and the non-isolated or unsealed side of the diaphragm may be exposed to the stimulus.

Though in one aspect of the invention, the bottom surface of the diaphragm may be exposed to the stimulus, in another aspect of the invention, the top surface or the top and bottom surfaces of the diaphragm may be exposed to the stimulus that deflects the diaphragm. For example, in one aspect, the aperture may not pass completely through the substrate, but may be of sufficient depth to permit the diaphragm to deflect under the influence of the stimulus to which the top surface of the diaphragm is exposed. For example, in one aspect, the aperture may be a blind hole having a depth of at least the thickness of the diaphragm, for instance the blind hole may have a depth of from about 0.05 micrometers to about 10 micrometers. In another aspect, the aperture may comprise a through hole completely through the substrate, again, this through hole may be sealed with a sealing agent or compound.

According to aspects of the invention, the stimulus may be one or more of elastic strain waves, compression waves, longitudinal waves, dynamic pressure waves, static pressure, acoustic emission waves, temperature, and acceleration, among other stimuli. In one aspect, providing an elongated open channel in the surface of the substrate may comprise providing an elongated v-groove in the surface of the substrate, for example, etching the top surface of the substrate.

In one aspect, positioning the diaphragm on the second surface comprises depositing a material on the second surface, for example, by growing or depositing one or more layers of polymeric material, for example, a multilayer diaphragm. The diaphragm may comprise a thin film, for example, a thin diaphragm having a thickness less than or equal to about 10 micrometers, or less than or equal to about 1.0 micrometer, for example, between about 0.2 micrometers and about 1.0 micrometers. However, in some aspects of the invention, the diaphragm thickness may be less than about 0.2 micrometers, for example, between about 0.05 to about 0.2 micrometers, though in some aspects, the diaphragm may have a thickness less than about 0.05 micrometers (that is, less than or equal to 50 nanometers).

It is envisioned that the method and devices of the invention may employ semiconductor manufacturing methods and/or micro-electromechanical systems (MEMS) manufacturing methods, for example, photolithography, masking, anisotropic etching, and removal of silicon and germanium and related semiconductor materials, and the like. Semiconductor manufacturing methods and/or MEMS manufacturing methods can provide relative dimensional precision and high dimensional tolerance, for example, of groove or channel depth, width, and/or position, which are uniquely suited for aspects of the invention. In one aspect, the methods of the invention provide more uniform diaphragms, for example, having more consistent thicknesses and properties, compared to the prior art.

The channel may have any conventional cross-section, for example, v-shaped, u-shaped, square, rectangular, or semi-circular, among other cross-sectional shapes. In one aspect, the channel comprises a cross section having at least one sidewall, for example, one vertical or inclined wall onto which the diaphragm can be formed and through which an aperture can be provided. In one aspect, the channel may have V-shaped cross-section with a substantially horizontal base, for example, at the bottom of the channel.

According to aspects of the invention, a method is provided in which the length of the interferometric cavity, for example, the length of an F-P cavity, can be held to a tolerance of +/−0.5 micrometers (μm) or finer; for example, held to a tolerance of +/−0.1 μm or finer; or even to a tolerance of +/−0.05 μm or finer. That is, a more repeatable and reliable sensor can be provided than can be provided by the prior art. In particular, in one aspect, the diaphragm may have a thickness of less than 0.05 micrometers or less, and the tolerance of the interferometric cavity length may be held to +/−0.05 micrometers or less.

According to aspects of the invention, one or more components of the sensor may be fabricated individually or substantially simultaneously. For example, one or more channels may be provided in one or more substrates, for instance, channels of varying, length, width, or depth. In addition, one or more recesses, one or more apertures, or one or more diaphragms may be formed or fabricated on one or more substrates. For example, two or more diaphragms of varying thickness and/or varying diameter for different sensor applications may be deposited in one or more channels. In one aspect, one or more apertures, for example, apertures of the same or varying depth and/or the same or varying diameter, or one or more diaphragms may be provided at substantially the same time, for example, by selective etching or selective deposition. For example, two or more sensors may be provided having different requirements but fabricated in substantially the same way, for instance, by the same anisotropic etching process. In one aspect, an array of sensors may be fabricated, for example, on a single substrate, to provide multiple sensors for monitoring and detecting an acoustic occurrence. According to these aspects of the invention, sensor inventories can be reduced and sensor handling and delivery, among other things, can be facilitated.

Another aspect of the invention is a sensor comprising or including: an elongated open channel in a top surface of a substrate, the open channel providing a first surface; a recess in at least a portion of the first surface of the open channel providing a second surface displaced from the first surface; a diaphragm positioned on the second surface, the diaphragm having a top surface and a bottom surface; a cavity in the second surface beneath the diaphragm exposing at least a portion of the bottom surface of the diaphragm; and an elongated wave-guide adapted to transmit electromagnetic radiation, the wave-guide having a beveled end positioned in the elongated open channel wherein an outer surface of the wave-guide contacts the first surface and wherein the beveled end is positioned over the diaphragm to transmit radiation to and receive radiation from the diaphragm and to define an interferometric cavity length between the diaphragm and an outer surface of the wave-guide. For example, the interferometric cavity length may be defined as the length from the top surface of the diaphragm, or the bottom surface of the diaphragm, and/or both the top and bottom surface of the diaphragm to the outer surface of the wave-guide, as discussed above with respect to the media the sensor is exposed to. In another aspect, the second surface displaced from the first surface may be located at a depth in the substrate deeper than the first surface, or be defined by a width wider than the width of the channel. In one aspect, the sensor may comprise one or more interferometric cavities, for example, the multi-cavity configuration discussed above having reflections from two or more surfaces, for instance, from three surfaces. Again, according to aspects of the invention, the stimulus may be one or more of elastic strain waves, compression waves, longitudinal waves, dynamic pressure waves, static pressure, acoustic emission waves, temperature, and acceleration, among other stimuli.

In one aspect, the cavity in the second surface may comprise a blind hole or a through hole or aperture in the substrate. The through hole or aperture may be positioned to expose at least some of the bottom surface of the diaphragm to the stimulus to be sensed by the sensor. The hole or cavity may have proximal end adjacent the diaphragm and a distal end, or example, an open or closed distal end. When the distal end of the hole or cavity is closed, the distal end may be closed with a rigid or deflectable diaphragm or membrane. For example, in one aspect, the deflectable membrane may be adapted to deflect under the influence of a stimulus and transmit the stimulus through a medium, such as, a gas or liquid, or another compressible material, in the closed cavity to the diaphragm at the proximal end of the hole or cavity. The closed cavity may comprise a sub-atmospheric (that is, a vacuum), an atmospheric, or a super-atmospheric pressure gas or liquid. In another aspect, the elongated waveguide may comprise an optical fiber having a beveled end. The electromagnetic radiation may be any available radiation, including infrared light, ultraviolet light, white light, and visible light, for example, provided by a laser or a diode.

Again, in one aspect, the sensor may provide an interferometric cavity length having a tolerance of +/−0.05 micrometers, or less. The diaphragm thickness may range from about 10 micrometers to about 0.05 micrometers, or less. In one aspect, a sensor is provided that is more reliable, more repeatable, and more sensitive sensor than the prior art.

In one aspect of the invention, the stimulus may be periodic or wave-like whereby the diaphragm may be pushed and pulled repeatedly or periodically. For example, the diaphragm may be pushed and pulled under a varying pressure or varying acoustic wave. In another aspect, the stimulus may be substantially constant, for example, a static pressure wave or a temperature. The pressure may also be superatmospheric or subatmospheric, for example, the stimulus may be a vacuum.

A further aspect of the invention is method for sensing a stimulus comprising or including: providing an elongated open channel in a top surface of a substrate, the open channel providing a first surface; providing a recess in at least a portion of the first surface to provide a second surface displaced from the first depth; positioning a diaphragm on the second surface of the recess, the diaphragm adapted to deflect in response to the stimulus; providing a cavity in the substrate beneath the diaphragm; positioning an elongated wave-guide having a beveled end in the elongated open channel wherein an outer surface of the wave-guide contacts the first surface and wherein the beveled end is positioned over the diaphragm to define an interferometric cavity length between the diaphragm and an outer surface of the wave-guide; transmitting a first electromagnetic signal from the beveled end upon the diaphragm; receiving a second electromagnetic signal reflected from the diaphragm; and comparing the second electromagnetic signal to a reference signal to detect deflection of at least a portion of the diaphragm to characterize the stimulus deflecting the diaphragm. The interferometric cavity length may be defined as the length from the top surface of the diaphragm, and/or the bottom surface of the diaphragm, and/or both the top and bottom surface of the diaphragm to the outer surface of the wave-guide, where the first and second electromagnetic signals may be received and reflected from the top surface of the diaphragm, and/or the bottom surface of the diaphragm, and/or both the top surface and the bottom surface. In addition the cavity in the substrate may be a through hole or an aperture having a distal end positioned to receive a stimulus and transmit the stimulus through the through hole or aperture to the diaphragm. The stimulus may impinge the top surface of the diaphragm, the bottom surface, or both the top and bottom surfaces.

Again, according to aspects of the invention, the stimulus may be one or more of elastic strain waves, compression waves, longitudinal waves, dynamic pressure waves, static pressure, acoustic emission waves, temperature, and acceleration, among other stimuli. In one aspect, positioning a diaphragm on the second surface of the recess may comprise depositing or growing a material on the second surface to form the diaphragm, for example, one or more layers of material. For example, the diaphragm may be grown by thermal oxidation of silicon. The aperture may have an open distal end positioned to receive a stimulus and transmit the stimulus through the aperture

In another aspect, transmitting a first electromagnetic signal from the beveled end may comprise transmitting the first electromagnetic signal along the wave-guide whereby the first electromagnet signal is emitted from the beveled end. In another aspect, receiving a second electromagnetic signal may comprise receiving the second electromagnetic signal by the beveled end and transmitting the second electromagnetic signal along the wave-guide. In one aspect of the invention, comparing the first electromagnetic signal to the reference electromagnetic signal to characterize deflection of at least the portion of the diaphragm may comprise transmitting the second electromagnetic signal to an interferometer signal analyzer or a photo detector. In another aspect of the invention, an intensity measurement may be made for a rapid signal response using only one wavelength and photo detector.

In another aspect of the invention, as shown below, the method may further comprise transmitting a source electromagnetic signal along the elongated wave-guide and reflecting the source electromagnetic signal from the beveled end toward the diaphragm; reflecting at least some of the source electromagnetic signal from a sidewall of the waveguide to provide the reference electromagnetic signal; and transmitting at least some of the source electromagnetic signal through the sidewall to provide the first electromagnetic signal.

According to some aspects of the invention, the second signal, for example, a light beam, reflected from the diaphragm will interfere, for example, optically interfere, with the reference beam reflected from the wave-guide sidewall. Any stimulus, for example, an acoustic wave, impacting and deflecting the diaphragm will vary the interference of the second signal with the reference signal whereby the stimulus can be detected and characterized, for example, measured. Two reflected beams—a reference signal reflected from the sidewall and a second signal reflected from the diaphragm—may be forwarded by the wave-guide to an interferometric detector to characterize the diaphragm deflection.

In another aspect, the distal end of the aperture may be an open distal end or a closed distal end. When the distal end of the aperture is closed, the aperture may comprise a sealed cavity, for example, at sub-atmospheric (that is, a vacuum), atmospheric, or super-atmospheric pressure, adapted to transmit the stimulus impacting the closed distal end to the diaphragm.

Again, in the method of the invention, the diaphragm may have a thickness less than 0.05 micrometers, or less, and the interferometric cavity length may have a tolerance of +/−0.05 micrometers, or less.

These and other aspects, features, and advantages of this invention will become apparent from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION

The following detailed description of the figures summarized above will be helpful in understanding the subject matter that is particularly pointed out and distinctly recited in the claims that appear at the conclusion of the specification.

FIG. 1is schematic diagram of a typical Fabry-Perot (F-P) cavity10according to the prior art that can be used for interferometric detection. F-P cavity10typically includes a wave-guide12, for example, an optical fiber, positioned adjacent a thin film14, for example, a diaphragm, to definite a cavity length16between the end of wave-guide12and thin film14. As is known in the art, in typical operation, deflection of thin film14, for example, by an acoustic emission or pressure wave, is detected from variation in the interference between the signal emitted by the wave-guide12and signal reflected from thin film14back into wave-guide12. As discussed above, sensors based upon an F-P cavity are sensitive to environmental noise and the geometry of the cavity, for example, as illustrated inFIG. 2.

FIG. 2is graphical representation20of the modulation of reflection intensity due to variation in Fabry-Perot (F-P) cavity length, for example, variation in cavity length16of F-P cavity10shown inFIG. 1. The graphical representation20includes an abscissa22of source wavelength, for example, laser wavelength, and an ordinate24of reflection intensity of the source reflected, for example, from thin film14. As shown inFIG. 2, when thin film14(FIG. 1) deflects, the reflection spectrum of the F-P cavity will shift correspondingly. If the laser wavelength is fixed, the reflected laser intensity will be modulated by the movement of thin film14. To avoid ambiguity and achieve high sensitivity and large dynamic range, according to aspects of the invention, the sensing wavelength may be tuned to the center of linear working zone as shown inFIG. 2. For a fixed source wavelength26, the linear zone28of intensity curve30defines an acceptable variation in intensity32. As also shown inFIG. 2, there is an acceptable deviation or departure34due to variation in cavity length16from the center of the linear zone28and the wavelength of the light source26. For example, according to an aspect of the present invention, the cavity length16can be accurate and not vary by more than +/−0.05 micrometers (μm) under a wavelength of 1.55 μm. For example, in one aspect of the invention, the cavity length analogous to cavity length16may range from bout 0.01 μm to about 500 μm, but typically ranges from about 0.5 μm to about 150 μm, and may have a tolerance of +/−0.05 μm, or less.

The intensity curves29shown inFIG. 2, that is, the F-P cavity interference fringe, can be described by the expression shown in Equation 1. In Equation 1,

ϕ∝cos⁡(4*π*n*Lλ)Equation⁢⁢1
ø is the intensity of the reflected light, λ is the wavelength, n is the index of the media in the F-P cavity, and L is the cavity length. For example, when a source laser's wavelength is 1.55 μm and the medium in the cavity is air, having a refractive index n=1, Equation 1 indicates that a cavity length change of 0.775 μm will introduce one period spectrum shift (one peak and one valley) at the wavelength position of 1.55 um. If a 1/16 period in the spectrum is chosen as a criterion for an acceptable departure between the center of linear zone28and the laser wavelength26, as shown inFIG. 2, the accuracy of cavity length L should be at least +/−0.05 μm for a laser wavelength of 1.55 μm.

FIGS. 3A,3B, and3C are axial views of sensors50,450, and550, respectively, according to aspects of the present invention that overcomes the disadvantages of the prior art and provides a sensing device that is more accurate, repeatable, and easier to manufacture compared to prior devices.FIG. 4is a transverse elevation view, partially in cross section, of the sensor50,450, and550shown inFIGS. 3A,3B, and3C, respectively, as viewed allow section lines4-4inFIG. 3A.

As shown inFIGS. 3A and 4, sensor50includes a wave-guide52, for example, a fiber optic, positioned in an elongated channel, groove, or recess54in a substrate56, for example, a channel54in a top surface58of substrate56. As shown most clearly inFIG. 4, wave-guide52may typically comprise a beveled end57, for example, a polished end beveled at anywhere from about 40 to about 50 degrees, but typically beveled at an angle of between about 43 degrees and about 47 degrees, for example, about 45 degrees. As shown inFIG. 3A, channel54has a width60at surface58and includes a direction of elongation62, as shown inFIG. 4. Channel54provides a first surface64, for example, at a first depth66from surface58.

According to aspects of the invention, at least a portion of the first surface64of channel54may be provided with a recess68having a second surface69displaced from first surface64. For example, in one aspect, second surface69may be, for example, second surface69having a second depth70from top surface58; second depth70is typically greater than first depth66. The first depth66of first surface64and the second depth70of second surface69inFIG. 3Aare shown for illustrative purposes only; however, it is to be understood that, according to aspects of the invention, the second surface69may simply be displaced from first surface64whereby a step is provided between the two surfaces to provide cavity length80. This step having cavity length80may be provided anywhere about the outside surface, or circumferential surface, of wave guide52, but is typically provided at a location upon which wave guide52impinges the sensing signal, for example, the laser beam. As shownFIGS. 3A and 4, according to aspects of the invention, second surface69is provided with a layer72from which a diaphragm73, for example, a circular diaphragm having a diameter74, may be formed (as will be discussed below), though any shaped diaphragm may be provided according to aspects of the invention, including square, rectangular, or ellipsoidal. During the formation of layer72, for example, as will be discussed below, one or more other surfaces of substrate56may receive a layer of material, for example, the formation of layer75on first surface64.

The size and dimensions shown inFIGS. 3A,3B,3C, and4are for illustration only and may not reflect the actual relative sizes of the features described. For example, the thickness and diaphragm73and the difference in depth, width, or position of surfaces64and69are illustrated much larger than their actual dimensions according to aspects of the invention, though in some aspects of the invention, the relative sizes shown are smaller than actual dimensions.

Diaphragm73typically includes a top surface76and a bottom surface77and substrate56includes at least one cavity78that exposes at least a portion of the bottom surface77of diaphragm73, for example, to be accessible to a wave79to be detected. As shown inFIGS. 3A and 4, in one aspect, cavity78may comprise a hole78extending from a bottom surface59of substrate56to the bottom surface77of diaphragm73. Hole78may be circular or non-circular, for example, square, rectangular, or ellipsoidal, and provide access to the wave79from a wave source (not shown), for example, an elastic strain wave generated by a fatigue crack, an impact, an inter-surface slippage, twinning, a phase transformations, a plastic deformation, or corrosion fatigue.

FIG. 3Aalso illustrates several dimensional features of aspects of the invention. For instance, aspects of the invention may include a shift distance81, a channel or groove angle82, and a distance from the center84. These dimensional features may also be applied to sensor450inFIG. 3B. As shown inFIG. 3A, shift distance81is the distance between the top extremity of the wave-guide52, for example, an optical fiber, (as indicated by phantom line83) and the top surface58of substrate56. According to aspects of the invention the value of shift distance81may vary broadly, and may be positive or negative. For example, when the value of shift distance81is positive, the top extremity of wave guide52is above top surface58of substrate56; when the value of shift distance81is negative, the top extremity of wave guide52is below top surface58of substrate56. According to one aspect of the invention, it can be advantageous to have a positive shift distance whereby the wave-guide52extends beyond the surface58of substrate56where wave-guide52can be contacted and firmly compressed to ensure contact between the wave-guide52and surface64of channel54. For example, a planar surface, such as, a silicon chip, can be compressed against the exposed wave-guide52to ensure contact between wave-guide52and surface64. Shift distance81may vary from about −500 μm to about +25 μm, but shift distance81typically ranges form about 5 μm to about 15 μm.

Groove angle82is the angle that a sidewall of channel54makes with the surface58of substrate56. Typically, the groove angle82is determined by the crystallographic geometry of the material of substrate56. For example, for a (100) single crystal silicon substrate, groove angle82of grooves along with [110] directions are typically 54.74 degrees, though groove angle82will vary for other materials. Groove angle82may vary broadly depending upon the substrate material and the etching process used. For example, groove angle82can be 90 degree (that is, “U type” groove) when the substrate comprises a (110) silicon wafer and the groove is formed by KOH etching. Also, the groove angle can be about 45 degrees when the substrate comprises fused silica or glass and the groove is formed by an isotropical etching method.

Distance from center84is the distance from the centerline of the channel54, for example, the apex of a V-groove, to the centerline of cavity78in substrate56. Typically, the distance to center84will be dependent upon the geometry of channel54, for example, dependent upon one or more of channel width60, depth66, angle of groove82, cavity length80, the diameter of wave guide52. In one aspect of the invention, distance to center84is chosen to optimize the location of diagram73, for example, to locate diaphragm73as near as possible to the center of the optical spot projected by the beveled end57of wave guide52onto diaphragm73in order to have the maximum sensitivity. However, according to aspects of the invention, the location of diaphragm73may deviate from optimum and still effectively function as described herein.

As shown inFIGS. 3B and 4, sensor450includes a wave-guide452, for example, a fiber optic, positioned in an elongated channel, groove, or recess454in a substrate456, for example, in a top surface458of substrate456having a recess468. According to the aspect of the invention shown inFIG. 3B, as will be discussed more fully below, the depth of recess468in channel454providing the second surface may vary while the width461of recess468may typically be larger than the width460of channel454. As shown most clearly inFIG. 4, wave-guide452may typically comprise a beveled end57, for example, a polished end beveled at anywhere from about 40 to about 50 degrees, for instance, from about 43 to about 47 degrees, but typically beveled at an angle of about 45 degrees. As shown inFIG. 3B, channel454has a width460at surface458and includes a direction of elongation62, as shown inFIG. 4. Channel454provides a first surface464, for example, at a first depth466from surface458, against which wave-guide452rests.

As shown inFIG. 3B, according to one aspect of the invention, at least a portion of the first surface464of channel454may be provided with a recess468having a second surface469displaced from first surface464. In one aspect of the invention, the displacement of second surface469from first surface464may be provided by the second surface468having a second depth470from top surface458, greater than first depth466; however, in contrast to the aspect to the invention shown inFIG. 3A, second depth470may vary from first depth466, as indicated by double arrow471. For example, second depth470may be greater than first depth466(for example, as is typical of the aspect shown inFIG. 3Aor, for example, providing a bottom surface467) or substantially equal to or the same as the first depth466. However, as discussed previously, according to aspects of the invention, the second surface469may simply be displaced from first surface464whereby a step is provided between the two surfaces to provide cavity length480. This step having cavity length480may be provided anywhere about the outside surface, or circumferential surface, of wave guide452, but is typically provided at a location upon which wave guide452impinges the sensing signal, for example, the laser beam.

According to one aspect of the invention, as shown inFIG. 3B, the relationship between the position of first surface464and second surface469may be more readily defined by the relative width461of recess468compared to the width460of channel454. For example, in one aspect, recess468and channel454may vary in width and thus provide a variation in the relative position of first surface464and second surface469, for example, second surface469may be displaced from first surface464.

As shownFIG. 3B, second surface469is provided with a diaphragm473, for example, a circular diaphragm having a diameter474, though, as discussed above with respect to diaphragm73, any shaped diaphragm may be provided according to aspects of the invention, including square, rectangular, or ellipsoidal.

As shown inFIGS. 3C and 4, sensor550includes a wave-guide552, for example, a fiber optic, positioned in an elongated channel, groove, or recess554having a first surface564. A recess568in channel554provides a second surface569displaced from first surface564. According to the aspect of the invention shown inFIG. 3C, and discussed previously, the displacement of second surface569from first surface564in channel may be established anywhere within channel554, but is typically provided at a location upon which wave guide552impinges the sensing signal, for example, the laser beam. As shown most clearly inFIG. 4, wave-guide552may typically comprise a beveled end57, for example, a polished end beveled at anywhere from about 40 to about 50 degrees, for instance, from about 43 to about 47 degrees, but typically beveled at an angle of about 45 degrees. As shown inFIG. 3B, channel554has a width560at surface558. In one aspect, channel554provides a first surface564, for example, at a first depth from surface558, against which wave-guide552rests.

As shown inFIG. 3C, according to one aspect of the invention, at least a portion of the first surface564of channel554may be provided with a recess568having a second surface569displaced from first surface564, for example, having a second depth from top surface558. As discussed above, according to aspects of the invention, the second surface569may simply be displaced from first surface564whereby a step is provided between the two surfaces to provide cavity length580. This step having cavity length580may be provided anywhere about the outside surface, or circumferential surface, of wave guide552, but is typically provided at a location upon which wave guide552impinges the sensing signal, for example, the laser beam. As shownFIGS. 3C and 4, according to aspects of the invention, second surface569is provided with a diaphragm573, for example, a circular diaphragm having a diameter574, though any shaped diaphragm may be provided according to aspects of the invention, including square, rectangular, or ellipsoidal.

However, as shown inFIG. 3C, in contrast to the aspects to the invention shown inFIGS. 3A and 3B, surface569may be provided on only a single surface of channel554. For example, on only one side of a V-shaped channel or groove.

The size and dimensions shown inFIGS. 3A,3B,3C, and4are for illustration only and may not reflect the actual relative sizes of the features described. For example, the thickness and diameter74,474, and574of diaphragms73,473, and573, respectably, and the difference in depth, width, and position of surfaces chancels54,454, and545, and recesses68,468, respectively, are illustrated much larger than their actual dimensions according to aspects of the invention, though in some aspects, the actual relative dimensions shown may be illustrated much smaller than their actual dimensions.

Similar to diaphragm73shown inFIG. 3A, diaphragms473and573inFIGS. 3B and 3Ctypically include a top surface and a bottom surface477,577and substrate456,556includes at least one cavity478,578that exposes at least a portion of the bottom surface477,577of diaphragm473,573for example, to be accessible to a wave or stimulus to be detected. As shown inFIGS. 3B and 3C, in aspects of the invention, cavity478,578may comprise a hole extending from a bottom surface459,559of substrate456,556to the bottom surface477,577of diaphragm473,573. Hole478,578may be circular or non-circular, for example, square, rectangular, or ellipsoidal, and provide access to the wave or stimulus from a wave source (not shown).

As shown inFIGS. 3A,3B,3C, and4, according to aspects of the invention, the placement of wave-guide52,452,552along first surface64,464,564and the providing of second surface68,468,568having diaphragm73,473,573provides a well-defined F-P cavity length80,480,580between the outer surface of wave-guide52,452,552and diaphragm73,473,573. As discussed above, the accuracy and repeatability of an F-P cavity is highly sensitive to variably of in cavity length80,480,580. However, according to aspects of the invention, cavity length80,480,580as will be discussed further below, can be closely toleranced during the fabrication process to minimize or eliminate variability of the reflective intensity of the F-P cavity due to variation in cavity length80,480,580.

The operation of sensors50,450, and550shown inFIGS. 3A,3B,3C, and4is best illustrated with the aid ofFIG. 4. As shown inFIG. 4, a reference signal90, for example, a reference laser beam having a wavelength λ, for example, a near infrared laser with wavelength of 1.55 μm, is directed along wave-guide52,452,552and reflects from beveled end57as reflected signal91. According to aspects of the invention, beveled end57is positioned adjacent diaphragm73,473,573for example, superjacent diaphragm73,473,573whereby reflected beam91is directed upon the upper surface76of diaphragm73,473,573. At the same time, at least some of the signal91reflected from beveled end57reflects off the surface of wave-guide52as reflected signal or beam92. Reflected signal92is reflected by beveled end57and propagates back down wave-guide52,452as reflected signal93. At least some of signal91is transmitted to diaphragm73,473,573and contacts and reflects from top surface76of diaphragm73and is reflected as reflected signal94and is reflected back along wave-guide52,452,552as reflected signal95. According to aspects of the invention, signal93reflected from the surface of the wave-guide and the signal95reflected from diaphragm73,473,573interfere with each other, for example, using conventional photo detectors (not shown) and conventional interferometric techniques, to define a characteristic baseline interference pattern for the undeflected or undisturbed diaphragm73,473,573. According to aspects of the invention, the deflection of diaphragm73,473,573for example, by wave79, for instance, from an acoustic emission, varies the phase of the reflected signal95which can be detected and compared to the baseline interference pattern to determine a characteristic interference pattern for the detected wave79. However, according to aspects of the invention, the accuracy and repeatability of the initial, baseline phase difference (that is, without stimulus) between signals93and95can be enhanced due to the improved control and tight tolerances that can be provided for cavity length80,480,580.

FIG. 5is a schematic illustration of a portion of the transverse elevation view shown inFIG. 4including the end of wave-guide52,452having beveled end57directing beam91upon diaphragm73(or diaphragm473inFIG. 3B) having a top surface76and a bottom surface77opposite top surface76. As shown inFIG. 5, in one aspect, one or more interferometric cavities, that is, a multi-cavity structure, may be provided. For example, as shown inFIG. 5, a interferometric cavity may be provided between the surface of the wave-guide52and the top surface76of diaphragm73(that is, “Cavity1”); between the surface of the wave-guide52and the bottom surface77of diaphragm73(“Cavity2”); and/or between the top surface77of diaphragm73and the bottom surface77of diaphragm73(“Cavity3”). According to aspects of the invention, one or more of these cavities may be utilized in detecting the stimulus. For example, in one aspect, cavity1or cavity2or both cavity1and cavity2may be used for an interferometric or F-P cavity.

In one aspect, the surface defining the length of the interferometric cavity length may be a function of the media, for example, air, water, an oil, etc., surrounding the sensor. For example, the refractive index, n, of the medium contacting the wave-guide52, top surface76of diaphragm73, and bottom surface77of diaphragm73may impact the reflectivity of the electromagnetic signal from the interface and thus affect the strength (for example, energy) of the reflected signal. This is illustrated schematically inFIG. 5.

For example, when diaphragm73is not coated, for example, not coated with a metal surface, and diaphragm73is substantially transparent, the optical refractive index between wave-guide52and diaphragm73may be designated n1; the refractive index of diaphragm73, n2; and the refractive index of the medium in the aperture78, n3. Then, as is known in the art, the intensity of the reflected radiation, Rxy, reflected from the interface at the top76of diaphragm73, R12, and at the bottom77of diaphragm73, R23, are provided by the relationships in Equation 2 below:
R12=[(n2−n1)/(n1+n2)]2andR23=[(n2−n3)/(n3+n2)]2Equation 2.
According to these relationships, if adjacent media have similar refraction indices, nxy, the reflection, Rxy, will be smaller, for example, smaller than the reflection from an interface having different or dissimilar refractive indices.

For example, in one aspect of the invention, sensor50,450may be immersed in an oil, for example, in a transformer having an oil, having an index n1=1.47; the diaphragm73may be silicon dioxide having an index n2=1.45, and the aperture78may be sealed and contain air having a refractive index n3=1.0. From the above relationships in Equation 2, due to the similarity of n1and n2, R12will be relatively less than R23, whereby the bottom surface77of diaphragm73will be more effective as a reflection surface for an interferometric cavity.

Note that in some aspects of the invention, should the reflection Rxybe insufficient, the top76or bottom77of diaphragm73and/or the surface of the wave-guide52may be coated with a reflection enhancing material, for example, a metal, to provide the desired reflectivity. In one aspect, should the refractive index of the wave-guide, nwg, approach the refractive index of the medium surrounding the wave-guide, n1, the wave-guide52may be coated to provide the desired reflection.

FIGS. 6 through 20illustrate methods of fabricating a sensor, for example, a sensor similar to sensor50,450,550according to aspects of the invention.FIGS. 6,7, and8are a perspective view, a cross-sectional view, and a plan view, respectively, of a channel, groove, or recess164, for example, an elongated channel, in a substrate156according to one aspect of the invention. The cross-section shown inFIG. 7is as viewed through section7-7shown inFIG. 6. In this and the following discussion reference numbers preceded by the numeral “1” may correspond to items and structures identified without the numeral “1” or with the numeral “4” or with the numeral “5” inFIGS. 1-5and described with respect toFIGS. 1-5above.

Substrate156may be made from any conventional material, for example, a metal, a metalloid, or a plastic. In one aspect of the invention, substrate156is a material that is conducive to conventional photolithographic or MEMS (Micro-Electrical-Mechanical Systems)-type processing, for example, wet etching, thin film deposition, and deep etching, among other processes. For example, in one aspect, substrate156may be made from silicon (Si), for example, single-crystal silicon. Prior to the formation of channel164, when channel164is formed by etching, the substrate156may be coated with silicon oxide or silicon nitride and photolithographed to open the anisotropic wet etching window in preparation for etching, as is conventional.

Etching, for example, wet anisotropic wet etching, of a silicon wafer is sensitive to defects in the substrate156, for example, defects in the single crystal silicon substrate. For example, it has been found that the etching rate around a defect is higher than that at other places in the substrate. Accordingly, it is preferred that high quality substrates be used for aspects of the invention. However, high temperature processing, such as, thermal oxidization, can introduce defects inside the substrate. Therefore, in one aspect of the invention, the mask layer for the etching process, for example, for KOH etching, may be a mask fabricated by a lower temperature CVD process, that is, compared to the temperature of thermal silicon dioxide mask fabrication. For example, a silicon nitride mask fabricated by a lower temperature LPCVD process may be used. The LPCVD process temperature is much lower than that of thermal oxidization, and can thus be less likely to introduce defects to the substrate that can typically be introduced by thermal oxidation processes, for example, the thermal oxidation of silicon dioxide.

Channel164may assume many different shapes according to aspects of the invention, for example, having a square, rectangular, polygonal, circular, semi-circular, u-shaped, v-shaped, or oval cross section, among other cross-sectional shapes. In one aspect of the invention, as shown most clearly inFIG. 7, channel164may have a triangular cross section with an apex directed into substrate156, that is, a “V groove” in the surface of substrate156. In one aspect, channel164may comprise a cross section having at least one sidewall, for example, one vertical or inclined wall onto which a diaphragm can be formed and through which an aperture can be provided. In one aspect, the channel may have V-shaped cross-section with a substantially horizontal base, for example, at the bottom of the channel.

However, according to aspects of the invention, as will become more apparent in the discussion below, the shape of channel164may assume any shape that this conducive to the shape of and optical properties of the wave guide positioned in channel164.

Channel164may have a width160ranging from about 100 μm to about 1500 μm, and typically may have a width160of about 200 μm to about 400 μm. Depth166will vary according to width160due to the crystal angle of the substrate, for example, the relatively fixed angle of a silicon (100) wafer, as discussed above. Channel width160is determined by, among other things, the thickness of the substrate156, for example, the thicker the substrate wafer, the larger width160may be. However, it is to be understood that it is not necessary to etch down to an apex of a V or U shape, as shown inFIG. 3B. In one aspect, the minimum width160and depth166are those dimensions that permit the wave-guide for example, the optical fiber (as discussed below), to settle into and firmly contact and be supported by the surfaces of the channel, for example, the two side surfaces of a V-shaped channel.

Channel164may be formed by any conventional forming process, for example, by conventional machining. However, in one aspect of the invention, channel164may be formed by one or more photolithographic or MEMS-type processing methods, for example, anisotropic wet etching or deep etching, among other methods.

In one aspect of the invention, channel164comprises a “V-groove” in the surface of substrate156. The inventors have found that when the wave-guide used comprises a standard, circular cylindrical shaped optical fiber, the V-groove shape of channel164is a preferred structure to hold the standard optical fiber. In one aspect, the channel164, regardless of shape, serves as the frame structure of the sensor. When substrate156comprises a silicon substrate, channel164, for example, a V-groove channel, may typically be fabricated by wet anisotropic etching on the (100) plane of the silicon substrate. The position, depth, and width of channel164may primarily be determined by the material removal method used, for example, by photolithography and wet anisotropic etching.

FIGS. 9,10, and11, are a perspective view, a cross-sectional view, and a plan view, respectively, of the channel, groove, or recess164shown inFIGS. 6,7, and8, respectively, having a recess168according to an aspect of the invention. The cross-section shown inFIG. 10is as viewed through section10-10shown inFIG. 9. It is to be understood that though recess168shown inFIGS. 9 through 20is similar to recess68shown inFIG. 3A, recess168may also be similar in structure to recess468or568shown inFIGS. 3B and 3C. In one aspect of the invention, recess168may be formed at substantially the same time as channel164, for example, in the same etching process.

Recess168may assume many different shapes according to aspects of the invention, for example, having a square, rectangular, polygonal, circular, or oval cross section. In one aspect of the invention, as shown most clearly inFIG. 10, recess168may be similar in shape to channel164and have a triangular cross section with an apex directed into substrate156, that is, again, a V-groove similar to channel164. However, according to aspects of the invention, as will become more apparent in the discussion below, the shape of channel168may assume any shape that this conducive to the shape of and optical properties of the wave guide positioned in channel164above recess168. In addition, according to aspects of the invention, regardless of the shapes of channel164and recess168, the difference in the size and/or position of at least one of the side walls of channel164and the size and/or position of at least one of the sidewalls of recess168may define the F-P cavity length between the optical fiber and the diaphragm fabricated on the sidewall of recess168.

Recess168may also be formed by any conventional forming process, for example, by conventional machining. However, in one aspect of the invention, recess168may be formed by one or more photolithographic or MEMS-type processing methods described with respect to channel164above, for example, anisotropic wet etching or deep etching, among other methods.

Recess168may have a first width161and a second width175. First width161may vary broadly depending upon the width160and position of channel164and the F-P cavity length180, among other things. In one aspect, first width161may vary from about 100 μm to about 2000 μm, and typically width161may vary from about 200 μm to about 500 μm. Second width175may range from about 50 μm to about 5000 μm, and typically second width175may range from about 200 μm to about 400 μm. Depth170will vary according to width161due to the crystal angle of the substrate, for example, the relatively fixed angle of a silicon (100) wafer, as discussed above.

In one aspect of the invention, channel164and recess168may both comprises V-grooves with two different widths. The V-groove a channel164may be used to accommodate an optical fiber, for example, a standard, circular cylindrical optical fiber, and the V-groove of recess168may be used as a platform to fabricate a thin diaphragm on its sidewall, as will be discussed below. Again, according to aspects of the invention, the difference in the size and/or position of the side walls of the V-groove of channel164and the sidewalls of V-groove of recess168may define the F-P cavity length between the optical fiber and the diaphragm fabricated on the sidewall of the V-groove of recess168.

When the wave-guide is a circular cylindrical optical fiber, the position of the optical fiber outer surface or sidewall is determined by the sidewall of the V-groove of channel164that holds the optical fiber. Consequently, in one aspect, the size and/or position of the V-groove of channel164and the size and/or position of V-groove of recess168determine the cavity length, L. In one aspect, when the channel164and recess168are formed by photolithographic methods, the sizes and positions of the V-grooves of channel164and of recess168are determined by the etching window. As a result, the size and/or position of the V-groove of channel164and the size and/or position of V-groove of recess168can be positioned and sized with enhanced accuracy, for example, with the accuracy of photolithography. For example, in one aspect, the size and/or position of the V-groove of channel164and the size and/or position of V-groove of recess168may be within several nanometers, for instance, +/−5 to 10 nanometers, or less. Accordingly, the size of the cavity lengths of the F-P cavity that can be provided when employing aspects of the present invention may be controlled within several nanometers, for instance, +/−5 to 10 nanometers, or less. That is, cavity length accuracies can be obtained by aspects of the invention that cannot be obtained by prior art methods, especially, not with the diaphragm thicknesses (and inherent AE sensing sensitivities) achievable with aspects of the invention.

FIGS. 12,13, and14, are a perspective view, a cross-sectional view, and a plan view, respectively, of the recess168in channel164shown inFIGS. 9,10, and11, respectively, having a layer172from which a diaphragm can be formed according to one aspect of the invention. The cross-section shown inFIG. 13is as viewed through section13-13shown inFIG. 12.

Layer172may comprise a thin film, for example, a thin layer having a thickness less than or equal to about 10 micrometers, or less than or equal to about 1.0 micrometer, for example, between about 0.2 micrometers and about 1.0 micrometers. However, in some aspects of the invention, the layer thickness may be less than about 0.2 micrometers, for example, between about 0.05 to about 0.2 micrometers, though in some aspects, the layer may have a thickness less than about 0.05 micrometers (that is, less than or equal to 50 nanometers). Layer172typically covers surface at least some of the surface of recess168, for example, at least one of the surfaces of a V-groove recess168. In one aspect, layer172may cover substantially the entire surface of recess168and the entire surface of channel164, for example, both surfaces of a v-groove surface and all surfaces of channel164, as indicated by layer177inFIGS. 13-14.

The inventors recognize, however, the thickness of the layer172may be limited by the structural integrity of the resulting diaphragm, as will be discussed below, which can be more fragile as diaphragm thickness decreases. In some aspects of the invention, not only a more repeatable and more reliable sensor is provided, but also a sensor with enhanced sensitivity due to the thinner diaphragm than can be provided in the prior art.

The relative size of layer172, and other features, shown inFIGS. 12-14, as well as in other figures herein, is not to scale, but is enlarged to facilitate illustration of aspects of the invention.

Layer172may be formed by depositing a material on the surface of recess168in channel164, for example, by growing or depositing one or more layers of polymeric material, for example, a multilayer diaphragm. Layer172may be formed by any conventional thin layer or diaphragm forming process, for example, by conventional means of applying a thin layer to the surface of recess168. However, in one aspect of the invention, layer172may be formed by one or more photolithographic or MEMS-type processing methods, for example, a depositing process, for instance, a vapor deposition process (VDP), a low-pressure chemical vapor deposition (LPCVD) process, an atmospheric pressure chemical vapor deposition (APCVD) process, a high-vacuum chemical vapor deposition (HVCVD), or a plasma-enhanced chemical vapor deposition (PECVD), among other methods. In one aspect, a LPCVD process may be used to deposit silicon nitride on recess168. The inventors have found that LPCVD-deposited silicon nitride has preferential etching qualities compared to other materials present. For example, when silicon dioxide is present (as will be discussed below with respect to the aspect of the invention described inFIGS. 24-26), LPCVD-deposited silicon nitride is removed during etching, for example, with a buffered oxide etchant (BOE) or hydrofluoric acid (HF), at a slower rate than the removal of silicon dioxide. Therefore, in one aspect of the invention, layer172may be deposited by a LPCVD process in order to facilitate exposure of the bottom surface of layer172at a later time to form a diaphragm. The inventors have also found that an LPCVD process can fabricate silicon nitride with better mechanical properties, for example, with lower pinhole density.

Layer172may be made from any material conducive to one or more the deposition processes listed above, for example, in one aspect, layer172may be made from a silicon nitride. Layer172may be a metal, for example, copper or aluminum; an alloy, for example, a chromium (Cr)/copper (Cu) alloy; a polymer, for example, a poly (methyl methacrylate) (PMMA) or its equivalent; a semiconductor material, such as, silicon or polysilicon; a dielectric material, for example, silicon dioxide (SiO2) or a silicon nitride; or a combination of two or more of these materials.

It will be understood by those of skill in the art that the expression “silicon nitride” is not limited to the material having the chemical formula Si3N4, but may include similar materials containing silicon and nitrogen. For example, one material that may be used for the layer may comprise a silicon nitride grown by PECVD and having at least some hydrogen in addition to the silicon and nitrogen. In one aspect, the material for the layer172may comprise materials that may not strictly adhere to the 3:4 ratio indicative of the formula Si3N4, for example, silicon and nitrogen containing materials having their composition varied in order to adjust the stress field in layer172and/or optical properties (such as, refractive index) of the material, such as, the presence of oxygen. For example, thermal gradients that may be generated during and after fabrication may cause stresses in layer172and in the subsequent diaphragm due to, for example, variation in thermal expansion coefficients of the mating materials. Varying the content of layer172may vary the expansion coefficient of layer172such that these stresses may be reduced.

Layer172may be, translucent, transparent, opaque, or at least partially reflective. For example, Layer172may comprise and at least partially transparent silicon dioxide or silicon nitride, and/or the top or bottom surface of layer172may comprise a thin metal layer, for example, a reflective thin metal layer, for instance, a sputtered gold layer.

In one aspect of the invention, prior to depositing layer172unto the surface of recess168, a dielectric material may be applied to the surface of recess168to act as an etching stop layer beneath layer172during subsequent processing, for example, during etching of a cavity beneath diaphragm173, as discussed below. For instance, a thin layer of dielectric, such as, a silicon oxide layer or a silicon nitride layer grown by PECVD, may be applied to the surface of recess168to function as an etching stop layer.

FIGS. 15,16, and17, are a perspective view, a cross-sectional view, and a plan view, respectively, of recess168and layer172shown inFIGS. 12,13, and14, respectively, having a cavity or hole178beneath the layer172according to one aspect of the invention whereby a diaphragm173is “released.” The cross-section shown inFIG. 16is as viewed through section16-16shown inFIG. 15.

According to aspects of the invention, any process may be used to remove at least some material from beneath layer172to provide a diaphragm173in layer172, for example, to “release the diaphragm”173from the surrounding substrate156. According to aspects of the invention, the released diaphragm173can have a surface that may be exposed to a stimulus, for example, an AE wave, whereby diaphragm173is deflected. Cavity178may be formed in substrate156by conventional processes, for example, by milling or drilling of substrate156. However, in one aspect of the invention cavity178may be formed by one or more photolithographic or MEMS-type processing methods, for example, an etching process, for instance, an anisotropic wet etching process, an isotropic wet etching process, an anisotropic dry etching process, or an isotropic dry etching process, among other methods. In one aspect, cavity178may be formed by wet anisotropic etching where the etchant may be a buffered oxide etchant (BOE) or hydrofluoric acid (HF). For example, a 49% concentration of HF may be used. In another aspect, a dry anisotropic etching, for example, Inductively Coupled Plasma Reactive Ion Etching (ICP-RIE), may be used. One preferred etching process is illustrated and described with respect toFIGS. 24,25, and26below.

Cavity178and, consequently, diaphragm173may have many different cross sectional shapes according to aspects of the invention, for example, having a square, rectangular, polygonal, circular, semi-circular, or oval cross section, among other cross-sectional shapes. The width or outside diameter and depth of cavity178and the outside dimension of diaphragm173may vary broadly depending upon the method used to produce it. For example, when cavity178is produced by the ICP-RIE method, the shape of cavity178may be an ellipse having a projection on surface168of a circle defining the diameter diaphragm173. The resulting diameter of diaphragm173may range from about 10 μm to about 300 μm, and typically may range form as a width or outside diameter of about 20 μm to about 100 μm. The relative size of cavity178shown inFIGS. 15-17, as well as in other figures herein, is not to scale, but is enlarged to facilitate illustration of aspects of the invention.

FIGS. 18,19, and20, are a perspective view, a cross-sectional view, and a plan view, respectively, of the groove164shown inFIGS. 15,16, and17, respectively, having a wave-guide152positioned in the groove164according to one aspect of the invention. The cross-section shown inFIG. 19is as viewed through section19-19shown inFIG. 18.FIGS. 18,19, and20illustrate a typical complete sensor assembly according to aspects of the invention.

Wave-guide152may be any conventional wave-guide adapted to transmit an electromagnetic beam or wave and direct it upon diaphragm173. The electromagnetic beam or wave transmitted by wave-guide152from a source (not shown) may comprise any form of electromagnetic radiation that can be directed along a wave-guide, including microwaves, T-rays, infrared light, visible light, ultraviolet light, X-Rays, gamma rays, or radio waves. However, in one aspect, the beam or waves transmitted by wave-guide152comprise a laser, for example, a near infrared laser with wavelength of 1.55 um.

In one aspect, wave-guide152comprises an optical fiber, for example, a conventional optical fiber having a circular cylindrical shape and a circular cross section. Wave-guide152may be a single-mode or a multimode optical fiber. In one aspect, wave-guide152may be a SMF-28 single mode optical fiber provided by Corning Inc., or its equivalent. Wave-guide152may be coated, for example, coated to vary the reflectivity of the wave-guide.

According to aspects of the invention, wave-guide152may be a an optical fiber having a end153that is beveled at an angle, for example, beveled and polished, whereby a beam or wave transmitted along wave-guide152is reflected from end153upon diaphragm173. The angle of the beveled end153of wave-guide152may vary from about 40 degrees to about 50, for example, about 45 degrees, to the axis of wave-guide152, that is, depending upon the relative location of the beveled end153to the diaphragm173. However, in one aspect of the invention, diaphragm173is positioned substantially beneath beveled end153whereby the angle of beveled end153is about 45 degrees to the axis of wave-guide152whereby beveled end153transmits light to and receives light from diaphragm173.

The outside diameter or sidewall of optical fiber wave-guide152typically contacts, for example, firmly contacts, the sidewall of channel164. According to aspects of the invention, as shown inFIG. 5, the sidewall of wave-guide152may define the boundary of a F-B cavity.

FIGS. 21,22, and23illustrate three perspective views of an EFPI sensor250fabricated according to aspects of the invention.FIG. 21is a perspective view, similar toFIG. 18, of sensor250.FIG. 22is a perspective view, partially in cross section of sensor250shown inFIG. 21andFIG. 23is a plan view of sensor250shown inFIG. 21. Similar to the aspect of the invention shown inFIGS. 3-20, sensor250shown inFIGS. 21-23, includes a substrate256, for example, a silicon substrate; a channel264in substrate256; a recess268in channel264; a layer272, for example, a silicon nitride diaphragm, positioned in recess268; a cavity278in substrate256beneath layer272forming diaphragm273; and a wave-guide252, for example, a fiber optic, having a beveled end253positioned in channel264whereby beveled end253is positioned over diaphragm273. The features and aspects of sensor250are similar to, if not identical to, the features of the corresponding structures shown inFIGS. 3-20but identified without the preceding numeral “2” or having the preceding numeral “1” or “4” or “5” instead of the preceding numeral “2.”

FIGS. 24,25, and26illustrate steps in a method of providing a cavity beneath the diaphragm according to one aspect of the invention. As shown inFIGS. 16,19, and22above, due to the orientation of the surface of recess168,268, the axis of cavity178,278in substrate156,256beneath layer172,272may typically form an angle with the plane of the surface of recess168,268. Accordingly, the use of conventional material removal processes, for example, anisotropic etching, can result in undesirable, non-uniform thicknesses of diaphragms173,273formed in layer172,272. The processes shown inFIGS. 24-26addresses this issue.

FIG. 24illustrates a section of a substrate356, for example, a single-crystal silicon substrate, having a representative surface368of a channel, such as, channel168or268above. Upon surface368is deposited a first material371, for example, a silicon dioxide, and then a second material372, for example, a silicon nitride. According to aspects of the invention, the materials371and/or372may be used to form a diaphragm, such as diaphragm173,273disclosed above, beneath which a cavity378is formed to expose at least a portion of the deposited materials371and/or372to provide a diaphragm373according to aspects of the invention. Materials371and372may be deposited on substrate356by any one or more of the conventional deposition methods mentioned above. As shown inFIG. 24, because the orientation of the surface368of the recess378is not perpendicular to the orientation of the surface368of substrate356, for example, a silicon wafer, and because the material of substrate356and first material371may typically not respond at the same rate to the material removal process, for example, etching, the depth of cavity378may vary due to the material removal process, as shown. As shown inFIG. 25, an etching process, for example, dry anisotropic etching process, may not remove material from material371at the same rate as substrate356, but as a result, the removal process may remove only a portion of first material371. As a result, the uneven removal of first material371, if not addressed, may typically result in a non-uniform thickness in the diaphragm comprising the remaining portions of first material371and second material372.

Since a non-uniform diaphragm thickness is undesirable, according to one aspect of the invention, the first material371is first deposited before second material372where first material371has an material removal rate that is different, for example, greater than, the second material372. For example, first material371may be a silicon dioxide and second material372may be a silicon nitride. In other words, according to aspects of the invention, first material371acts as a “buffer layer,” for example, protecting the second material372from the material removal process.

As shown inFIGS. 25 and 26, after a first material removal step, for example, dry anisotropic etching, removes the material of substrate356, for example, a silicon, and at least some of first material371, for example, a silicon dioxide, a second material removal step removes first material371to expose at least a portion of second material372, while removing little or none of second material372, to provide a substantially uniform diaphragm373of second material372. In one aspect, the second material removal process may be an etching process employing hydrofluoric acid (HF). The removal rate of silicon dioxide371is greater with HF etching than the removal rate of silicon nitride372with HF. According to an aspect of the invention, the HF etching process removes first material371to yield a second material372of substantially uniform thickness to provide a diaphragm373of substantially uniform thickness on the surface368. In one aspect, second material372may be a silicon nitride produced by a LPCVD process, though a silicon nitride produced by a PECVD process may also be used.

According to another aspects of the invention, the accuracy of the etching process may be enhanced by what is known in the art as “salient compensation” or “convex corner compensation.” For example, as described by Chu and Fang in “A Novel Convex Corner Compensation for Wet Anisotropic Etching on (100) Silicon Wafer,”Micro Electro Mechanical Systems,2004, 17th IEEE International Conference on MEMS, (2004), pp. 253-256 (the disclosure of which is incorporated by reference herein in its entirety), various methods are disclosed for controlling the shape and positioning of etched structures. One method of convex corner compensation that may be applied to aspects of the present invention is illustrated inFIGS. 27 and 28.

FIG. 27is a schematic illustration of the typical size and shape of a mask for an etched structure, for example, a V-groove as discussed above, when etching is practiced without convex corner compensation and the typical resulting structure.FIG. 28is a schematic illustration of the typical size and shape of a mask for an etched structure, for example, a V-groove, when etching is practiced with convex corner compensation. According to aspects of the invention, the top image inFIG. 27represents a top plan view of a channel564, for example, similar to channel64,164, or464described above, having a recess568, for example, similar to recess68,168, or468described above. For instance, the top image inFIG. 27may represent the appearance of the desired structures after etching or the image of the substrate as masked. According to aspects of the invention, the size and location of the transition between channel564and recess568, indicated by arrow570in the top image ofFIG. 27, is typically significant, if not critical, to the formation of the desired sensor according to aspects of the invention. However, without convex corner compensation, due to, for example, convex corner undercutting, the conjunction between larger and smaller structures, for instance, grooves, will be etched towards the smaller one. For example, the resulting structures created by etching, for example, with potassium hydroxide (KOH), are illustrated in the lower image ofFIG. 27. The lower image ofFIG. 27represents the top plan view a channel664, recess668, and transition670produced by etching without convex corner compensation. Clearly, a comparison of the two images inFIG. 27reveals that the shape and location of transition670has varied markedly from the desired shape and location of the desired transition570.

In contrast to the images shown inFIG. 27, the images shown inFIG. 28illustrate the desired shape of a mask for an etched structure and the resulting structure obtained by aspects of the invention. According to aspects of the invention, the top image inFIG. 28represents a top plan view of a channel764, for example, similar to channel64,164, or464described above, having a recess768, for example, similar to recess68,168, or468described above. According to aspects of the invention, in order to more precisely provide the desired size and location of the transition between channel764and recess768, indicated by arrow870in the bottom image ofFIG. 28, at least one salient compensation structure is provided to reduce or even eliminate the movement of the structure or feature towards the smaller structure. For example, as shown in the upper image ofFIG. 28, at least one protection bar770may be provided to minimize or prevent movement of the transition, for example, due to convex corner undercutting, for example, as disclosed by as Chu and Fang. The lower image ofFIG. 28represents the top plan view a channel864, recess868, and transition870produced by etching, for example, with potassium hydroxide (KOH), with convex corner compensation, that is, with protection bars770. Clearly, a comparison of the two images inFIG. 28reveals that the shape and location of transition870is much more consistent with the desired shape and location of the transition.

In another aspect of the invention, the accuracy of the position and dimensions of etched features may be improved. As is known in the art, the accuracy and quality of position and dimensions of a channel, groove, or recess in a substrate after etching, for example, wet anisotropic etching, is sensitive to the alignment accuracy between the etching window edge and the orientation of the crystal planes of the substrate, for example, of the single crystal silicon. Typically, the commercial standard for etching window edge alignment accuracy with the crystal planes of the substrate comprises an off-orientation accuracy of +/−1 degree. Aspects of the present invention can improve this off-orientation accuracy compared to prior art methods.

FIGS. 29 and 30illustrate one aspect of the invention that provides “orientation assistance” to the patterning of etched features. Aspects of the invention can provide etching window edge alignment accuracy with the crystal planes of the substrate with an off-orientation accuracy of greater than the industry standard+/−1.0 degrees.FIG. 29is a schematic illustration of an etched pattern202produced by a process of etching a substrate in a first step to indicate the correct orientation of the crystal planes of the substrate, for example, by anisotropic wet etching of single crystal silicon.FIG. 30is a schematic illustration or an etched pattern204on the substrate206, for example, a silicon wafer, after etching, for example with potassium hydroxide (KOH). The lines in pattern204with non-correct orientation with respect to the substrate crystal planes will be etched away while the lines with correct orientation, that is, consistent with the substrate crystal planes, will survive the etching process and can then be used for reference in subsequent etching window placement and alignment. For example, if not aligned properly, the surfaces of the resulting channel or groove may not be smooth, but have undesirable steps, for example, like a staircase. By applying aspects of the invention, the dimensions and positions of etched features can be improved and smoother feature surfaces can be provided. For example, aspects of the invention can provide a dimensional and positional accuracy of +/−1.0 degree or finer, for instance, as fine a tolerance as +/−0.1 degrees has been realized by employing aspects of the invention.

Experimental Validation—Static Test

Aspects of the invention were validated in laboratory trials.FIG. 31is a schematic diagram of an experimental setup210used by the inventors to validate aspects of the invention. Setup210includes a pressurized gas source212, such as an air tank, that is used to provide a high-pressure air to pressurize a pressure chamber214. The flow of air is directed through a conduit216and is regulated by a control valve218. A sensor220according to aspects of the invention having optical fiber221was tested side by side with a commercially available pressure transducer222, specifically, an Omega model PX303 pressure transducer. The output of the reference pressure sensor222was assumed to be the true pressure value applied to the testing chamber214to evaluate the performance of the optical sensor220. The outputs of sensor220and222were forwarded to a data acquisition and recording system including a spectrum measurement system224and a computer226. The results of static testing are displayed inFIG. 32.

FIG. 32is a graphical representation230of the spectrum shifts of the sensor220for data collected with the setup210ofFIG. 31. Solid black squares inFIG. 22represent the measured values of sensor220when pressure was increased consistently and the red (open black) squares are the measured values for sensor220when pressure was decreased consistently.FIG. 32displays the comparative test of sensitivity, linearity, and hysteresis for the invention220. The graph230indicates a sensitivity of spectrum shift of sensor220according to the invention is about 0.15 nanometers/kiloPascal (nm/kPa). The linearity of sensor220measurements is good with a correlation coefficient (R) of 0.99987. The maximum shift difference for bidirectional running at the same pressure is about 0.05 nm, which means a maximum hysteresis is about 0.32%. The sensitivity of the diaphragm deformation of the invention is about 4.64 nm/kPa, which is similar with the calculation results of 4.404 nm/kPa. Clearly, aspects of the present invention agree very well with a commercially available sensor.

Experimental Validation—Dynamic Test with Pressure Release

FIG. 33is a graphical representation240of the comparison of the dynamic response of sensor220compared to reference sensor222using the setup210shown inFIG. 31, but with a modified data acquisition system. The data acquisition system of system210was modified to measure the pressure change when the gas was released suddenly. In particular, a laser with fixed wavelength and a photo detector were used instead of the spectrum measurement system224. The data was sampled at200kHz.FIG. 33displays the voltage output by reference sensor222and optical sensor220according to the invention when the pressure is released quickly. As shown inFIG. 33, the reference sensor222(shown as a dashed line) responds to a fast pressure change from 90% to 50% within 7 milliseconds (ms) approximately. The optical sensor220(shown as a solid line) according to the invention, exhibits a similar response compared to reference sensor222. However, based upon calculations, the bandwidth of the optical sensor220according to the invention has a dynamic performance that is much higher than the reference sensor222. Quantifying the high-end dynamic range of optical sensor220is limited by the physical test setup210(for example, the speed of the pressure release) and the dynamic range of the reference sensor222. High frequency performance testing of aspects of the invention are also underway.

Experimental Validation—A Balloon Explosion

An investigation of the comparative response of a sensor according to the present invention and a reference sensor for a blast event was also undertaken using a punctured balloon as a blast source.FIG. 34provides a schematic diagram of the testing system250used in this investigation. A 4″ polyvinyl chloride (PVC) tube251was used to confine the acoustic wave generated by a balloon253. An electrical reference sensor255and the optical acoustic sensor257according to an aspect of the invention were mounted together on a Poly(methyl methacrylate) (PMMA) sheet259at the end of the PVC tube251. The outputs of reference sensor255and optical sensor257were connected to a data acquisition system (DAC)261.

The balloon253was placed into the center of the PVC tube251and then punctured262. The pressure wave generated by the popped balloon propagated to sensors255and257mounted at the end of tube251. The sampling rate of the DAC system261was set to 500 kHz.FIG. 35illustrates the results of the balloon testing for both sensors.

FIG. 35is a graphical representation260of the voltage output by reference sensor255and optical sensor257to the balloon blast simulation illustrated inFIG. 34. In order to facilitate comparison of the output data, in graph260, the signal265of reference sensor255was shifted −0.15 Volts (V) in comparison to the signal267of sensor257according to the invention. As seen in viewingFIG. 35, the responses from the two sensors are similar, though the output signal265of reference sensor255is somewhat smoother. The difference in quality of the signals may be due to a broad range of factors, including the lower response time of reference sensor255(that is, about 1 ms), the faster response time optical sensor257, and the different mounting locations of the sensors, among other sources, which are being investigated.

Experimental Validation—Accuracy of Cavity Length

The inventors also investigated the quality or uniformity of the F-P cavity length that can be achieved according to aspects of the invention. For example, the accuracy of the length of “Cavity1” shown inFIG. 5. In the testing summarized inFIG. 36, three nominal F-P cavity lengths were used: 15.643 μm, 23.368 μm, and 31.093 μm.

Thirteen v-groove type channels were fabricated on the same silicon wafer and then measured. The deviations of the cavity length from a nominal design value are shown the inFIG. 36.FIG. 36is a bar graph270of the deviation from nominal dimension of an F-P cavity length according to aspects of the invention. As shown inFIG. 36, six (6) out of the 13 samples are in the +/−0.05 μm range of nominal length (which, as discussed above, corresponds to a 1/16 period shift of the spectrum). As also shown inFIG. 36, ten (10) out of the 13 samples are within the +/−0.1 μm range of nominal length. The inventors believe that since imperfectness or deviations from the desired nominal cavity length may mainly come from defects in the substrate, for example, a silicon crystal, the precision of the cavity length of aspects of the invention may be improved by using high quality silicon wafers for substrates, for example, silicon wafers fabricated especially for Microelectromechanical System (MEMS) application and using high purity etchant.

FIG. 37is a top plan microscope photo280of a v-groove-type channel having circular diaphragms according to aspects of the invention. The dark horizontal band282in photo280is the flat bottom surface of the V-groove, for example, as shown inFIG. 3B, having sloping sidewalls283,284and circular diaphragms (which appear elliptical in this view). In photograph280inFIG. 39, the top side wall283includes one diaphragm285and the bottom side wall284includes one larger diaphragm286and one smaller diaphragm287. In the actual diaphragms shown, the silicon oxide in the substrate beneath the diaphragms was removed by 49% solution of HF.

As shown inFIG. 37, the dark central circles shown for diaphragms285,286, and287are the through holes of the cavity (cavity378inFIGS. 24-26) and the bright circular bands about the central circles are the areas about the through holes (378) where the silicon oxide (371inFIGS. 24-26) between the silicon nitride (372inFIGS. 24-26) and silicon substrate (356inFIGS. 24-26) has been removed by the HF etching (seeFIGS. 24-26and the associated discussion). As indicated byFIG. 37, the high-concentration hydrofluoric acid (HF) can reach the silicon oxide (371) via the small deep holes (378) etched from the back side of silicon wafer (356) according to aspects of the invention to provide the desired diaphragm release.

Though aspects of the present invention were developed for use in fabricating AE sensors, it is recognized that aspects of the invention are not limited to AE sensor fabrication, but can also be applied in the fabrication of other diaphragm-type sensors, especially, in diaphragm-based optical fiber sensor. Examples include pressure sensors, accelerometers, and temperature sensors. It is also recognized that aspects of the invention are also applicable to non-sensor technologies, for example, to the fabrication of any type of diaphragm or membrane that may be desired in the MEMS fabrication art.

It will be clear from the above description to those of skill in the art that aspects of the present invention include the fabrication of silicon-based anisotropic wet etched V-grooves as a diaphragm base structure; the use of photolithographic methods to achieve precise F-P cavity length control and high yield; the use of angled and polished wave guides to deliver and collect light to and from the diaphragm; and the fabrication of very thin and high quality diaphragms. Among other advantages, aspects of the present invention provide many advantages over the prior art methods and sensors. Specifically, among other things, aspects of the present invention provide:a simple fabrication process employing substantially standardized MEMS processing techniques;precise cavity length control based on a novel technology that can precisely control F-P cavity length to better than +/−0.05 micros;precise diaphragm thickness control;ultra-high sensitivity sensors having diaphragm thicknesses that can be fabricated to tens of nanometers;wide ranging applications, including high pressure measurement, for example, using thick diaphragms of tens of microns in thickness;a flexible design that can be adapted to specific applications, including a metal layer can be easily added by thermal evaporation or sputtering method;simplified assembly using, for example, V-groove channels and optical fiber can be easily assembled and aligned to the substrate;a robust structure that provides hard contact between the wave guide and substrate;good temperate stability due to the small cavity length;good leak-proof quality since sensors according to the invention can be sealed using standard MEMS bonding technology; anda high yield and low cost manufacturing process where different sizes of diaphragms can be fabricated on a single substrate at the same time and selected for assembly on different sensors having varying specifications.