Patent Document

TECHNICAL FIELD 
     The present invention generally relates to micromachined devices, and particularly microelectromechanical system (MEMS) devices formed by etching processes. More particularly, this invention relates to a micromachining process and design elements for a MEMS device using a deep reactive ion etching (DRIE) process to improve yields and device reliability. 
     BACKGROUND OF THE INVENTION 
     A wide variety of MEMS devices are known, including accelerometers, rate sensors, actuators, motors, microfluidic mixing devices, springs for optical-moving mirrors, etc. As an example, rotational accelerometers that employ MEMS devices are widely used in computer disk drive read/write heads to compensate for the effects of vibration and shock. Other applications for rotational accelerometers that use MEMS devices include VCR cameras and aerospace and automotive safety control systems and navigational systems. Rotational rate sensors and accelerometers have been developed whose MEMS devices are fabricated in a semiconductor chip. Notable MEMS devices that employ a proof mass for sensing rotational rate or acceleration include a plated metal sensing ring disclosed in U.S. Pat. No. 5,450,751 to Putty et al., and an electrically-conductive, micromachined silicon sensing ring disclosed in U.S. Pat. No. 5,547,093 to Sparks, both of which are assigned to the assignee of this invention. Sparks&#39; sensing ring is formed by etching a chip formed of a single-crystal silicon wafer or a polysilicon film on a silicon or glass handle wafer. A sensor disclosed in U.S. Pat. No. 5,872,313 to Zarabadi et al., also assigned to the assignee of the present invention, employs a sensing ring and electrodes with interdigitized members. The positions of the interdigitized members relative to each other enable at least partial cancellation of the effect of differential thermal expansion of the ring and electrodes, reducing the sensitivity to temperature variations in the operating environment of the sensor. Each of the above sensors operates on the basis of capacitively sensing movement of their rings. The sensing rings are supported by a central hub or pedestal. Surrounding the rings are drive electrodes that drive the rings into resonance, while sensing electrodes that also surround the rings serve to capacitively sense the proximity of the ring (or nodes on the ring) which varies due to Coriolis forces that occur when the resonating ring is subjected to rotary motion. 
     Another notable MEMS device that employs a silicon proof mass for sensing rotational acceleration is disclosed in U.S. patent application Ser. No. 10/410,712 now U.S. Pat. No. 6,257,062 to Rich, incorporated herein by reference. Rich discloses a disk-shaped proof mass supported above a cavity formed in a substrate. Instead of being centrally supported by a pedestal, Rich&#39;s proof mass is suspended from its perimeter with tethers anchored to the substrate rim surrounding the proof mass. The tethers allow the proof mass to rotate about an axis perpendicular to the plane containing the proof mass and tethers. Fingers extend radially outward from the proof mass and are interdigitized with fingers extending radially inward from the substrate rim. The cantilevered fingers of the proof mass and rim are capacitively coupled to produce an output signal that varies as a function of the distances between adjacent paired fingers, which in turn varies with the angular position of the proof mass as it rotates about its axis of rotation in response to a rotational acceleration. 
     Sensors of the type described above are capable of extremely precise measurements, and are therefore desirable for use in a wide variety of applications. However, the intricate proof masses and associated sensing structures required for such sensors must be precisely formed in order to ensure the proper operation of the sensor. For example, Rich&#39;s device requires a sufficient gap between paired interdigitized fingers to prevent stiction and shorting, yet paired fingers must also be sufficiently close to maximize the capacitive output signal of the sensor. Rich employs stiction bumps formed on the proof mass fingers to inhibit stiction between closely-spaced fingers. Increasing the area of the fingers to achieve greater capacitive coupling would result in increased capacitive output for a given finger gap. However, traditional etching techniques have not generally been well suited for mass-producing silicon micromachines with high aspect ratios necessary to etch closely-spaced fingers in a relatively thick substrate. For example, with conventional etching techniques it is difficult to achieve a 10:1 aspect ratio capable of forming interdigitized fingers spaced three micrometers apart in a silicon substrate that is thirty micrometers thick. In addition to operational considerations, there is a continuing emphasis for motion sensors that are lower in cost, which is strongly impacted by process yield, yet exhibit high reliability and performance capability. Consequently, improvements in the processing of MEMS devices for sensing and other applications are highly desirable. Deep reactive ion etching (DRIE) is a process known as being capable of performing deep, high aspect ratio anisotropic etches of silicon and polysilicon, and is therefore desirable for producing all-silicon MEMS of the type taught by Rich. However, DRIE is a young technology practiced largely for research and development. Accordingly, to take advantage of the unique capabilities of the DRIE process, its etch idiosyncrasies must be determined and reconciled to render it suitable for mass production. 
     SUMMARY OF THE INVENTION 
     The present invention provides a process and design elements for a microelectromechanical system (MEMS) device by a deep reactive ion etching (DRIE) process during which a substrate overlying a cavity is etched to form trenches that breach the cavity to delineate suspended structures. The invention is particularly useful in the fabrication with a DRIE process of semiconductor MEMS devices used to sense motion or acceleration, which typically include a proof mass suspended above a cavity so as to have an axis of rotation perpendicular to the plane of the proof mass, as taught by Rich, Sparks and Zarabadi et al. While the invention will be illustrated in reference to a MEMS device with a proof mass, the invention is applicable to essentially any suspended structure that can be fabricated by forming a trench in a substrate overlying a cavity. 
     According to the invention, in addition to a relatively large member such as a proof mass, MEMS devices also may include additional and smaller structures that are suspended above the same cavity, such as the tethers and cantilevered fingers of Rich. A first general feature of the invention is the ability to define suspended structures with a DRIE process, such that the dimensions desired for the suspended structures are obtained. A second general feature of the invention is the ability to define specialized features, such as stiction bumps that, if delineated by DRIE, must be properly located between suspended structures in order to be effective in improving the reliability of the MEMS device. Yet another general feature of the invention is the control of the environment surrounding suspended structures delineated by DRIE in order to obtained their desired dimensions. 
     A significant problem identified and solved by the present invention is the propensity for the DRIE process to etch suspended features at different rates. DRIE has been determined to etch wide trenches more rapidly than narrower trenches. According to the invention, DRIE etches or, more accurately, erodes suspended structures more rapidly at greater distances from anchor sites of the substrate being etched, which occurs when a suspended structure becomes isolated from the bulk substrate when the trench(s) that delineates the structure breaches the cavity. (As used herein, an anchor site is a location on the bulk of the substrate from which the suspended structure is ultimately supported from the bulk of the substrate.) Consequently, though two suspended structures are separated by a gap of constant width, DRIE processes have been determined to more rapidly erode the suspended structure located farther from an anchor site. Using Rich&#39;s MEMS device as an example, once the proof mass is separated from the bulk of the substrate using a DRIE process. the proof mass fingers etch more rapidly than the rim fingers because the rim fingers are anchored (cantilevered) directly from the rim of the bulk substrate surrounding the proof mass, while the proof mass fingers are ultimately anchored to the rim of the bulk substrate through the tethers that suspend the proof mass from the same rim of the bulk substrate. A consequence of this more rapid etch is backside erosion and lateral thinning of the proof mass fingers. 
     In view of the above, in order to DRIE etch a substrate above a cavity to form suspended structures above the cavity, in which a first of the suspended structures is farther from the substrate anchor site than the second suspended structure, the present invention exploits the greater propensity for backside and lateral erosion of certain structures farther from substrate anchor sites so that, at the completion of the etch process, all suspended structures have acquired their respective desired widths. In this example, first and second surface regions of the substrate corresponding to the first and second suspended structures are masked in preparation for the DRIE etching process, leaving exposed those surface regions of the substrate corresponding to the trenches that will surround and delineate the suspended structures. The first masked surface region is intentionally wider than the desired width of the first suspended structure, thus resulting in the adjacent exposed surface region being narrower than the width desired for the trench that will delineate the first suspended structure. The first and second suspended structures are then concurrently DRIE etched. According to the invention, as a result of the first suspended structure being a greater distance from the anchor site than the second suspended structure, the first suspended structure is subject to backside and lateral erosion after the cavity is breached, causing the first structure to narrow and eventually acquire its desired lateral width during completion of the etch. As a result, the first masked surface region is intentionally undercut so that, at the completion of the etch process, the first and second suspended structures have acquired their respective desired widths. 
     The tendency for DRIE to etch wider trenches more rapidly than narrower trenches, a phenomenon which may be termed “etch lag,” is also detrimental to the formation of suspended features for the same reasons explained above. An example is where first and second suspended structures are to be DRIE etched in a substrate over a cavity, in which the first structure is delineated by a wider trench than the second structure. Because the wider trenches of the first structure etch more rapidly during DRIE etching, the wider trenches breach the underlying cavity before the narrower trenches of the second structure, leading to backside erosion and lateral narrowing as explained previously. This phenomenon would be compensated for by masking the substrate so that the masked surface region corresponding to the first structure is intentionally wider than desired, resulting in the adjacent exposed surface regions being narrower than the width desired for the trench that will delineate the first structure, yet wider than the exposed surface regions of the substrate corresponding to the trench that will surround and delineate the second structure. The trench formed in the wider exposed surface regions of the first structure breaches the underlying cavity before the narrower trench of the second structure, with the resulting backside and lateral erosion of the first structure causing the first structure to narrow and eventually substantially acquire its desired width during completion of the etch of the narrower trench surrounding the second structure. However, this scenario is complicated by the findings of the present invention that backside and lateral erosion occur more rapidly with those suspended structures located farther from an anchor site. This invention provides two approaches for addressing this problem. A first is to taper the width of the mask for a suspended structure while maintaining a constant gap width for the exposed surface area in which the trench will be etched to delineate the structure. The mask is tapered to be wider with increasing distance from the anchor site, so that as the width of the mask increases, backside and lateral erosion is correspondingly more rapid to eventually produce a substantially uniform width for the structure. Alternatively, the width of the mask for a suspended structure is maintained constant while tapering the gap width for the exposed surface area in which the trench will be etched to delineate the structure. The gap is tapered to be wider with decreasing distance from the anchor site, so that backside and lateral erosion of the structure that occurs more rapidly with increasing distance from the anchor site is balanced by the more rapid etch rate associated with the increasingly wider gap near the anchor site. As a result, a substantially uniform width for the structure can again be obtained. 
     The teachings of this invention concerning the relationship between distance to an anchor site and backside and lateral erosion is also pertinent to other aspects of DRIE etching a MEMS device. As previously noted, a feature of the invention is the ability to properly define specialized elements, such as stiction bumps. According to the invention, stiction bumps must be defined in regions of the substrate away from those areas in which accelerated backside and lateral erosion will occur. Also a feature of this invention is maintaining a proper environment surrounding suspended structures, such as by eliminating unnecessary variations in trench width. An important example is avoiding intersecting trenches that would create a localized wider gap prone to more rapid etching and subsequent backside and lateral erosion, resulting in vertical notches at the intersections. 
     In view of the above, it can be seen that the present invention provides a DRIE etching process by which suspended structures of desired widths can be more precisely formed. As a result, the present invention is able to take advantage of the deep etching capability of the DRIE process, while compensating for etch idiosyncrasies that would otherwise adversely affect the structural integrity and durability of a MEMS device, so as to improve yields and device reliability. 
     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a MEMS device in accordance with a preferred embodiment of this invention. 
     FIGS. 2 and 3 are plan and cross-sectional views, respectively, of a portion of the MEMS device of FIG.  1 . 
     FIG. 4 is a detailed plan view of several interdigitized proof mass and rim fingers of the MEMS device of FIG.  1 . 
     FIG. 5 is a plan view of the floor of the cavity in which the MEMS device of FIG. 1 is located. 
     FIGS. 6 and 7 are plan views showing portions of an etch mask used in the fabrication of the fingers and tethers, respectively, of the MEMS device of FIG.  1 . 
     FIG. 8 is a plan view showing a portion of an alternative etch mask used in the fabrication of the tethers of the MEMS device of FIG.  1 . 
     FIGS. 9 and 10 schematically illustrate a DRIE etch idiosyncracy identified with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 1 through 5 represent a MEMS device  10  fabricated with a DRIE process in accordance with the present invention, with masking steps of the DRIE process being represented in FIGS. 6 through 8. The device  10  is represented as a rotational accelerometer of the type disclosed by Rich, which is incorporated herein by reference. However, those skilled in the art will appreciate that the device  10  could be employed and modified for a variety of applications, including the rate sensors and accelerometers taught by Putty et al., Sparks and Zarabadi et al. 
     As illustrated, the device  10  includes a proof mass  14  formed in a sensing die  12 . The die  12  is shown as including a semiconductor layer  12   b  on a substrate  12   a  (FIG.  3 ). A preferred material for the semiconductor layer  12   b  is epitaxial silicon and a preferred material for the substrate  12   a  is single-crystal silicon, though it is foreseeable that other materials could be used. For example, the substrate  12   b  could be formed of quartz, glass or any other advantageous substrate to which the semiconductor layer  12   b  could be bonded. In a preferred embodiment, a wafer containing the die  12  is processed by a known bond-etchback process, by which the substrate  12   a  is etched to form a cavity  20  and then oxidized to form a bond oxide layer (not shown) on its surface including the cavity  20 , and the semiconductor layer  12   b  is epitaxially grown on a second wafer (not shown) and then bonded to the bond oxide layer of the substrate  12   a . The second wafer is then selectively removed to leave only the epitaxial layer  12   b  on the substrate  12   a  and overlying the cavity  20 , as shown in FIG.  3 . While a bond-etchback process is preferred, it is foreseeable that other techniques could be used to produce the die  12  and enclosed cavity  20  of FIG.  3 . 
     As also seen in FIGS. 1 through 3, the proof mass  14  is defined in the semiconductor layer  12   b  so as to be suspended above the floor  18  of the cavity  20  between a central hub  16  and a rim  22  formed by the semiconductor layer  12   b . The proof mass  14  is attached to the bulk of the die  12  (through the semiconductor layer  12   b ) with four equiangularly-spaced tethers  30 , and is completely separated from the hub  16  by a trench  17 , whose width may be on the order of about seven micrometers. The tethers  30  provide that the primary and desired translational mode of the proof mass  14  is rotation within the plane of the proof mass  14  about the hub  16 . 
     As seen in FIGS. 1 and 2, electrode fingers  24  radially extend outward from the proof mass  14 , and are interdigitated with electrode fingers  26  that radially extend inward from the rim  22 . The fingers  24  and  26  are separated by trenches  28  and roughly equiangularly spaced around the perimeter of the proof mass  14 . In the preferred embodiment, the trenches  28  are of alternating greater and lesser widths, as is most readily apparent from FIG. 4, though it is foreseeable that a constant trench width could be used. Each of the narrower trenches  28  defines a capacitive gap between a pair of smooth capacitor plates defined by the pair of fingers  24  and  26  it separates. In contrast, the wider trenches  28  (which may be, for example, twice the width of the narrower trenches  28 ) provide air gaps that separate each pair of capacitively-coupled fingers  24  and  26  from adjacent paired fingers  24  and  26 . These air gaps are effectively parasitic gaps, in that they do not positively contribute to device performance as do the capacitive gaps. The capacitor plates provided by the fingers  24  and  26  are preferably large relative to the width of the narrower trench  28  therebetween, which preferably has a uniform width of, for example, about three micrometers. When a voltage potential is present between pairs of capacitively-coupled fingers  24  and  26 , the rim fingers  26  capacitively sense the proximity of the proof mass fingers  24 , which will vary when the proof mass  14  is subjected to rotary motion. The large number of interdigitated fingers  24  and  26  of the device  10  produce a capacitive signal that is sufficiently large to measure and manipulate. 
     The operational requirements of the device  10  and its conditioning circuitry (not shown) will be appreciated by those skilled in the art, especially in reference to Rich, and therefore will not be discussed in any detail here. It is sufficient to say that the performance of the device  10  is generally enhanced by increasing the number of pairs of fingers  24  and  26 , and improving the uniformity of the capacitive gaps (narrow trenches  28 ) while also minimizing the widths of the gaps. Other configurations for the device  10  are foreseeable, depending on the intended application and operating natural mode of the device. 
     As also shown in FIG. 1, each of the four tethers  30  extends from the interior of the proof mass  14 , being separated from the proof mass  14  by trenches  32 . The trenches  32  are typically on the order of about seven micrometers in width, similar to the trench  17  separating the proof mass  14  from the hub  16 . The hub and tether trenches  17  and  32 , respectively, can be termed structural gaps (as opposed to the capacitive and parasitic gaps formed by the finger trenches  28 ) in that they are inactive to device signal. However, the tether trenches  32  are important to device performance in that they affect the compliance and response of the device  10  to some stimulus. The opposite end of each tether  30  is anchored to a portion  31  of the semiconductor layer  12   b , thereby supporting the proof mass  14  and, to a lesser extent, physically limiting rotation of the proof mass  14  relative to the rim  22 . Because the tethers  30  provide the structural support for the proof mass  14 , they are required to have specified widths (as measured in the plane of the proof mass  14 ) and thicknesses (as measured in the direction perpendicular to the plane of the proof mass  14 ) to achieve proper rotational compliance. The tethers  30  should also be free of irregularities, such as notches and other surface flaws that would weaken the tether  30 , increase their compliance, and provide nucleation sites for cracks. 
     Stiction between the fingers  24  and  26  may still occur in view of the very narrow trenches  28  separating them. In Rich, stiction bumps were formed on the proof mass fingers, so as to face the adjacent rim fingers. In the event the proof mass rotates sufficiently to bring one or more of the proof mass fingers in contact with their adjacent rim fingers, stiction bumps prevent stiction, in which the fingers would permanently stick together as a result of electrostatic forces. However, in an investigation leading to the present invention, stiction bumps formed by DRIE on proof mass fingers in accordance with Rich were found to be ineffective in preventing stiction in the event of an extraordinary rotational translational stimulus. During the investigation, a second and unexpected source of stiction was determined to be an undesirable translational mode of the proof mass  14  in the Z-direction, i.e., perpendicular to the plane of the proof mass  14 . The proof mass  14 , which is relatively large compared to the gap separating it from the floor  18  of the cavity  20 , can permanently stick to the cavity floor  18  if a requisite condition is met to cause Z-direction translation, such as a large static charge build-up on either the cavity floor  18  or the proof mass  14  during the DRIE etch, or water that wicks under the proof mass  14  and evaporates, pulling the proof mass  14  down into contact with the floor  18 . The investigation leading to this invention resulted in solutions to both of the stiction problems. For reasons to be more fully explained below, the device  10  of this invention is preferably fabricated to have stiction bumps  34  formed only on the rim fingers  26 , as depicted in FIGS. 3 and 4, and stiction bumps  36  formed at certain locations on the floor  18  of the cavity  20  directly beneath the proof mass  14 , as shown in FIGS. 3 and 5. 
     The investigation was directed to the use of DRIE processing to form the hub trench  17 , finger trenches  28  and tether trenches  32  that delineate the proof mass  14  fingers  24  and  26  and tethers  30  of the device  10  shown in FIG.  1 . While having different widths, the structural trenches (hub trench  17  and tether trenches  32 ), the capacitive gaps (the narrower portions of the finger trenches  28 ), and the parasitic gaps (the wider portions of the finger trenches  28 ) preferably have a constant width along their lengths, though it is foreseeable that trenches with variable widths could be used. In the course of the investigation, it was determined that DRIE consistently caused certain suspended features to etch more rapidly than others. Specifically, the hub and tether trenches  17  and  32  (which are wider than the finger trenches  28 ) consistently etched more rapidly than the finger trenches  28 . Conversely, the capacitive gaps formed by the narrower finger trenches  28  (which are narrower than the tether trenches  32  and the parasitic gaps formed by the wider finger trenches  28 ) were consistently the slowest to etch. Consequently, as a result of etch lag associated with DRIE, the hub trench  17  breached the cavity  20  beneath the proof mass  14  first, followed by the tether trenches  32  and the parasitic gaps formed by the wider finger trenches  28 , and finally the capacitive gaps formed by the narrower finger trenches  28 . However, etch lag could not account for all etch idiosyncracies observed with the device  10 . 
     From FIG. 1, it can be appreciated that the suspended structures of the device  10  are not equidistant from the bulk of the die  12 , the bulk being in reference to those portions of the die  12  other than the proof mass  14 , fingers  24  and  26 , and tethers  30  micromachined from the die  12 . For example, the proof mass fingers  24  are farther from the bulk of the die  12  than the rim fingers  26 , since the rim fingers  26  are anchored (cantilevered) directly from the rim  22 , which is an integral portion of the bulk of the die  12 , while the proof mass fingers  24  are ultimately anchored to the rim  22  through the proof mass  14  and the tethers  30  that suspend the proof mass  14  from the rim  22 . Surprisingly, from the investigation it was also determined that the DRIE process more rapidly etched suspended structures as their distances from their anchor site (wafer bulk) increased. While not wishing to be held to any particular theory, it is believed that this phenomenon was caused in part by heat transfer and the highly charged environment of the DRIE process. As the proof mass  14  becomes increasingly isolated from the remainder of the semiconductor layer  12   b  as a result of the trenches  17 ,  28  and  32  breaching the cavity  20 , heat transfer from the proof mass  14  to the bulk of the die  12  decreases, resulting in a temperature increase of the proof mass  14  and proof mass fingers  24  that may increase the etching rate. Similarly, the opportunity for charge build-up in the MEMS device  10  shown in the Figures is great, with static build-up resulting in uneven charge levels in different active regions within the die  12 . In particular, a charge build-up is likely to occur in the proof mass  14  and its fingers  24  and tethers  30 , as compared to the rim fingers  26  and the surrounding substrate (including the rim  22 ), particularly as the proof mass  14  becomes increasingly free from the remainder of the semiconductor layer  12   b  as the trenches  17 ,  28  and  32  are completed. It was shown that once one of the more rapidly etched trenches (e.g., the hub and tether trenches  17  and  32 ) breaches the cavity  20 , the anisotropic nature of DRIE etching may cause highly directional and highly energetic physical etchant species to be reflected by the floor  18  of the cavity  20  onto the undersides of the immediately adjacent suspended structures (portions of the proof mass  14 , fingers  24  and  26 , and tethers  30 ), causing both backside erosion of these structures and unintentional lateral thinning. 
     The above-described phenomenon is represented in FIGS. 9 and 10, which show a masked substrate  50  over a cavity  52 . In FIG. 9, a relatively wider trench  54  (such as a tether trench  32  or a parasitic gap  28  between fingers  24  and  26 ) has breached the cavity  52  before a relatively narrower trench  56  (such as a capacitive gap between fingers  24  and  26 ). A first portion  58  of the substrate  50  (similar to the proof mass  14  of the device  10 ) is separated from the bulk of the substrate  50  (though still suspended above the cavity  52  and from the bulk of the substrate  50  by some suitable structure not shown in FIGS.  9  and  10 ). Second and third portions  60  and  62  of the substrate  50  (similar to the proof mass and rim fingers  24  and  26  of the device  10 ) remain attached to the bulk of the substrate  50  (and are therefore closer to an anchor site on the substrate  50  than the first portion  58 ). FIG. 9 also shows highly directional and highly energetic physical etchant species  64  being reflected by the floor of the cavity  52  onto the undersides and sidewalls of the first and second portions  58  and  60 . FIG. 10 represents the substrate  50  of FIG. 9 immediately after the narrower trench  56  has breached the cavity  52 . In FIG. 10, the walls of the first and second portions  58  and  60  of the substrate  50  delineated by the wider trench  54  are both shown as having been subjected to backside and lateral erosion, but the first portion  58  exhibits greater erosion and lateral thinning as a result of being farther from the bulk of the substrate  50  than the second portion  60 . In FIG. 10, the walls of the first and second portions  58  and  60  of the substrate  50  delineated by the wider trench  54  are both shown as having been subjected to backside and lateral erosion as a result of being adjacent the wider trench  54 , which breached the cavity  52  while the narrower trench  56  was being etched to completion. However, the first portion  58  is shown as having sustained greater backside erosion and lateral thinning than the second portion  60  as a result of being subjected to more rapid etching after breaching of the wider trench  54 , since the first portion  58  is farther from the bulk of the substrate  50  than the second portion  60  and therefore experiences greater heating and/or charging during the DRIE process. 
     In accordance with the above etch phenomenon, the wider hub and tether trenches  17  and  32  (especially the portions of the tether trenches  32  nearer the proof mass  14 ) and the wider portions of the trenches  28  that define the parasitic gaps between fingers  24  and  26  were found to etch at a faster rate than the remaining portions of these trenches  28  and  32 , and therefore breached the cavity  20  first. As etching progressed, erosion on the backside of the suspended structures occurred, causing thinning of the fingers  24  and  26  and tethers  30  in the z axis (perpendicular to the plane of the proof mass  14 ) and thinning of the fingers  24  and  26  and tethers  30  in the x-y axis (in the plane of the proof mass  14 ). However, those suspended structures farther from an anchor site to the bulk of the die  12  (e.g., the proof mass fingers  24  and the portions of the tethers  30  farthest from the substrate rim  22 ) were observed to be more susceptible to backside and lateral erosion than those suspended structures nearer an anchor site (e.g., the rim fingers  26  and the portions of the tethers  30  nearest the substrate rim  22 ). The overall effect was that the proof mass fingers  24  and portions of the tethers  30  farthest from the rim  22  were significantly narrower than desired or acceptable. In addition, any stiction bumps placed on the proof mass fingers  24  were eroded by excessive etching to the point that they were completely removed, or at least their effectiveness was drastically reduced. In addition, notches and other surface flaws were observed during the investigation. Notching was particularly seen near the distal ends of the fingers  24  and  26  and tethers  30  due to energetic etch species reflection from the angled walls of the cavity  20 . Additionally, vertical notches were noted on the sidewalls of suspended structures (fingers  24  and  26 , tethers  30 , etc.) in locations where the trenches delineating the structures (e.g., trenches  28  or  32 ) were intersected by a second trench. All of the etch idiosyncrasies described above are believed to be associated to some degree with essentially all DRIE processes in which a suspended structure is delineated by a trench that breaches an underlying cavity. 
     The present invention addresses the above defects at the masking level by the manner in which those features of the device  10  prone to DRIE overetching are masked. In a preferred process for fabricating the device  10  by DRIE, a suitable etch is first performed to form the cavity  20  in the surface of the substrate  12   a  (in the form of a wafer of the desired material). A suitable technique is a wet etch of a type known in the art. Following oxidation of the substrate  12   a , the semiconductor layer  12   b  (previously grown on a second wafer) is then bonded to the substrate  12   a , with the result that the cavity  20  in the substrate  12   a  is enclosed by the semiconductor layer  12   b . Following selective removal of the second wafer, the remaining substrate  12   a  and semiconductor layer  12   b  yield the die  12 . The surface of the die  12  is then processed in a manner well known in the art to form layers of the MEMS device  10 , after which the surface is masked to protect surface regions of the die  12  corresponding to the proof mass  14 , the fingers  24  and  26 , the tethers  30  and the surrounding rim  22 . A suitable DRIE process for use with this invention employs an Alcatel 601 DRIE machine and a pulsed gas process in accordance with U.S. Pat. No. 6,127,273 to Laermer et al. Another suitable process employs an Alcatel 602 DRIE machine operated at a cryogenic temperature in accordance with Research Disclosure No. 42271, dated June 1999. 
     FIG. 6 is a detailed plan view of masking in what will be the interdigitized finger region of the device  10 , with masks  40  and  42  patterned for etching the fingers  24  and  26 , respectively (the fingers  24  and  26  and their after-etch widths are indicated in phantom). Regions of the semiconductor layer  12   b  remaining exposed between the masks  40  and  42  are also visible in FIG.  6 . Notably, while the width of the mask  42  corresponds to the width of the rim finger  26  that will be defined by the mask  42 , the mask  40  is significantly wider than the width of the proof mass finger  24  that it will define. As a result, the region of the semiconductor layer  12   b  that will be exposed by the masks  40  and  42  to the DRIE process to form one of the trenches  28  is narrower than the desired trench  28 . According to the invention, once the cavity  20  is breached, the region of the semiconductor layer  12   b  corresponding to the proof mass finger  24  beneath the mask  40  will etch more rapidly than the region of the semiconductor layer  12   b  corresponding to the rim finger  26  beneath the mask  42 , causing greater undercutting beneath the mask  40  to the extent that the desired width of the proof mass finger  24  (shown in phantom) will be obtained. The ratio of the width of the mask  40  relative to the desired width for the finger  24  will vary depending on various DRIE parameters, the desired width of the trench  28 , the width of the trench  28  as compared with the widths of other trenches defining the structure, and the thickness of the finger  24 . For a proof mass finger  24  having a desired width of about seven to eight micrometers a thickness (based on the thickness of the semiconductor layer  12   b ) of about thirty micrometers, separated from its capacitively-coupled rim finger  26  by a trench  28  having a width of about three micrometers, having structural trench widths of about seven micrometers, and parasitic trench widths of about six micrometers, the mask  40  preferably extends beyond each desired edge of the proof mass finger  24  by about 0.5 micrometers. 
     In reference to FIGS. 7 and 8, if the width of a mask  44  on the semiconductor layer  12   b  over what will be a tether  30  (the tether  30  and its after-etch width are indicated in phantom) were uniform along the entire length of the tether  30 , the tether  30  would be significantly narrower adjacent the proof mass  14  than where the tether  30  attaches to the rim  22 , attributable to the greater rate of backside and lateral erosion of the tether  30  with distances farther from the anchor site (rim  22 ) of the tether  30 . To counter these effects, the DRIE process of the present invention entails masking the semiconductor layer  12   b  to account for the nonuniform etching along the length of the tether  30 . 
     In FIG. 7, the width of the mask  44  for the tether  30  is maintained constant while tapering the gap width of the surface area  46  exposed by the mask  44  and through which the trenches  32  will be etched to delineate the tether  30 . The gap width of the exposed surface area  46  is tapered to be wider with decreasing distance from the rim  22  (the anchor site for the tether  30 ), so that backside and lateral erosion of the tether  30  that occurs more rapidly with increasing distance from the rim  22  is balanced by the more rapid etch rate associated with the increasingly wider gap near the rim  22 . As a result, a substantially uniform width for the tether  30  can be obtained. The appropriate ratio of the width of the exposed surface area  46  relative to the desired width of the trenches  32  will vary. 
     In contrast, the mask  44  for the tether  30  is shown in FIG. 8 as being tapered to increase in width toward the proof mass  14 , with the width nearest the proof mass  14  (i.e., farthest from the tether&#39;s anchor site at the rim  22 ) being wider than the width desired for the tether  30 , while the width of the mask  44  adjacent the rim  22  (i.e., nearest the rim anchor site) can be patterned to have approximately the width desired for the tether  30 . A constant gap width is maintained for the surface area exposed by the mask  44  through which the trenches  32  will be etched to delineate the tether  30 . As the distance from the rim  22  increases, backside and lateral erosion is correspondingly more rapid to eventually produce a substantially uniform width for the tether  30 . The appropriate ratio of the width of the mask  44  relative to the desired width of the tether  30  will vary, as will the amount of taper in the tether  30  itself along its length. 
     Significantly, the device  10  represented in FIG. 1 is also configured to maintain a proper environment surrounding the suspended structures (e.g., proof mass  14 , fingers  24  and  26 , and tethers  30 ), by avoiding unnecessary variations in trench width. An important example is the avoidance of intersecting trenches that would create a localized wider gap prone to more rapid etching and subsequent backside erosion, lateral erosion and vertical notches. Accordingly, the only intersecting trenches are the finger and tether trenches  28  and  32 , which necessarily intersect along the length of each tether  30  in a manner that minimizes the trench width variation. 
     As noted above, the DRIE process of the present invention also entails appropriate placement of the stiction bumps  34  to avoid being eroded by the etching phenomenon associated with the energetic, highly charged environment of the DRIE process. Because the DRIE process more rapidly etches the proof mass fingers  24 , with the result that the mask  40  is undercut, any bumps formed on the fingers  24  would also be rapidly etched and rendered ineffective. As a solution, the present invention relocates the interfinger stiction bumps  34  to the rim fingers  26  by appropriately masking the rim fingers  26 , as shown in FIG.  6 . 
     The highly charged environment of the DRIE process has been shown to increase the likelihood of undesirable translation and stiction in the Z-direction of the device  10 , possibly as a result of the static charge build-up discussed above. Though the proof mass  14  of the device  10  is particularly stiff in the Z-direction because of the tether design, stiction of the proof mass  14  to the floor  18  of the cavity  20  has been unexpectedly found to be a major yield problem when etching is performed by DRIE. Accordingly, as a direct result of implementing the DRIE process for a mass-produced MEMS device, the present invention provides the stiction bumps  36  formed on the floor  18  of the cavity  20 . According to the invention, the bumps  36  are preferably placed directly beneath the proof mass  14 , as shown in FIG. 5. A difficulty with forming the bumps  36  is the proper positioning and sizing of the bumps  36  as a result of the highly-directional cavity etch and the DRIE etch. The present invention has shown that the bumps  36  should be placed on the cavity floor  18  away from the edges of the cavity  20  and the trenches  17 ,  28  and  32  in order to prevent energetic etch species reflection off of the bumps  36  toward the backsides of the cantilevered fingers  24  and  26  and tethers  30 , the result of which would be undesirable notching of the structures. 
     Also a factor in location of the bumps  36  is the high temperature bond oxidation process following the cavity etch, by which the semiconductor layer  12   b  is bonded to the substrate  12   a . The bond process creates a vacuum within the initially sealed cavity  20  as the die  12  returns to room temperature. As a result, the semiconductor layer  12   b  is elastically pulled downward into the cavity  20  until the cavity  20  is breached by one of the trenches  17 ,  28  or  32  during the DRIE etch; most preferably, the cavity  20  is breached first by the hub trench  17 , which can be readily sized to provide for controlled venting of the cavity  20  away from the relatively fragile fingers  24  and  26  and tethers  30  without adversely affecting the etching process and device performance. The bumps  36  formed during the cavity etch are preferably placed away from regions of maximum deflection of the semiconductor layer  12   b  so as not to be a source of plastic deformation of the proof mass  14  later defined by etching the semiconductor layer  12   b . Accordingly, the stiction bumps  36  of this invention are located uniformly around the inner and outer perimeters of the future proof mass  14 , but not beneath any trench or other opening formed through the semiconductor layer  12   b . The bumps  36  are also preferably sized to prevent stiction of the proof mass  14  during severe Z-direction translations, while not being so large as to contact the semiconductor layer  12   b  when deflected as a result of the vacuum within the cavity  20 . For this reason, an optimal height for the stiction bumps  36  is believed to be on the order of about one-fourth to about three-fourths of the depth of the cavity  20 , which in practice is about eleven micrometers, though shallower or deeper cavities could also been successfully used. Because wet etches suitable for forming the cavity  20  are highly directional in silicon, resulting in different etch rates along different crystalline planes, it will be appreciated by those skilled in the art that the rate at which the height of a stiction bump  36  decreases during the wet etch is a fairly complex function of the etch rates of the exposed facets (silicon directions). Thus, suitable modeling is preferably employed to obtain stiction bumps  36  having the prescribed height. 
     Those skilled in the art will appreciate that conventional silicon processing techniques and materials can and would be employed in the fabrication of a MEMS device, beyond those discussed above. In addition, while a particular configuration is shown for the proof mass  14 , fingers  24  and  26  and tethers  30 , various modifications could be made by one skilled in the art. More particularly, the present invention is applicable to essentially any suspended or cantilevered structure that is DRIE etched over a cavity. Finally, it is foreseeable that the present invention could be utilized to encompass a multitude of applications through the addition or substitution of other processing or sensing technologies. Therefore, while the invention has been described in terms of a preferred embodiment, other forms could be adopted by one skilled in the art. Accordingly, the scope of the invention is to be limited only by the following claims.

Technology Category: b