Method for Fabricating Three-Dimensional Microstructures and a High-Sensitivity Integrated Vibration Sensor Using Such Microstructures

The present invention deals generally with micromechanical structures which function as transducing elements in semiconductor devices. More particularly, the present invention relates to methods for fabricating these three-dimensional microstructures and to vibration sensors in which these microstructures are utilized.

Materials which have traditionally been used in microelectronic devices due to their electrical properties have recently been utilized to form mechanical structures. Techniques for micromachining these materials to produce miniature three-dimensional mechanical structures such as cantilever beams, trestles, pedestals and other similar structures have been developed. Hence, it is now possible to fabricate micromechanical structures on silicon chips which respond to inertial and other forces.

These micromechanical structures have been used as transducing elements to "sense" vibration and acceleration. In these devices, a micromechanical structure, such as a cantilever beam, is formed such that it responds to an applied force. For example, a cantilever beam can be formed which deflects in response to a force applied normal to its principal surface. The direction and magnitude of a force applied to the beam is proportional to the beam deflection. Thus, micromechanical structures can be fabricated which are excellent transducing elements for vibration sensors and accelerometers. Micromechanical transducing elements can be rapidly and reliably mass-produced using batch fabrication techniques adapted from microelectronics technology. The quality of the cantilever beam or other transducing element is critical to the performance of the sensor.

In order to provide greater packing density and to reduce parasitic capacitances which may interfere with signal transmission, many vibration sensors and accelerometers are designed such that the micromechanical transducing element and the detection circuitry are provided on a signal chip. Since the magnitude of deflection of the micromechanical structure is very small, it is essential that parasitic capacitances be kept to an absolute minimum so that they do not overwhelm the signal generated by the transducing element. By integrating electrical and mechanical components in a single silicon body to produce a "smart sensor", parasitic capacitances can be substantially reduced.

In the past, solid-state accelerometers and vibration sensors were based primarily on the piezo-resistive effect. Piezo-resistive sensors relate a stress-induced change in resistivity of a diffused resistor to the magnitude of the stress-producing force. Other solid-state accelerometers utilize changes in the electrical characteristics of a pn junction when stressed. In general, however, piezo-effect devices are only accurate when measuring extremely large stresses. Moreover, when these devices are highly stressed, the materials of which they are formed often fracture under the strain. Other known accelerometers include cantilever beams which function in the nature of electromechanical switches. The beams deflect in response to accelerations, but the beam movement is used only to close or open an electrical contact, much like a mechanical switch. Some of these mechanical, switch-like devices have an array of cantilever beams. Each beam is weighted or "loaded" to respond to an incrementally greater acceleration by virtue of a greater inertial mass. The devices are capable of measuring a range of acceleration thresholds.

Sensors have also been devised which include a cantilever beam or other transducing element which is integrated with metal oxide semiconductor (MOS) detection circuitry in a single chip. In these smart sensor, accelerations of the chip induce motions in the beam that produce capacitance variations which drive the detection circuitry. The changes in capacitance are extremely small, and the signal so produced is thus susceptible to being overwhelmed by parasitic capacitances. As mentioned above, parasitic capacitances are most easily reduced by placing the detection circuitry "on-board" with the micromechanical transducing element.

Conventional techniques for fabricating smart sensors having one or more micromechanical transducing elements, do not provide a convenient procedure for fabricating both the detection circuitry and the micromechanical structure. One acute problem with prior art techniques is that conventional chemical etchants used to undercut micromechanical structures such as cantilever beams and the like etch the preformed detection circuitry along with the micromechanical structure. Hence, the use of prior art chemical etchants such hydrofluoric acid and ethylenediamine pyrocatechol require that the detection circuitry be protected by complicated masking techniques during formation of the micromechanical structures. It would therefore be desirable to provide a process for fabricating three-dimensional structures such as cantilever beams and trestles which affords a high degree of selective etch control and which does not require complicated masking procedures to protect the detection circuitry. In United States patent application entitled "Method for Patterning Silicon Dioxide With High Resolution in Three Dimensions", Serial No. 836,900, filed Mar. 6, 1986, which is assigned to the assignee of the present invention, a method is disclosed in which an aqueous ammoniacal hydrogen peroxide solution is used to preferentially etch ion-damaged silicon dioxide. We have now discovered a method for producing micromechanical structures which utilizes an ammoniacal hydrogen peroxide solution as an etchant. We have also discovered three-dimensional microstructures for use as transducing elements in vibration sensors. We have further discovered a high-sensitivity integrated vibration sensor which uses standard MOS circuit components to measure extremely small motions of microstructures.

It is an object of the present invention to provide a reliable and convenient method for fabricating micromechanical structures.

It is a further object of the present invention to provide a method for preferentially etching phosphosilicate glass without substantially etching silicon, silicon dioxide and polysilicon.

It is a still further object of the present invention to provide a method for forming a cantilever beam having its center of gravity at the end point of the free end of the beam.

Another object of the present invention is to provide a method for measuring extremely small deflections of a beam or other micromechanical structure.

In accordance with one aspect of the present invention, there is provided a method for preferentially etching a layer of phosphosilicate glass 100 to 1000 times faster than underlying or surrounding layers of silicon, silicon dioxide, or polysilicon. The method includes contacting the layers with an aqueous ammoniacal hydrogen peroxide solution to selectively remove the phosphosilicate glass layer without substantially etching silicon, silicon dioxide or polysilicon layers. In one embodiment, the novel etching method is used to fabricate a micromechanical structure. According to the novel method, a phosphosilicate glass layer is first deposited over a substrate. The substrate may include silicon, silicon dioxide, and polysilicon. One or more vias are formed in the phosphosilicate glass layer to expose a portion of the underlying substrate. A micromechanical structure is then formed overlying the phosphosilicate glass layer and extending through the via in rigid attachment with the underlying substrate. The phosphosilicate glass layer is then preferentially removed in an aqueous ammoniacal hydrogen peroxide solution. The resulting micromechanical structure has an attachment point to the substrate at the location of each via, but is otherwise free to deflect toward or away from the substrate. In the preferred embodiment, the method of the present invention is used to form a micromechanical structure on a substrate on which detection circuitry has been formed. Due to the selectivity of the etchant, a micromechanical structure can be formed without substantially etching the pre-formed circuitry.

In another aspect, the present invention provides a method for fabricating a loaded or massed micromechanical cantilever beam such that the center of gravity of the beam is located at the end point of the free end of the beam. According to this method, a fully formed cantilever beam is encased in a layer of photoresist through which an opening is made to expose a portion of the top or principal surface of the beam. A mass is then formed overlying the photoresist and extending through the opening in contact with the principal surface of the cantilever beam. The opening through the photoresist and the mass are formed such that the mass extends beyond the free end of the cantilever beam in an overhanging fashion. The photoresist is then removed with an oxygen plasma freeing the loaded beam. By placing the mass in overhanging relationship to the beam, the center of gravity of the structure can be placed at the end point of the free end of the beam, thereby eliminating its reverse moment.

The present invention further comprehends a vibration sensor having at least one cantilever beam which forms an integral part of a variable capacitor in a high frequency oscillator. Oscillation is stopped or started by an enable signal and, as the beam deflects, the change in capacitance changes the frequency of oscillation of the circuit. Small frequency changes are detected by feeding the output of the circuit into a high performance counter which is clocked by a high frequency reference clock. By enabling the counter for a long period of time compared to the period of the oscillator, very small frequency changes can be converted into a different count produced by the counter. The oscillator thus produces a digital output which is fed into a digital counter.

Other objects, features and advantages of this invention will become more apparent from the following description of the preferred embodiments thereof and from the drawings, in which:

FIGS. 1 through 5 and FIG. 7 show cross-sectional views of a device in intermediate fabrication stages made in accordance with this invention.

FIG. 6 is a graph illustrating the relative etch rates of various materials etched in accordance with the present invention.

FIGS. 8 through 13 show cross-sectional views of an alternate device in intermediate fabrication stages made in accordance with this invention.

FIG. 14 is a schematic circuit of an oscillator having a frequency modulated by a variable capacitor in accordance with this invention.

FIG. 15 is a block diagram of an integrated detected system in accordance with the present invention.

FIG. 16 are graphs of outputs of the oscillator under various simulated accelerations.

Referring now to FIG. 1 of the drawings, substrate 20 has formed thereon detection circuitry 22 which is illustrated here as standard MOS circuit elements. Substrate 20 may include any material which resists etching in the etchant solution of the present invention, such as silicon, silicon dioxide and polysilicon. Detection circuitry 22 will also typically include one or more of these materials. As will be explained more fully herein below, in some applications substrate 20 is formed of single-crystal silicon and serves as one plate, usually ground, of a variable capacitor. It is to be understood that the detection circuitry 22, as shown in FIG. 1, is fully fabricated but lacks an overlying metal interconnect layer. As will be described, the metal interconnect layer is formed subsequent to the formation of a micromechanical structure. Also, during the formation of detection circuitry 22, a pedestal 24 may be formed, preferably of silicon dioxide. In this embodiment of the invention, pedestal 24 will serve as the point of attachment by which the micromechanical structure is rigidly connected to substrate 20. In some applications, it may be desirable to attach the micromechanical structure directly to substrate 20. Although detection circuitry 22 is illustrated as including several MOS circuit elements, other well known detection circuitry may be suitable for use in the present invention.

Referring now to FIG. 2, a layer of phosphosilicate glass 26 is formed overlying substrate 20, detection circuitry 22 and pedestal 24. Phosphosilicate glass layer 26 is suitably applied by spin-coating or by chemical vapor deposition. In order to obtain the rapid preferential etch of phosphosilicate layer 26 which is essential to the present invention, the phosphosilicate glass layer should contain from about 5.5 percent to about 8.0 percent by weight P.sub.2 O.sub.5. It is most preferred that phosphosilicate glass layer 26 contain about 6.0 percent by weight P.sub.2 O.sub.6. As the concentration of P.sub.2 O.sub.5 decreases below 6.0 percent, the time required to etch phosphosilicate glass layer 26 increases significantly. Although increasing the concentration of P.sub.2 O.sub.5 within phosphosilicate glass layer 26 augments the etch rate in the etchant of the present invention, the increased P.sub.2 O.sub.5 content also makes phosphosilicate glass layer 26 more hydroscopic. When the concentration of P.sub.2 O.sub.5 exceeds approximately 8 percent, phosphosilicate glass layer 26 becomes so hydroscopic that further processing becomes difficult.

Following the formation of phosphosilicate glass layer 26, a contact opening or via 28 is formed using conventional photolithographic and etching techniques. As shown in FIG. 3 of the drawings, via 28 is formed above and extends through phosphosilicate glass layer 26 to pedestal 24. Via 28 provides access for a subsequent layer 30 (FIG. 4) which is shown in this embodiment as a layer of polysilicon. The dimensions of via 28 are not critical to the invention. However, via 28 at pedestal 24 should be of sufficient size to expose adequate surface area for layer 30 to be rigidly attached to pedestal 24. POlysilicon layer 30 is then formed overlying phosphosilicate glass layer 26 and extending through via 28 to pedestal 24 as shown in FIG. 4. Although polysilicon works well in forming layer 30, it may be suitable and desirable in some applications to form layer 30 of metal such as aluminum, gold, tungsten or molybdenum, or possibly a silicide or polycide, or combinations thereof. Other materials may be used in forming layer 30, and those materials which have sufficient strength to deflect without fracturing and which resist substantial etching in the etchant of the present invention are most appropriate for use herein. It is also to be understood that materials which are suitable for forming layer 30 must also adhere well to pedestal 24 or directly to substrate 20 in some devices.

Once formed, layer 30 is patterned to form the desired micromechanical structure, illustrated in FIG. 5 as a cantilever plate or beam 32. Beam 23 is formed using a patterning technique which is compatible with the material from which layer 30 is formed. Cantilever beam 32 is shown attached to pedestal 24 through via 28 and, in this intermediate state of fabrication beam 32 is wholly supported by underlying phosphosilicate glass layer 26. It may be desirable in some applications to form an array of cantilever beams 32 or to form micromechanical structures of other configurations such as a trestles or a trampoline-like structures. These alternative arrangements can be formed simply by forming multiple pedestals and vias.

The shape of beam 32 is dictated in part by the topography of the layer on which it is disposed, e.g., phosphosilicate glass layer 26. By forming phosphosilicate glass layer 26 using a spin-coating technique, its top or principal surface is essentially even, lacking dips or valleys which may be present in underlying layers. Hence, when layer 30 is formed, a non-conformal beam (not shown) can be fabricated. Alternatively, a conformal beam 32 can be fabricated as shown in FIG. 5 by forming phosphosilicate glass layer 26 by chemical vapor deposition wherein phosphosilicate glass layer 26 assumes the topography of underlying layers.

After cantilever beam 32 has been formed, phosphosilicate glass layer 26 is removed by immersing substrate 20 with its overlying layers in the etchant solution of the present invention. The preferred etchant solution which is used herein to selectively etch phosphosilicate layer 26 contains, in parts per volume, about one part of an aqueous ammonium hydroxide solution (NH.sub.4 OH), about one part of an aqueous hydrogen peroxide solution (H.sub.2 O.sub.2), and from about 2.5 parts to about 4 parts water. The aqueous ammonium hydroxide solution used herein to prepare the etchant solution contains from about 28 to about 30 percent NH.sub.3 by weight. The aqueous hydrogen peroxide solution contains about 30 percent H.sub.2 O.sub.2 by weight. The most preferred etchant solution contains one part aqueous ammonium hydroxide solution, one part aqueous hydrogen peroxide solution and three parts water. This corresponds to an actual initial aqueous composition containing about 3.25 mole percent hydrogen peroxide and about 6.5 mole percent ammonium hydroxide. This preferred initial concentration can be increased or decreased by at least 50 percent when used under the conditions used herein. It may be possible to vary this preferred concentration even more if the principles of the present invention are followed. In these proportions, the etchant solution etches phosphosilicate glass layer 26 about 100 to 1000 times faster than it etches silicon, silicon dioxide or polysilicon.

The device as illustrated in FIG. 5 is then immersed in the etchant solution of the present solution, preferably in a reflux apparatus to minimize evaporation of the etchant constituents. Due to the rapid rate at which the etchant solution removes phosphosilicate glass layer 26, the temperature of the etchant solution can be quite low, preferably about 80.degree. C. By maintaining the etchant solution at this relatively low temperature, hydrogen peroxide is conserved. The etch rate of phosphosilicate glass layer 26 can be increased by increasing the temperature of the etchant solution up to about 90.degree. C. However, etching at these higher temperatures may require that the hydrogen peroxide content of the etchant solution be supplemented by adding hydrogen peroxide during the etching procedure.

The composition of the etchant solution does not remain constant throughout the etching process. Hydrogen peroxide, in particular, decomposes significantly during the etching process due to the relatively high temperatures encountered in the etch bath. Accordingly, the ratio of hydrogen peroxide to ammonium hydroxide decreases dramatically from the initial 1:1 ratio. Hence, it is preferred that the etchant solution be freshly prepared at room temperature to minimize thermal decomposition of the solution components. Etching should be commenced immediately after the etchant reaches about 80.degree. C. It should also be noted that additives which inhibit thermal decomposition of hydrogen peroxide are known. These inhibitors may be useful in the present invention to reduce the preferred initial concentration of hydrogen peroxide or to reduce the frequency or amount of hydrogen peroxide which may be added to the etchant solution during etching.

Depending upon the thickness of phosphosilicate glass layer 26, which will usually be in the range of about 0.5 micrometers to 2 micrometers, the complete removal of phosphosilicate glass layer 26 in the etchant solution usually requires several minutes to several hours. As stated, the etchant solution of the present invention removes phosphosilicate glass layer 26 two to three orders of magnitude faster than silicon, silicon dioxide and polysilicon. Hence, substrate 20, detection circuitry 22, and pedestal 24 are not substantially affected by the etchant. The device remains in the etchant bath until substantially all of phosphosilicate glass layer 26 has been removed. Therefore, the etch rate of phosphosilicate glass layer 26 can be controlled by varying the amount of P.sub.2 O.sub.5 in the phosphosilicate glass, by changing the etchant solution or by adjusting the temperature of the etchant bath. As illustrated in FIG. 6, the etch rate of a phosphosilicate glass layer which was formed by chemical vapor deposition and which contained approximately 6.0 percent P.sub.2 O.sub.5, etched in the preferred etchant solution at 80.degree. C. in a reflux bath is greater than 100,000 angstroms per hour. By comparison, the same etchant bath etches silicon and silicon dioxide at a rate of only about 100 angstroms per hour. Therefore, phosphosilicate glass layer 26 is etched three orders of magnitude faster than silicon or silicon dioxide, providing an extremely accurate preferential etching method. As shown in FIG. 7, after phosphosilicate glass layer 26 is etched away, cantilever beam 32 extends laterally from its attachment to pedestal 24 across substrate 20 from which it is spaced apart by air gap 34. Air gap 34 may serve as the dielectric of a variable capacitor having cantilever beam 32, and substrate 20 and capacitor plates.

Finally, the appropriate metallization (not shown) is applied and patterned to interconnect cantilever beam 32, detection circuitry 22, and substrate 20. In operation, accelerations normal to the principal surface of cantilever beam 32, i.e., along the axis designated by the numeral 31, cause cantilever beam 32 to deflect toward silicon substrate 20 which reduces the thickness of air gap 34, resulting in a change in capacitance. As will be explained more fully, detection circuitry 22 detects these extremely small changes in capacitance which can then be related to the force acting on cantilever beam 32.

In another aspect, the present invention provides a method by which a cantilever structure can be loaded with a mass to attain a preselected sensitivity and response and in a manner such that the center of gravity of the cantilevered structure is located at the end point of the free end of the beam. Referring now to FIG. 8 of the drawings, a cantilever beam 36 is formed having one end 38 secured to pedestal 40 which, in turn, is attached to substrate 42. Cantilever beam 36 extends longitudinal across substrate 42, terminating at free end 44. Cantilever beam 36 is spaced apart from substrate 42 by intervening air gap 46. Next, as shown in FIG. 9, cantilever beam 36 is encased in a layer of photoresist 48 which also overlies pedestal 40 and substrate 42. Photoresist layer 48 fills air gap 46 supporting cantilever beam 36. Many positive and negative photoresists are available which are suitable for use in forming photoresist layer 48 and which are well known to those skilled in the art. Photoresist layer 48 is then patterned to form contact opening or via 50 above and extending through to cantilever beam 36 as shown in FIG. 10. The position of via 50 is dictated by a number of factors including the length and weight of cantilever beam 36 and the weight of the mass to be formed thereon.

Following the formation of via 50 in photoresist layer 48, photoresist layer 50 is blanket exposed in ultraviolet light and then further hardened by exposure to a CF.sub.4 plasma for approximately one minute. Next, photoresist layer 48 and the underlying device are baked at about 150.degree. C. for approximately one hour. Photoresist layer 48 is further hardened by implanting it with 80 keV at 1.0.times.10.sup.15 silicon ions cm.sup.-2. At this stage, photoresist layer 48 is still essentially hydrophobic. In order to improve the adhesion of a subsequent metal or silicide layer, the entire surface topography of photoresist layer 48 is made hydrophilic by exposing photoresist layer 48 to an oxygen plasma for approximately 30 seconds.

Referring now to FIG. 11, a layer of metal 52, such as aluminum, is formed over photoresist layer 48 and extends through via 50 to make rigid contact to pedestal 40. As with cantilever beam 36, layer 52 can be formed of polysilicon, aluminum, gold, tungsten, molybdenum, silicide, polycide and combinations thereof. In addition, it should be understood that gold would be preferred because of its high mass. However, aluminum might be preferred because of convenience it provides in allowing use of existing deposition and etching steps of an existing manufacturing process. After deposition, layer 52 is patterned to form mass 54 which, as shown in FIG. 12, extends beyond free end 44 of cantilever beam 36. Next, photoresist layer 48 is substantially completely removed in an oxygen plasma. As shown in FIG. 13, the resulting structure includes cantilever beam 36 to which mass 54 is attached. By forming mass 54 such that it overchanges free end 44 of cantilever beam 36, the center of gravity of cantilever beam 38 can be located precisely at the end point 56 of free end 44. This configuration eliminates the negative bending moment associated with a cantilever beam which does not have the overhanging mass of the present invention. This method for attaching a mass to a cantilevered structure can be used to place the center of gravity of the beam at any desired, predetermined location.

According to another aspect of the present invention, a method is provided for detecting extremely small motions of a cantilevered or other micromechanical structure. In order for the micromechanical structures fabricated in accordance with the present invention to operate as transducing elements in accelerometers and vibration sensors, extremely small beam deflections must be detected by the detection circuitry. As stated, accelerations normal to the principal surface of the beam cause the beam to deflect due to its inertial mass. These beam motions produce a capacitance variation, but the magnitude of the capacitance signal is generally only a few femtofarads. The present invention provides a method for detecting these extremely small beam movements and provides a digital output which can be processed easily by a microprocessor or other digital logic.

In the method of the present invention, the micromechanical structure and the underlying substrate form a variable capacitor and a high frequency oscillator as shown in FIG. 14, which schematically illustrates an oscillator 56 which frequency is modulated by a variable capacitor 58. An enable signal starts or stops the oscillator 56. Movement of the cantilevered structure causes a change in capacitance in the variable capacitor 58 which is used to change the frequency of oscillation of the circuit. A capacitance change of only a few femtofarads will change the frequency of oscillation. Extremely small changes in the frequency of oscillation are detected in the following manner. The change in frequency of oscillation of the circuit is fed into a high performance counter 60 which is clocked by a high frequency reference clock as shown in FIG. 15. By enabling the counter 60 for a long time period compared to the period of the oscillator output, a very small change in frequency can be translated into a different count produced by the counter 60. Since the oscillator produces a digital output which is fed into a digital counter, the final count can be used directly by digital signal processing or other logic circuitry without first converting it from an analog signal to a digital signal. As is known, analog to digital converters often introduce noise and errors into the signal.

In a micromechanical variable capacitor having a beam with a 30.times.50 micron plate and having a sensitivity of about 0.5 microns/g, where the on-board detection circuitry includes an oscillator designed in 5 micron NMOS technology using easy-to-process circuit parameters, FIG. 16 shows the response of the oscillator for 1g accelerations. The data is FIG. 16 was produced using a circuit simulator to simulate an accelerator having the foregoing specifications. Each set of plots gives voltages at three nodes of the oscillator circuit. The signal designated by the letter f was assumed to be the input to the counter. The simulation is performed for 100 nanoseconds. The change in frequency of oscillation was detectable even within this short time period. The frequency of the oscillator with the beam simulated at rest was about 57.4 MHz and the frequency for simulated 1g of acceleration in the positive and negative directions was, respectively, 58.5 MHz and 55.0 MHz. These differences were translated to count differences of over one million over a period of a second. By enabling the oscillator for about 10 milliseconds, which is a relatively short time period for mechanical changes of micromechanical devices, accelerations of less than 0.001 g could be detected as a change in the count value over 10.

Since a cantilever beam is more sensitive to accelerations in one direction, it may be possible to fabricate an accelerometer or vibration sensor having an array of beams to detect different components of acceleration. Furthermore, it may be possible to fabricate accelerometers and vibration sensors which compensate for temperature differences.