|Publication number||US20040065638 A1|
|Application number||US 10/267,082|
|Publication date||Apr 8, 2004|
|Filing date||Oct 7, 2002|
|Priority date||Oct 7, 2002|
|Also published as||WO2004033365A2, WO2004033365A3|
|Publication number||10267082, 267082, US 2004/0065638 A1, US 2004/065638 A1, US 20040065638 A1, US 20040065638A1, US 2004065638 A1, US 2004065638A1, US-A1-20040065638, US-A1-2004065638, US2004/0065638A1, US2004/065638A1, US20040065638 A1, US20040065638A1, US2004065638 A1, US2004065638A1|
|Original Assignee||Bishnu Gogoi|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (17), Referenced by (24), Classifications (15), Legal Events (3)|
|External Links: USPTO, USPTO Assignment, Espacenet|
 This invention relates generally to electronics, and more particularly to electronic components and methods of manufacture.
 As electronic systems and technology becomes more sophisticated, market demand shifts increasingly toward smaller, more sensitive electronic devices. In an attempt to meet that demand, manufacturers have integrated mechanical devices with semiconductor technology to produce sophisticated Microsystems capable of sensing and controlling their environment. This technology is known as micro-electromechanical systems, or MEMS. MEMS technology is used in the fabrication of a variety of devices, including, among others, motion sensors, pressure sensors, flow controllers, and flow sensors. Motion sensors, also called inertial sensors, are widely used in advanced mechanical systems such as, among others, robotics, vehicle guidance systems, and space- and ground-based tracking, location, and positioning devices. Gyroscopes and accelerometers are two of the most successful motion sensors in terms of performance and of market acceptance and demand.
 MEMS devices require very high electrical isolation in order to prevent substrate feedthrough, i.e., the transmission of signals from one layer of the sensor device to another. Because of the high isolation requirement, many fabrication techniques make use of a substrate comprising glass or quartz, both of which provide good isolation, rather than a silicon substrate, which may not provide sufficient isolation. Other high isolation substances, such as ceramic and silicon carbide, may also be used. A layer of epitaxial silicon (or “epi layer”) formed over the high-isolation substrate may be used as the device layer. The epi layer is used, at least in part, because it has the same crystal orientation as the directly underlying material, thus allowing high quality, uniform processing. Current MEMS gyroscope and accelerometer technology uses epitaxial silicon overlying a silicon substrate, where the difference in doping between the silicon substrate and the epi layer is used to provide etch selectivity between the silicon substrate and the epi layer. Existing technology requires the use of ethylene diamine pyrocatechol (EDP) as an etchant in this setting. EDP is highly selective to heavily-doped silicon, but not selective to lightly-doped silicon, thus providing a precise etch stop layer. This makes EDP very well-suited to the existing motion sensor fabrication techniques. The existing methodology is flawed, however, in that EDP is very corrosive and highly carcinogenic, thus requiring costly safety measures that discourage its use and greatly limit its usefulness. Additionally, EDP is incompatible with metal-oxide-semiconductor (MOS) and complimentary MOS (CMOS) processing, which further limits its usefulness. Accordingly, a need exists for a method of forming a MEMS device that does not require the use of EDP, but still provides the benefit of a precise etch stop layer.
 The invention will be better understood from a reading of the following detailed description, taken in conjunction with the accompanying figures in the drawings in which:
FIG. 1 is a flow diagram illustrating a method of forming a sensor according to an embodiment of the present invention;
FIG. 2 illustrates a cross-sectional view of a portion of a silicon-on-insulator substrate in accordance with an embodiment of the invention;
FIG. 3 illustrates a cross-sectional view of a portion of the silicon-on-insulator substrate of FIG. 2 after subsequent processing steps in accordance with an embodiment of the invention;
FIG. 4 illustrates a cross-sectional view of a portion of a support substrate in accordance with an embodiment of the invention;
FIG. 5 illustrates a cross-sectional view of the support substrate of FIG. 4 after subsequent processing steps in accordance with an embodiment of the invention;
 FIGS. 6-8 illustrate cross-sectional views of a portion of a sensor after various processing steps in accordance with an embodiment of the invention; and
FIG. 9 is a flow diagram illustrating another method of forming a sensor according to an embodiment of the present invention.
 For simplicity and clarity of illustration, the drawing figures illustrate the general manner of construction, and descriptions and details of well-known features and techniques are omitted to avoid unnecessarily obscuring the invention. Additionally, elements in the drawing figures are not necessarily drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention. Furthermore, the same reference numerals in different figures denote the same elements.
 Furthermore, the terms first, second, third, fourth, and the like in the description and in the claims, if any, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is further understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in other sequences than illustrated or otherwise described herein. Moreover, the terms left, right, front, back, top, bottom, over, under, and the like in the description and in the claims, if any, are used for descriptive purposes and not necessarily for describing permanent relative positions. It is understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in other orientations than illustrated or otherwise described herein. The term coupled, as used herein, is defined as directly or indirectly connected in an electrical or non-electrical manner.
 A particular embodiment of the sensor formation method disclosed herein includes the step of providing a silicon-on-insulator (SOI) substrate containing a device layer, an insulator layer, and a handle layer. The device layer may be patterned to form a device structure therein in accordance with the requirements of a particular device to be created. A support substrate is also provided and patterned, and an electrically conductive layer is formed over the support substrate. The SOI substrate and the support substrate are bonded together, and the handle layer and the insulator layer are removed from the SOI substrate, thus releasing the device structure formed in the device layer.
 Referring now to the figures, and in particular to FIG. 1, a method 100 illustrates a method of forming a device according to an embodiment of the present invention. A first step 110 of method 100 is to provide a silicon-on-insulator substrate. In accordance with an embodiment of step 110 in FIG. 1, FIG. 2 depicts a silicon-on-insulator (SOI) substrate 200 comprising a device layer 210, an insulator layer 220, and a handle layer 230. Device layer 210 has a device layer surface 240. As will be seen hereinafter, insulator layer 220 serves as a sacrificial layer used in sensor fabrication methods. Device layer 210, in one embodiment, may comprise silicon. In this same embodiment, handle layer 230 may also be comprised of silicon, and insulator layer 220 may be comprised of an oxide, nitride, or other electrically insulating material. For example, insulator layer 220 may comprise silicon dioxide that is thermally grown in an oxidation furnace. In one embodiment, SOI substrate 200 can be formed using a wafer-to-wafer bonding process whereby device layer 210 and handle layer 230 are bonded together with insulator layer 220 in the middle. As will be readily apparent to one of ordinary skill in the art, insulator layer 220 may be provided with intentionally-patterned cavities, not shown, included to facilitate or simplify device fabrication.
 Referring again to FIG. 1, a second step 120 of method 100 is to pattern the device layer of the SOI substrate. In FIG. 3, device layer 210 is shown after being patterned with a first pattern according to an embodiment of the present invention to provide a device structure 310 therein. Device layer 210 may be patterned with various silicon etching techniques including, for example, reactive ion etching (RIE) and wet etching. As an example, the wet etching technique can use potassium hydroxide (KOH) or tetra-methyl-ammonium-hydroxide (TMAH). Deep RIE systems can be used to avoid problems associated with forming trenches and holes having different aspect ratios in device layer 210. Ion beam milling can also be used to pattern device layer 210.
 Device structure 310 may be, in one embodiment, a portion of a sensor such as a gyroscope or an accelerometer for detecting motion. As an example, device structure 310 can be a single seismic mass for a two- or three-axis accelerometer, thus providing a more compact sensor. As another example, device structure 310 can be a seismic mass for a vibration sensing device or a switch. In an embodiment where device structure 310 is a seismic mass for a gyroscope, device structure 310 may comprise beams 320 separated by gaps 330 to form a finger-like or cantilever structure. Gaps 330 are etched into device layer 210 so as to reach an insulator layer surface 340.
 Gyroscopes, as opposed to accelerometers, are active devices, meaning they are actively driven. Gyroscopes fabricated using MEMS technology conventionally make use of the Coriolis effect, meaning the device is made to oscillate at a fixed amplitude in one plane or along an axis of the gyroscope. The turning rate experienced by the gyroscope becomes an input signal for the gyroscope. The interaction of the fixed amplitude oscillation of the seismic mass, including beams 320, along a first axis and the turning rate of the gyroscope provides a displacement of beams 320 along a second axis, where such displacement along a second axis is measured as an electrical output signal of the gyroscope.
 Turning back to FIG. 1, a third step 130 of method 100 is to provide a support substrate, and a fourth step 140 of method 100 is to pattern the support substrate. Referring to FIG. 4, a support substrate 410 is shown to have been etched with a second pattern in accordance with an embodiment of the present invention. Support substrate 410 may comprise a glass or quartz wafer, which, because of the high electrical isolation characteristics of those materials, helps prevent substrate feedthrough and thereby contributes to greater device sensitivity. The same effect may be achieved, in another embodiment, by providing support substrate 410 comprised of silicon with an overlying insulating layer. In this embodiment the insulating layer may comprise, for example, an oxide or a nitride. Support substrate 410 may also comprise other high isolation substances, such as ceramic and silicon carbide, that also provide simultaneous electrical insulation and mechanical support. Support substrate 410 further comprises a support substrate surface 430 in which a recess 420 is formed.
 Referring back to FIG. 1, a fifth step 150 of method 100 is to form an electrically conductive layer over the support substrate. FIG. 5 depicts a conductive layer 510 that has been patterned on support substrate 410 in accordance with an embodiment of the present invention. Conductive layer 510 may comprise a variety of electrically conductive materials including, for example, aluminum, gold, copper, or other metals. It may also comprise doped polysilicon or other doped conductive layers. Gold is used in many instances, despite its relatively greater cost, because it is resistive to being etched by conventional oxide etchants that have poor etch selectivity to aluminum and other metals. Furthermore, gold is preferred when the sensor to be formed is a switch. This preference is due, at least in part, to the ability of gold to reduce contact resistance within the sensor. In a different embodiment of the invention, conductive layer 510 comprises aluminum, which is both cheaper than gold and more compatible with conventional semiconductor fabrication methods.
 Again referring back to FIG. 1, a sixth step 160 of method 100 is to bond together the SOI substrate and the support substrate. FIG. 6 shows how this may be done according to one embodiment of the invention. Support substrate surface 430 and device layer surface 240 are brought together so as to be in physical contact with each other. SOI substrate 200 and support substrate 410 are thus in an inverted relationship with respect to each other. The substrates are then bonded together using any suitable bonding technique. For example, a surface activated bonding technique may be used. In a different embodiment, anodic bonding can be used to bond together SOI substrate 200 and support substrate 410. After bonding, a combined wafer 600 is formed.
 A seventh step 170 of method 100 in FIG. 1 is to remove the handle layer from the SOI substrate. Combined wafer 600 after handle layer 230 (see FIG. 6) is removed is depicted in FIG. 7 in accordance with an embodiment of the invention. Seventh step 170 in FIG. 1 leaves support substrate 410, device layer 210, insulator layer 220, and conductive layer 510 as the main remaining components of combined wafer 600 in FIG. 7. Handle layer 230 (FIG. 6) may be removed using an appropriate etching method that uses insulator layer 220 as an etch stop layer. If insulator layer 220 comprises a buried oxide layer, for example, a precise etch stop definition for certain etchants is provided. The etch stop does not depend on the thickness or doping level of device layer 210, thus removing any constraints on those parameters that would otherwise need to be observed for etching purposes. Gyroscopes, in particular, are very sensitive to process variations, making the precise etch stop an advantageous feature of method 100 in FIG. 1. As an example, a wet etchant such as KOH or TMAH or an RIE technique can be used to remove the handle layer.
 Referring again to FIG. 1, an eighth step 180 of method 100 is to remove the insulator layer from the SOI substrate. The result of eighth step 180 in FIG. 1 is shown in FIG. 8, where combined wafer 600 comprises support substrate 410, device layer 210, and conductive layer 510 in accordance with an embodiment of the invention. The combination comprises a sensor 800, having undergone the complete fabrication process associated with an embodiment of the invention. The removal of insulator layer 220 (FIG. 7) releases device structure 310. Device layer 210 acts as a precise etch stop layer halting the removal of material at beams 320. In one embodiment, a dry etchant such as an RIE may be used for this etching or removal process. The types and uses of such dry etchants are well known in the art. The use of a dry etchant can reduce a common problem known as stiction for device structure 310. It should be noted that, during the removal of insulator layer 220 (FIG. 7), a slight undercut may be formed in the oxide used for surface activated bonding of support substrate 410 and device layer 210.
 In a different embodiment, insulator layer 220 (see FIG. 7) is removed using a wet etchant comprised of acetic acid, acetic anhydride, water, and hydrofluoric acid. The concentrations of the foregoing components within at least one particular embodiment of the etchant are given in U.S. Pat. No. 5,824,601, which is hereby incorporated herein by reference. Other wet etchants like hydrofluoric acid (HF) or buffered HF may also be used. The etchant disclosed in U.S. Pat. No. 5,824,601 is preferred over HF when conductive layer 510 comprises aluminum because of its higher etch selectivity between aluminum and oxide.
 In at least one embodiment, an integrated circuit may be formed in device layer 210 and electrically coupled to device structure 310. The integrated circuit is illustrated by dashed region 810 in FIG. 8 and can be formed in device layer surface 240 of device layer 210 before, during, or after the patterning of the device layer in method 100 (FIG. 1). Depending on the requirements for sensor 800, integrated circuit 810 can be electrically coupled to device structure 310 during the formation of the electrical interconnect of integrated circuit 810. Integrated circuit 810 can be electrically coupled to conductive layer 510 during the bonding step in method 100 (FIG. 1). In this embodiment, the bonding step can use an anodic bonding technique.
 As an example, FIG. 9 illustrates a method 900 of forming a device according to an embodiment of the invention in which an integrated circuit is formed in a device layer of an SOI substrate. Method 900 includes a step 920 to pattern the device layer to form a device structure, to form an integrated circuit in the device layer, and to electrically couple the device structure to the integrated circuit. Method 900 also includes a step 960 to bond together the silicon-on-insulator substrate and the support substrate and to electrically couple the conductive layer to the integrated circuit.
 Although the invention has been described with reference to specific embodiments, it will be understood by those skilled in the art that various changes may be made without departing from the spirit or scope of the invention. Accordingly, the disclosure of embodiments of the invention is intended to be illustrative of the scope of the invention and is not intended to be limiting. It is intended that the scope of the invention shall be limited only to the extent required by the appended claims. Additionally, benefits, other advantages, and solutions to problems have been described with regard to specific embodiments. The benefits, advantages, solutions to problems, and any element or elements that may cause any benefit, advantage, or solution to occur or become more pronounced, however, are not to be construed as critical, required, or essential features or elements of any or all of the claims.
 Furthermore, the terms “comprise,” “include,” “have,” and any variations thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements, but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Moreover, embodiments and limitations disclosed herein are not dedicated to the public under the doctrine of dedication if the embodiments and/or limitations: (1) are not expressly claimed in the claims and (2) are or are potentially equivalents of express elements and/or limitations in the claims under the doctrine of equivalents.
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|International Classification||B81C1/00, G01P15/125, G01C19/56, G01P15/08, B81B3/00|
|Cooperative Classification||G01P15/0802, G01C19/5719, B81C1/00142, G01P15/125, B81C2201/0191|
|European Classification||G01C19/5719, G01P15/125, G01P15/08A, B81C1/00C4B|
|Feb 7, 2003||AS||Assignment|
Owner name: MOTOROLA, INC., ILLINOIS
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:GOGOI, BISHNU;REEL/FRAME:013753/0233
Effective date: 20030130
|May 7, 2004||AS||Assignment|
Owner name: FREESCALE SEMICONDUCTOR, INC.,TEXAS
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:MOTOROLA, INC;REEL/FRAME:015360/0718
Effective date: 20040404
|Feb 2, 2007||AS||Assignment|
Owner name: CITIBANK, N.A. AS COLLATERAL AGENT,NEW YORK
Free format text: SECURITY AGREEMENT;ASSIGNORS:FREESCALE SEMICONDUCTOR, INC.;FREESCALE ACQUISITION CORPORATION;FREESCALE ACQUISITION HOLDINGS CORP.;AND OTHERS;REEL/FRAME:018855/0129
Effective date: 20061201