WO2008080004A1 - Gravage d'isolants assisté par rayons x - Google Patents

Gravage d'isolants assisté par rayons x Download PDF

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Publication number
WO2008080004A1
WO2008080004A1 PCT/US2007/088422 US2007088422W WO2008080004A1 WO 2008080004 A1 WO2008080004 A1 WO 2008080004A1 US 2007088422 W US2007088422 W US 2007088422W WO 2008080004 A1 WO2008080004 A1 WO 2008080004A1
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Prior art keywords
substrate
electrolyte
current
etch
series
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PCT/US2007/088422
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Ville Kaajakari
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Louisiana Tech Research Foundation
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Publication of WO2008080004A1 publication Critical patent/WO2008080004A1/fr
Priority to US12/479,482 priority Critical patent/US20100236940A1/en

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    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25F—PROCESSES FOR THE ELECTROLYTIC REMOVAL OF MATERIALS FROM OBJECTS; APPARATUS THEREFOR
    • C25F3/00—Electrolytic etching or polishing
    • C25F3/02—Etching
    • C25F3/14—Etching locally

Definitions

  • Quartz crystal resonators are a multibillion dollar a year industry. Crystalline quartz resonators are widely used in high tech electronics, such as communications devices and sensors. These resonators are used as frequency references and clock generators. Quartz is the preferred material for resonators because of its piezoelectric nature, high quality factor, and excellent thermal stability. Where other materials, such as silicon, require external temperature compensation, properly cut quartz resonators are insensitive to variations in environment temperature. As the devices which employ these resonators continue to shrink in size, miniaturization of the resonators is important for reduced size, weight, and power consumption.
  • quartz processing for high frequency resonators is still a process of repeated mechanical and/or chemical polishing of the crystals to achieve the desired shapes.
  • This processing takes significant time and is a barrier to further miniaturization because of increasing production costs.
  • This process is also difficult to scale to larger production levels, since resonator fabrication must be individually monitored.
  • MEMS fabrication techniques allow for wafer level processing which can produce hundreds of devices on a single wafer making large scale production more efficient. Unfortunately most traditional MEMS fabrication techniques are not well suited for use on crystalline quartz because of its anisotropy and resistance to chemical etching.
  • Quartz is an anisotropic material and chemical etching in most crystal orientations tends to be slow with low aspect ratios, but is routinely undertaken. See, for instance, Suda P, Zumsteg A, Zingg W Anisotropy of etching rate for quartz in ammonium biflouride. In: 33rd Annual Symposium on
  • Electrochemical etching is a form of wet etching which exploits current passing through the material in order to control the etch rate. Fundamentally, chemical etching is facilitated by charge transfer between the material surface and the etchant disassociating the surface atoms. By passing current through the material, the number of electrons available at the etch front is changed. This effect has been exploited in silicon micromachining to vary the etch rate, affect the aspect ratio, and create etch stops. See U.S. patent number 6511915.
  • FIG. 6 In a typical arrangement, shown in Figure 6 some parts of the Si sample are exposed to the electrolyte, typically an acid. Electrical contact is made with the sample (considered an electrode). A counter electrode, typically platinum, is immersed within the liquid. By applying electrical bias in the form of an applied voltage between the sample and counter electrode, current flows through the sample-liquid interfaces. The current affects the availability of electrons to take part in the silicon dissolution process. Typically, it is observed that small currents increase the etching rates while higher currents decrease the etching rate, or may even stop the etching.
  • a typical etch rate versus bias voltage for electrochemical etching is shown in Figure 7. Depending on bias voltage, the etch rate can vary by almost two orders of magnitude. This can be used for etch stops and to make microstructures such as membranes. Additionally, visible light has been used to generate excess carriers in semiconductors to enhance electrochemical etching. See U.S. patent number 6,51 1,915.
  • Quartz is a typical example of an insulating piezoelectric crystal.
  • Figure 2 shows measured current and resistance through a quartz sample with bias ranging from 10 V to 500 V. As can be seen from this Figure, the current through the sample is small and consequently the resistance is high. The resistance also remains approximately constant over the measured bias range. This is to be expected for electric fields less than 25 kV/mm. At very high voltages, dielectric breakdown will occur and high currents can flow through the sample. However, these high voltages are not a practical way to force large currents through the sample as the breakdown is largely uncontrollable and it is difficult to prevent arcing. Thus, insulators, such as quartz, will not sustain a significant electrical current with just a voltage bias, and consequently are not susceptible to the classical electrochemical etching process.
  • Visible light is characterized by photon energy less than 3 eV which is not sufficient to overcome the band gap E G in insulators. Hence, other techniques are needed if electrochemical etching of an insulator is to be undertaken.
  • the invention is a method of inducing a sufficient current in a insulator to allow electrochemical etching to take place.
  • the current can be induced by illuminating the insulator with sufficiently high energy electromagnetic radiation in order to lift electrons from the valence band to the conductance band and applying a voltage bias to the insulator to direct the current as desired.
  • the insulator can also be made conductive by illuminating the insulator with charged particles, such as electrons. The absorbed charged particles can be used to create a current in the insulator for electrochemical etching.
  • the method can be used with etch masks and x-ray or electron masks.
  • Figure 1 depicts the classification of materials based on the electronic band structures.
  • Figure 2 graphs the experimental current through a quartz plate and sample resistance.
  • Figure 3 shows the energy spectrum for electromagnetic radiation.
  • Figure 4A depicts a quartz sample irradiated X-rays.
  • Figure 4B shows the variation of current in the quartz sample with x-ray tube cathode voltage variation.
  • Figure 5A is a graph of the variation of the current in the sample of figure 4 with variation of the biasing voltage, while the x-ray radiation energy is held constant.
  • Figure 5B is a graph of the variation of quartz resistance with the biasing voltage, while the sample is under a constant x-ray flux with average energy of 2.5 keV and intensity of approximately 1 W/cm 2 .
  • Figure 6 shows a typical electrochemical etching set-up (PRIOR ART).
  • Figure 7 shows a typical etch rate vs. bias voltage curve for electrochemical etching (PRIOR ART).
  • Figure 8 depicts a set-up to electrochemically etch normally insulating materials such as quartz using photons such as x-rays.
  • Figure 9 depicts a set -up to electrochemically etch normally insulating materials such as quartz using electrons (9c) and X-rays, (9b).
  • Figure 10 shows a graph depicting etch rate and current for a sample irradiated with an electron beam.
  • FIG. 11 shows a typical arrangement for the electrochemical etching of an insulator (hereafter referred to as a quartz sample), using photons such as x-rays.
  • Figure 12 depicts electrochemical etching (EE) using a masks on each side of crystal material and no bias is uses, simply a grounded platinum electrode
  • Figure 13A depicts the arrangement of Figure 12, but employs a biasing voltage
  • Figure 13 B depicts an EE application of a back bias voltage to steer electrons to reduce the etch rate at particular locations. .
  • Figure 13 C depicts EE using a back bias to steer the electrons towards to middle to obtain angled etch.
  • Figure 14 depicts the method using an x-ray mask is positioned between the sample and the x-ray source.
  • the etchant can be alternatively on the side of or opposite to the x-ray source.
  • the reference electrode is not needed but can be used to monitor the x-ray generated current through the sample.
  • Figure 15 depicts the method using a variety of biasing configurations shown in figures (b) and (c) to selectively vary the etch rate.
  • Figure 16 depicts the method using a conductive electrode is positioned over a first side of the crystal material and etch masks placed on the side of the crystal exposed to a etching solution.
  • An x- ray mask is positioned between the sample and the x-ray source.
  • a variety of bias electrode configurations can be used on both sides of the crystal as shown in figures (b), (c), (d), and (e) to selectively vary the etch rate.
  • the back bias is used to steer the current flow to obtain sloped etch.
  • selective biasing is used to obtain different etch rates in different locations.
  • the back bias is used to steer electrons to obtain reduced side wall etch rate.
  • FIG. 17 depicts the method using a conductive mask is positioned over both sides of the crystal material.
  • a variety of bias electrode configurations can be used on both sides of the crystal as shown in figures (b), (c), (d), and (e) to selectively vary the etch rate.
  • Figure 18 depicts the method using multiple electrodes positioned on the sample
  • Figure 19 depicts the method using masking layers on each side of the crystal material. Both sides of the crystal are exposed to a hydrofluoric acid etching solution. A bias current is induced across the crystal by connecting a voltage source between lead electrodes placed in the etching solutions
  • Figure 20 depicts the penetration of X-rays into a quartz sample, showing (a) Soft x-rays, below
  • Figure 22 is a graph showing the measured etch rate of a quartz sample as control, using not electrochemical etching.
  • Figure 23 is a graph showing the measured etch profiles using an electrochemical etch .
  • Figure 24 depicts the selective etching through controlled charge carrier injection (or radiation illumination) using the electrochemical etching method, as opposed to chemical etching of an insulator. Etching in the direction of the current is 3X faster than other directions allowing for sharper defined wall structures.
  • Figure 25 depicts one process for creating flat resonators.
  • Figure 26 depicts one process for creating inverted mesa resonators
  • Figure 27 depicts one process for creating mesa resonators
  • the invention is electrochemical etching of an electrically insulating material by assisting the production of current in the insulator.
  • the number of current conducting carriers should be increased.
  • Two possible ways to generate free carriers(electrons or holes) in insulators are:
  • Valence band electrons must be provided with sufficient energy to lift them to the conductance band.
  • the natural division between low band gap and high band gap materials is 3 eV.
  • an "insulator" is a material having a band gap greater or equal to 3 eV. This energy level corresponds to the edge of the visible light spectrum.
  • Suitable electromagnetic radiation will be ultraviolet light or higher frequency radiation, such as x-rays.
  • the energy of the photons needs to be larger than the band gap energy.
  • optical photons visible light
  • photons with higher energy are required.
  • lithium niobate crystals LiNbO 3
  • quartz crystal which has a band gap of 9 eV. This translates to photon wavelength of about 135 nm and thus photon wavelength should be 135 nm or smaller to excite electrons in this material.
  • Fluorescent lamps produce UV radiation by ionizing low-pressure mercury vapor.
  • UV light into any material is usually very low and UV is therefore not preferred as the illumination radiation.
  • X-rays can be readily produced with available sources, such the x-ray tubes or synchrotrons, and x-ray sources are a ready source of the high energy photons needed.
  • the high energy photons or x-rays provide the benefit of penetrating deeper inside the material before being absorbed. This penetrating property of x-rays is useful for samples that employ a metal sheet or metal conductor used as the electrode for the electrochemical etching process. Lower energy photons would be easily absorbed by a sufficiently thick metal sheet, thus blocking the photons from entering the sample.
  • the higher energy x-rays can more readily penetrate through the electrode into the sample, thus making the sample more conductive.
  • Figure 4A shows the experimental set-up used to verify photon induced conductivity in an insulating quartz crystal.
  • the sample used was a quartz crystal plate with thickness of 380 ⁇ m and is biased electrically between two gold conductors with thickness of 100 nm.
  • the sample was exposed to x-rays with energies ranging from 100 eV to 40 keV using x-ray tube.
  • Figure 4B shows resulting current vs. x-ray tube cathode voltage variation. Due to photon generated carriers, the currents are significantly larger than that shown in Figure 2 for a quartz sample that was not exposed to x-rays.
  • Figure 5A shows the variation of the current in the sample of Figure 4 with variation of the biasing voltage, keeping the x-ray radiation energy constant with average energy of 2.5 keV and
  • the electrochemical etching process can hence be used with insulating material if coupled with illumination of the sample with sufficiently energetic electromagnetic radiation.
  • the known chemical etching process parameters etchant chemical, PH, and process temperature
  • the biasing voltage and energy level and intensity of the illumination radiation chosen to produce the desired current levels in the material.
  • increasing current levels will generally increase the etch rates.
  • Figure 10 shows the experimental data showing the variation of etch rate with the current density. Although the data shown in this figure arises from the electron illumination method described later, it is equally applicable to illumination with photons such as x-rays.
  • an x-ray tube biased 35 keV and operating at 1 mA electron beam current gives sufficient x-ray flux so that quartz crystal sample that is 380 ⁇ m thick will have current of -10 nA/cm 2 to +10 nA/cm 2 when biased between -2000 V and 2000 V.
  • This current variation through the sample results in similar etch rate variations as obtained with the e-beam set-up.
  • the etch rate can be either accelerated or decelerated.
  • a maximum variation of approximately 600% is seen, +/-300% from baseline.
  • the optimum current density to be generated through the crystal material will depend on numerous factors, including the etching solution, the solution temperature, the solution pH, the material being etched, and the structure to be created by the process.
  • quartz being etched with hydrofluoric acid at 32 0 C exhibits large etch rate variations at a current density between 0-5 nA/cm 2 .
  • a rapid etch rate may be desired to create fine grained shape features in the etched material.
  • the rates may be varied over time to produce three dimensional structures in the insulating material.
  • the inventive process thus allows for a degree of control of the etching process not possible with chemical etching alone.
  • the technique can be employed in a variety of equipment configurations.
  • Etching apparatus typically includes a container for containing a supply of electrochemical electrolyte or etchant (etchant and electrolyte will be used interchangeably).
  • the container generally has an open side or bottom which is sealable against the insulator to be etched, allowing the electrochemical etchant to contact the substrate.
  • An electromagnetic radiation source for generating electromagnetic radiation in excess of 3 eV is positioned adjacent to container.
  • the radiation source will be x-rays as x-ray sources are readily available, and x-rays have the necessary energy. Radiation energy of greater that 10OeV is preferred, and greater that 1 keV more preferred.
  • Intensity of the radiation should exceed 1 uW/cm 2 , with a preferred intensity of about 1 mW/cm 2 .
  • the radiation source directs the radiation through a window onto the sample for generating electron-hole pairs (hence forth simply referred to as "electrons") (see Figure 9C)
  • An electrical bias is applied to the substrate by connecting a bias source (a voltage source) by wire to a contact (typically a metallic film) on the back side of the substrate, and by wire to a reference electrode immersed within the etchant.
  • a bias source a voltage source
  • the bias generated is from a DC source, although in limited circumstances, an AC generated bias can be used.
  • a reverse bias (or back bias) source can be applied to the substrate by connecting an electrical bias source (it may be the same source) to a contact on the front side of the substrate (typically a metallic film) to counter or reduce the bias applied to the substrate from the back side of the substrate.
  • an electrical bias source it may be the same source
  • the bias is created between the contact and the electrolyte, but the electric field created also extends into the substrate and has effects in the substrate.
  • the combination of radiation and electrical bias generate a current, and the current density needed will depend on the activity of the electrolyte and the desired
  • the illumination radiation can be sourced to illuminate the sample from any angle, through the etchant, or using direct exposure of a portion of the sample or substrate isolated from the etchant.
  • a typical setup is shown in Figure 9C. If the radiation source must pass through a substantial volume of the etchant to reach the sample, the radiation flux or radiation energy levels may have to increase to account for energy absorption by the etchant (variation in the etchant temperature may also be effected in this instance).
  • the process can be used with etch masks and radiation masks, as further described, and other configurations commonly used in electrochemical etching. In the following examples, the radiation is shown illuminating a portion of the sample that is isolated from the etchant.
  • the polarity of the applied electrical bias is shown as negative on the substrate and positive on the reference electrode. This is shown for convenience only and the desired polarity of the bias will depend on the composition of the substrate, the reference electrode, and the electrolytic materials, and the effect to be achieved. For instance, when etching quartz using HF, if very fine structure details are desired that would necessitate reducing the etch rate, the polarity of the applied bias could flip (+ on the substrate) and (-) on the reference electrode.
  • Figure 8 shows a typical arrangement for the electrochemical etching of an insulator (hereafter referred to as a quartz sample), using photons such as x-rays.
  • a quartz sample an insulator
  • One side of the quartz is in contact with the etching solution. Electrical contact is made to a region on the other side of the sample to enable current flow through the sample.
  • By biasing the sample electrically limited current will flow through the sample. This current is normally too small to significantly affect the etch rate.
  • the quartz sample hereafter referred to as a quartz sample
  • the preferred etching solution depends on the material of interest.
  • fluoride based acids are commonly used as fluoride is more electronegative than oxygen and can break to oxygen bonds.
  • acids are hydrofluoric acid (HF), ammonium fluoride (NH4F), and ammonium bifluoride (NH4HF2).
  • HF hydrofluoric acid
  • NH4F ammonium fluoride
  • NH4HF2 ammonium bifluoride
  • the pH of the fluoride solution is known to strongly affect the etching rate and this effect can be used in optimizing the electrochemical etching.
  • the electrochemical etching can be used to adjust the etch rate variations across the insulating wafer sample to obtain better control of the final product, such as individual resonators.
  • Figure 11 shows an example where individual electrodes are positioned at two regions of the substrate and used to control the etch rates in two different locations on the exposed surface.
  • the current can be used to speed up or slow down the etch rate in different locations on the substrate's surface. This degree of control was unavailable in prior techniques, and can be used to produce a variety of differing structures, or mass produce a number of uniform structures.
  • the prior typical quartz resonator manufacturing process first uses chemical etching to thin down a quartz wafer. This process is not accurate as the initial wafer thickness and the etch rate across the wafer may not be uniform. After the rough etching, the quartz wafer is typically divided into individual resonators for individual etching. This is a costly process. Using the current technique, it should be possible to mass produce a quantity of resonators from one quartz wafer using a single etch process. It is possible to individually compensate each resonator structure on the wafer if the electrode at the location of each resonator is biased individually. To further increase the control over each resonator (e.g. thickness) it is possible to continuously measure the sample thickness and adjust the etch rate by modifying the bias voltage. One possible way to measure the sample thickness
  • ⁇ B0482967.1 ⁇ is to use optical methods such as the time of flight measurement, or ultrasonic transducer for thickness measurement by time of sound propagation through each resonator structure.
  • optical methods such as the time of flight measurement, or ultrasonic transducer for thickness measurement by time of sound propagation through each resonator structure.
  • Another method would be to directly (or indirectly) measure the current through each individual structure by monitoring voltage drop, resistance or current through each sample in the region(s) of interest.
  • the biasing voltages can be modified (including modification by initializing or increasing an applied back biasing voltage).
  • the biasing voltage directed through that structure such as a cavity or well
  • a back biasing voltage or rear bias voltage can be used to reduce the current flow through the structure, such as shown in Figure 13B.
  • Electrodes could be positioned across the wafer substrate and used not to create multiple structures, but a single structure of uniform thickness. As can be seen, the degree of control that is possible with the present technique is orders of magnitude greater than that previously available.
  • Figures 12-19 In these figures, x-rays are considered the illumination source, but any high energy radiation can be employed. Also shown are electrodes, x-ray masks and etch masks. The electrode is simply a conductor, and if the electrode is immersed in the etchant, the electrode should be inert to the etchant. Gold is a typical electrode for use in quartz etching using HF as the etchant.
  • An etch mask is a substance placed on the insulator that is inert to the etchant, and hence, reduces the etching process wherever the etch mask is located.
  • the etchant is HF acid
  • an etch mask can be made of gold.
  • An x-ray mask is a material designed to block x-Rays. A patterned mask, opaque to the radiation employed, is applied either to the substrate, or supported adjacent to the substrate. For finer control, the "opacity" of the mask could be varied, providing regions where the mask is semi transparent, allowing a certain
  • ⁇ B0482967.1 ⁇ reduced flux of radiation through (hereafter, we will refer to x-ray mask).
  • X-Ray masks and mask composition are well known in the art, and have considerable use in lithography.
  • Gold can also be used as an x-ray mask if thick enough, as well as lead and other metal compositions.
  • the required thickness of the x-ray mask will be dependent on incident energy of the illuminating radiation, and the material of the mask. As shown in Figure 2OA, low energy photons (soft X-rays, ⁇ lkeV) are effectively blocked by the metal electrode on the sample (depending on thickness) and will not assist in current generation.
  • the attenuation length for 100 eV photons in gold is just 20 nm and for 1 keV photons the attenuation length is 100 nm.
  • higher energy photons can be stopped by a thicker mask but can penetrate through the a thin electrode on the sample.
  • 5 keV photons have attenuation length of 1 ⁇ m in gold and 20 ⁇ m in quartz.
  • these x- rays can pass through on electrode that is a few hundred nanometers thick and generate carriers within a sample.
  • Even higher energy X-rays can be used as shown in Figure 2OC, these X-rays penetrate deep into the sample but are harder to mask.
  • the X-ray mask can be positioned directly on the sample, or positioned off the sample as there will be only minor diffraction effects using X-rays.
  • the preferred range of X-rays will depend upon the applied bias and material thickness, but 1 keV- 1000 keV are believed to be suitable for many applications..
  • Figure 12 shows a variation using an X-ray mask (shown as a gold mask) and an etchant mask. No biasing voltage is provided. Instead, the Reference voltage is grounded, providing a sink for the X- ray generated electrons. This is usually not preferred as the current generated will be low for a comparable apparatus using a biasing voltage, such as shown in Figure 13.
  • FIG. 13 A Shown in Figure 13 A is arrangement of Figure 12, but using a biasing voltage applied to the X-ray masks (typically, the X-ray mask will be several micrometers thick).
  • a biasing voltage (preferred range is about 100-3000 volts) is used to induce a current across the crystal material by connecting a voltage source across the sample by applying a voltage to the electrodes (one of which is preferably is in contact with the insulator sample).
  • gold is used as the bias electrode (typically, about 100 nanometers thick).
  • Figure 13B is similar, but a second biased voltage is applied
  • a back bias or reverse is used to steer electrons to reduce the etch rate at the sidewalls.
  • a reverse bias is applied to the front middle or center structure and is used to steer the electrons towards to middle etchant mask to obtain an angled etch pattern.
  • a reverse bias has the same polarity as that of the bias applied (that is, the bias conductors applied to the substrate have the same polarity (as shown, a negative polarity), but he bias is "back” or “reverse” as it is applied on the opposite side of the substrate, thereby countering the bias applied on the irradiated side of the substrate. It is possible to use a bias using revered polarity, such as shown in Figure 16B.
  • Figure 14 shows photons used to make certain portions of the sample conductive, but no biasing voltage is applied, similarly to Figure 12.
  • Figure 14A shows a ground on the reference electrode, and also shows a ground on the back of the insulator.
  • FIG. 14B shows a similar arrangement, except the x-rays must traverse a volume of etchant prior to incidence on the insulator.
  • Figure 15 shows variations of the arrangement using a biasing voltage, but without employing an etch mask.
  • the negative electrode covers one side of the crystal, while in Figure 15 A, the negative electrode is selectively placed in multiple locations on the crystal (generally opposite the well or cavity to be formed).
  • Figure 15B shows an arrangement where the biasing electrode is surrounded by another electrode biased at different potential. The second electrode will change the electric field lines and steer the current flow. For example, if the second electrode has the same negative polarity as the first electrode but is biased at higher potential, the electric field will focus the electrodes in the region of the first electrode.
  • Figure 15C shows another arrangement employing two different biasing voltages on the crystal at different locations, resulting in differing currents and variations in the etch rate across the crystal surface.
  • Figure 16 depicts comparable configurations to Figure 15, but further shows how the electrochemical etching combined with the traditional physical etch masking.
  • Figure 16A shows a conductive electrode positioned over a first side of the crystal material, with a mask placed on the side of the crystal exposed to a hydrofluoric acid etching solution.
  • An x-ray mask is positioned between the sample and the x-ray source. X-rays are directed past the gold mask to the crystal material.
  • a current is induced across the crystal material by connecting a voltage source between the conductive mask and a lead electrode placed in the etching solution.
  • Figure 16B shows the arrangement of Figure 16A but employs a back voltage applied to the etch masks.
  • Figure 16 depicts a variety of bias electrode configurations that can be used on both sides of the crystal, as shown in Figures 16A, C, D, and E to selectively vary the etch rate.
  • B the back bias is used to steer the current flow to obtain a sloped etch.
  • D selective biasing is used to obtain different etch rates in different locations.
  • E the back bias is used to steer electrons to obtain reduced side wall etch rate. Note that the back bias voltage used in Figure 16B could be used with the reverse polarity.
  • Figure 17 shows variations in the equipment using etch masks, but without X-ray masks. Arrows within the crystal approximate the current paths (the field gradients). Note that Figure 17A shows a common electrode across the back of the crystal, while Figure 17B shows select locations of the negative electrode on the back of the crystal. By selective placement of the electrodes (and select use of back bias such as shown in Figures 17A and 17D it is possible to customize the electric field within the crystal to best steer the current to the desired etch area. Note that the arrangements using a back bias ( Figure 17A and B are more directed to the desired etch area. In Figure 17, electrons will be produced across the entire back of the crystal structure as no x-ray masks are used.
  • FIG. 17 shows how the local etch rates can be varied without masking the x-rays. The current can only flow from the electrodes placed on the sample and thus the etch rate around the electrode can be increased or decreased.
  • Figure 18 also depicts current paths in the crystal, but the arrangement in Figure 18 lacks etch masks. Note the current tines using a back bias (Figure 18B).
  • Figure 18 shows how this local etch rate control can be obtained without electrodes at the etchant side of the sample.
  • Figure 19 shows an arrangement where both biasing electrodes are positioned remotely from the crystal (as shown, both electrodes are located within the etchant), allowing etching to be undertaken at each side of the crystal, although the rate of etching should be faster at the side of the crystal closest to the positive electrode, as the electrons will be steered to that side due to the applied bias.
  • etch masks use of back bias, selective application of the forward bias using a predetermined pattern for the electrodes, an the energy of the incident radiation, the ability to steer the radiation induced electrons to the area desired to be etched is possible.
  • Such control is not feasible using standard chemical etching of insulators.
  • the techniques allow fine control of steering the generated current, and by varying the radiation energy, the amount of current induced can be controlled. Variation in the current density allows control of the etch rate, and steering of the current (and/or the use of masks) allows for control of areas to be etched. The system thus allows for simple electrical control of the etching.
  • Complex geometries can be formed in a substrate by substitution or replacing of masks (either radiations masks or etching masks) with a second mask, and performing the radiation assisted electrochemical etching a second time.
  • Micro devices that may be manufactured with this technique include those devices currently created out of primary for non-insulating materials, and include perforated membranes, cantilevered beams, mass balances, microbridges, a tethered proof mass, a micro plates, micro mirrors, and other structures. Examples of use of the techniques to create multiple quartz resonator structures on a single wafer substrate are shown in Figures 25 -27. As used, a resonator is a physical device that oscillates at
  • Quartz resonators are commonly used in cell phones and other RF devices, as quartz is piezoelectric, a crystal, and has excellent frequency stability across a wide range of temperature. Current quartz generators are millimeters in lateral dimensions. Smaller dimensions are needed for integration in RF micro-devices or integrated chips.
  • the first step is electro chemical etching of the substrate; after completion of the electrochemical etching process, a quartz wafer is formed having a series of resonator precursor structures.
  • the remaining steps of the process depicted are standard techniques to produce a final resonator, generally creating the electrical contacts to activate the resonator structure, and isolating individual completed resonators. In each process, no radiation masks are shown, but could be employed.
  • bias electrode is connected to the substrate, one for each resonator to be formed on the wafer.
  • starting wafer thickness are on the order of 200- 400 ⁇ m, machined to specification by mechanical polishing and possibly also by chemical etching. Spacing between resonators on the wafer can be on the order of the starting substrate thickness to the final
  • the spacing allows the bias produced at one resonator to have minimal effects on adjacent resonator structures.
  • the wafer thickness at the location of the resonators is monitored such as using optical time domain reflectometry techniques (or laser interferometric techniques) or monitoring of current levels near the etch surface (monitoring in the electrolyte) or near the etch surface (monitoring on an adjacent conductive etch mask), or monitoring the resistance or the voltage drop across or near the area of concern, For piezoelectric crystals, it may easiest to measure the sample resonant frequency to determine the wafer thickness and the location of the electrode.
  • the individual bias is adjusted to speed up or slow down the etch rate at each resonator structure to achieve a uniform thickness (or a desired thickness if it varies across the structure) of the finished product.
  • Each of these techniques is considered “monitoring the etch rate" by monitoring a parameter related to the etch rate and correlating the detected parameter (or evolution of the recorded parameter) with a thickness of the substrate or directly correlating to the etch rate (allowing a calculation of the thickness by integrating the etch rate over the time of etching).
  • the thickness is then compared with the desired thickness, and may be compared with the thickness of the surrounding structures, in order to modify the process parameters at the particular structure (generally the bias DC voltage).
  • This will generally be under computer control, where the computer can access a database having stored an experimentally determined etch rate versus process parameter look up table.
  • metal electrodes about 100 nm thick are deposited to the wafer on the opposite sides, such as vapor deposition or sputtering. Prior to deposition of the metal electrodes, typically any bias electrodes and etch mask are removed. A final trim may be achieved by adjusting metal thickness. As shown in Figure 25C, the wafer is cut to pieces to form individual resonators. The typical dimensions for the common "AT-strip" resonators are thickness of 10-100 ⁇ m and lateral dimensions of 1-3 mm.
  • lateral dimensions can be greatly reduced allowing for much smaller quartz resonators, such as on the order of 10-100 ⁇ m, lateral dimensions, allowing quartz resonators to be more readily used in micro RF devices and integrated circuits, although larger resonators can be made as well. .
  • Figure 26 shows the process to make multiple "inverter mesa" resonators on a single wafer. Shown in Figure 26A is the electrochemical etching of the wafer. The wafer is masked to form wells that will be used as the resonators. Again, there is an electrode for each resonator on the wafer. The wafer thickness at the location of the resonators is monitored and the bias is adjusted to speed up or slow down the etch rate. In Figure 26B, after etching, metal electrodes about 100 nm thick are deposited to the wafer on the opposite sides. A final trim may be achieved by adjusting metal thickness. The wafer is cut to pieces to form individual resonators Figure 26C.
  • the typical dimensions for the common "inverted mesa" resonators are thickness of 1-100 ⁇ m and lateral dimensions of 100 ⁇ m -3 mm.
  • the advantage of the inverted mesa structure is that as the resonator is supported by a thicker frame, a thinner resonator structure is possible. It is also possible to make high frequency filters by connecting several resonators together.
  • Figure 27 shows the process to make multiple "mesa" resonators on a single wafer.
  • the wafer is masked to etch pits or cavities that will form individual resonators.
  • the electrode has a smaller footprint then the cavity's floor so that it can be used to slow down the etch rate at the center of the pit.
  • additional smaller electrodes could be positioned adjacent to the center electrode, and aligned with the desired "side" wells. After formation of the central pit, these electrodes can be activated (or remain activated) while the center electrode is deactivated.
  • the central cavity could be created as in Figure 26, and the wafer removed.
  • a new etch mask would be deposited, covering a portion of the central cavity, and the wafer re-inserted into the electrochemical etching apparatus to etch the side cavities (with or without a reverse bias applied to the new etch mask, not shown).
  • variations in the cavity depth can also be achieved by using X-ray masks that vary in opacity - more transmissive at the "edge" of the cavity, less transmissive at the center of the cavity, to vary in the current in the substrate accordingly (not shown). These techniques is will form a mesa structure where the resonating portion of the wafer surrounded by a thinner portion to trap the vibrations. The wafer thickness at the location of the resonators is monitored (preferably near the center of the cavity) and
  • the bias is adjusted to speed up or slow down the etch rate to control the etch rates at the location of the resonators.
  • metal electrodes about 100 nm thick are deposited to the wafer on the opposite sides ( Figure 27B).
  • a final trim may be achieved by adjusting metal thickness.
  • the wafer is cut to pieces to form individual resonators.
  • the typical dimensions for the "mesa" resonators are thickness of 5-100 ⁇ m and lateral dimensions of 100 ⁇ m -3 mm ( Figure 27C).
  • etch rate dependency on temperature is given by R oz e- r ⁇ ⁇ , where E A is the activation energy and T is the temperature. Due to the exponential temperature dependency, even a small change in temperature can have a significant effect in the etch rate.
  • the heating is accomplished by passing current through the sample.
  • current can be created by illuminating the insulator (or parts of the insulator) with an electron beam, injecting energetic electrons into the insulator's conduction band making the crystals temporarily conductive.
  • ⁇ B0482967 1 ⁇ solution to remove any defects in the surface layer, and then thoroughly cleaned with isopropyl alcohol to remove any organic material from the etch surface.
  • the quartz sample was sealed between a UHV chamber and Teflon etch vessel using viton gaskets (see Figure 9B).
  • the sample was aligned to an electron gun, (such as shown in Figure 21) which was used to bombard the backside of the quartz with high energy electrons during the etching process.
  • the front surface of the quartz was placed in contact with the etchant, which was grounded using a platinum electrode. Current passed from the backside to the grounded material-etchant interface, and was measured using a Keithley 487 Picoammeter connected in series with the Pt reference electrode.
  • the etch rate can be either accelerated or decelerated.
  • a maximum variation of approximately 600% is seen, +/-300% from baseline.
  • the observed electrochemical effect is primarily a function of the current density through the sample. Roughness of the etched surface was approximately the same for control and experimental samples and compared well to the initial surface roughness of the quartz. Also no visible damage to the crystal was noted as a result of the electron bombardment. Electrochemical manipulation of the etch rate thus allows good etch rate control with minimal side effects to the quartz.
  • the etch rate is not sensitive to the energy of the injected electrons, the thickness of any electron mask will be highly dependent on the energy of the injected electrons.
  • Heavy metals such as gold or lead, can be uses as an e-mask, and grounding of the E-masks is preferred.
  • this invention should have practical application to any material with band gap larger than 3.0 eV, but the invention necessarily exclude materials with a band gap of less than 3.0. However, with low energy band gaps, the conventional etching processes may be more efficient.

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Abstract

L'invention concerne un procédé de gravure électrochimique d'un isolant non conducteur. Le procédé implique l'induction d'un courant dans l'isolant par l'excitation d'électrons dans la bande de conduction en fournissant l'énergie nécessaire par une irradiation de l'isolant. En variante, des électrons peuvent être fournis de manière externe par un canon à électrons. L'isolant est soumis à une polarisation électrique et les électrons induits ou fournis créent alors un courant dans l'isolant qui réalise la gravure.
PCT/US2007/088422 2006-12-21 2007-12-20 Gravage d'isolants assisté par rayons x WO2008080004A1 (fr)

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Citations (5)

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US4926086A (en) * 1988-07-07 1990-05-15 Centre Suisse D'electronique Et De Microtechnique S.A. Piezoelectric resonator
US5805626A (en) * 1995-09-20 1998-09-08 Mitsubishi Materials Corporation Single-crystal lithium tetraborate and method making the same, optical converting method and converter device using the single-crystal lithium tetraborate, and optical apparatus using the optical converter device
US6511915B2 (en) * 2001-03-26 2003-01-28 Boston Microsystems, Inc. Electrochemical etching process
US6579068B2 (en) * 2000-08-09 2003-06-17 California Institute Of Technology Method of manufacture of a suspended nitride membrane and a microperistaltic pump using the same
US20050241933A1 (en) * 1999-06-22 2005-11-03 President And Fellows Of Harvard College Material deposition techniques for control of solid state aperture surface properties

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JPS61103725A (ja) * 1984-10-25 1986-05-22 Inoue Japax Res Inc ワイヤカツト放電加工方法
DE3706124A1 (de) * 1987-02-25 1988-09-08 Agie Ag Ind Elektronik Verfahren zum elektroerosiven bearbeiten von elektrisch schwach oder nicht leitenden werkstuecken sowie elektroerosionsmaschine zur durchfuehrung des verfahrens

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Publication number Priority date Publication date Assignee Title
US4926086A (en) * 1988-07-07 1990-05-15 Centre Suisse D'electronique Et De Microtechnique S.A. Piezoelectric resonator
US5805626A (en) * 1995-09-20 1998-09-08 Mitsubishi Materials Corporation Single-crystal lithium tetraborate and method making the same, optical converting method and converter device using the single-crystal lithium tetraborate, and optical apparatus using the optical converter device
US20050241933A1 (en) * 1999-06-22 2005-11-03 President And Fellows Of Harvard College Material deposition techniques for control of solid state aperture surface properties
US6579068B2 (en) * 2000-08-09 2003-06-17 California Institute Of Technology Method of manufacture of a suspended nitride membrane and a microperistaltic pump using the same
US6511915B2 (en) * 2001-03-26 2003-01-28 Boston Microsystems, Inc. Electrochemical etching process

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