Electronic apparatus having improved scratch and mechanical resistance

Embodiments of the invention include a sensor such as a capacitive sensor with improved scratch resistance. The sensor has a substrate and a layer of sensing elements formed thereon that are formed from materials having a greater mechanical firmness than conventional aluminum or other soft metal materials. Sensor interconnects also are made of such materials. The increased mechanical firmness of the sensing elements and interconnects improves the scratch and mechanical resistance thereof by reducing scratches, mechanical stress and cracks by reducing the deformation and consequently the bridge and/or gap effects of the sensing element material. Such effects plague conventional electronic devices, integrated circuits and sensors. Alternatively, the inventive sensor includes, e.g., a dielectric region operably coupled to the sensing elements and interconnects, thus forming a capacitive sensor or other electronic devices. The sensing elements and interconnects are formed beneath, within or on top of the dielectric region, and their improved mechanical firmness improves the scratch and mechanical resistance of the dielectric region or other material region formed thereon.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The invention relates to electronic devices including integrated circuit
 sensors, imagers and other sensing devices. More particularly, the
 invention relates to improved scratch and mechanical resistance for
 integrated circuit devices including integrated circuit sensors, other
 sensing devices and other electronic devices.
 2. Description of the Related Art
 Sensing devices including integrated circuit (IC) devices such as those
 used for capacitive sensors desire scratch resistant surfaces to protect
 the integrated circuit from repeated contact by external elements. For
 example, in capacitive sensors such as fingerprint sensors, the ridges and
 valleys on the finger tip making contact with the surface of the sensor
 (e.g., a CMOS chip) form variations in capacitance across a dielectric
 material in the chip that can be measured by an array of sensing elements
 arranged within the chip. The capacitance variations are then output by
 the sensor and used to form an image of the fingerprint.
 Conventional sensors of this kind typically provide one or more layers
 including mechanically firm dielectric materials, e.g., silicon nitride
 (Si.sub.3 N.sub.4), silicon dioxide (SiO.sub.2), and aluminum oxide
 (Al.sub.2 O.sub.3). Also, the sensing elements and interconnect wires,
 which typically are formed on top of, within or below the dielectric
 region, typically are made of aluminum, copper, chromium, alloys of these
 metals, or multi-layers of these metals, because of their relatively low
 resistivities. For example, see the capacitive sensors disclosed in U.S.
 Pat. No. 4,353,056, issued to Tsikos, and U.S. Pat. No. 5,325,442, issued
 to Knapp.
 However, sensors including integrated circuit sensors having such
 arrangements are still susceptible to scratches from the stress of
 repeated touch and from sharp, pointed external elements (e.g.,
 fingernails and writing instruments). Such scratches and mechanical stress
 effects degrade the performance and/or functionality of the device and
 often lead to the complete failure of the device. Therefore, it is
 desirable to provide a sensor in general and an integrated circuit sensing
 device in particular having an improved scratch resistance and mechanical
 resistance over existing devices.
 SUMMARY OF THE INVENTION
 The invention is embodied in a sensor such as a capacitive sensor with
 improved scratch resistance. The sensor has a substrate and a layer of
 sensing elements formed thereon that are formed from materials having a
 greater mechanical firmness than conventional aluminum or other soft metal
 materials. Sensor interconnects also are made of such materials. The
 increased mechanical firmness of the sensing elements and interconnects
 improves the scratch and mechanical resistance thereof by reducing
 scratches, mechanical stress and cracks by reducing the deformation and
 consequently the bridge and/or gap effects of the sensing element
 material. Such effects plague conventional electronic devices, integrated
 circuits and sensors. Alternatively, the inventive sensor includes, e.g.,
 a dielectric region operably coupled to the sensing elements and
 interconnects, thus forming a capacitive sensor or other electronic
 devices. The sensing elements and interconnects are formed beneath, within
 or on top of the dielectric region, and their improved mechanical firmness
 improves the scratch and mechanical resistance of the dielectric region or
 other material region formed thereon.

DETAILED DESCRIPTION
 In the following description like reference numerals indicate like
 components to enhance the understanding of the invention through the
 description of the drawings.
 Although specific features, configurations and arrangements are discussed
 hereinbelow, it should be understood that such is done for illustrative
 purposes only. A person skilled in the relevant art will recognize that
 other configurations and arrangements can be used without departing from
 the spirit and scope of the invention.
 Embodiments of the invention are based on the realization that scratches,
 stress, and other defects inflicted upon sensors by external elements are
 exacerbated by the use of sensing elements and interconnects that are made
 of aluminum, aluminum alloys and other relatively soft materials. For
 example, in capacitive sensors having a dielectric region formed over the
 sensing elements, soft metals such as aluminum or multi-layer metal
 structures with aluminum are useful as sensing elements because of their
 relatively low resistivities. However, such soft metals often become
 deformed and develop bridges and/or gaps beneath the thin dielectric
 region and thus cause the dielectric region to act as membranes, which are
 easily scratched or broken by stress inflicted thereon. Also, in sensors
 or other electronic devices in which these soft metal interconnects and
 sensing elements are formed on top of other harder materials, the soft
 metals often form cracks and/or gaps that lead to broken or deteriorated
 elements and/or interconnect wires resulting from the stress inflicted
 thereon.
 According to embodiments of the invention, a scratch resistant region
 having one or more layers including the sensing elements, contacts and
 interconnects, are made of materials having a me cal hardness much greater
 than that aluminum, many aluminum alloys and other soft metals. In this
 manner, the device exhibits greater resistance to scratches and defects,
 including arrangements where the scratch resistant region has formed
 thereon a dielectric region of one or more dielectric layers. The scratch
 resistant regions are made of titanium nitride (TiN), metals such as
 tungsten (W), tantalum (Ta), titanium (Ti) and alloys of these metals.
 Some materials such as TiN are called conducting ceramics or metals. For
 purposes of discussion herein, titanium nitride (TiN) will be referred to
 and treated as metal.
 Referring now to FIG. 1, a sensor 10 according to an embodiment of the
 invention is shown. Sensor 10 includes a substrate 12 and a scratch
 resistant region 14 formed on substrate 12. Typically, scratch resistant
 region 14 is a sensing region having a sensing surface 16 for receiving
 the object being sensed. Scratch resistant region 14 is operably coupled
 to sensing circuitry (not shown), which, for example, often is contained
 within substrate 12.
 Substrate 12 is any substrate suitable for use with scratch resistant
 region 14. For example, substrate 12 is a semiconductor substrate
 containing, e.g., sensing circuitry and other device elements, that is
 coupled to scratch resistant region 14 by appropriate means.
 Alternatively, substrate 12 is a piezoelectric substrate or an insulating
 material such as glass having the appropriate coupling to scratch
 resistant layer 14.
 Scratch resistant region 14 comprises one or more conducting ceramics,
 metals, metal alloys and/or layers thereof that provide a suitable
 mechanical hardness to the sensor. For example, scratch resistant region
 14 includes at least one layer or region of titanium nitride (TiN),
 tungsten (W), tantalum (Ta), titanium (Ti) and alloys of these metals.
 According to embodiments of the invention, scratch resistant region 14 has
 a thickness within the range, e.g., of approximately 100 to approximately
 6000 angstroms (.ANG.). Embodiments of the invention include one or more
 scratch resistant regions 14 having a thickness within the range, e.g., of
 approximately 100 to approximately 5000 .ANG.. Typically, the thickness of
 scratch resistant region 14 is within the range from approximately 1000 to
 approximately 3000 .ANG..
 For sensors as capacitive sensors, a dielectric region is formed on scratch
 resistant region 14. Accordingly, dielectric region 22 has a sensing
 surface 24 for receiving an object that is being sensed. In this manner,
 when the object to be sensed comes into contact with sensing surface 24, a
 capacitor is formed with the object and scratch resistant region 14
 serving as upper and lower capacitor plates on either side of dielectric
 region 22, which is positioned therebetween. Dielectric region 22 is made
 of, e.g., silicon nitride (Si.sub.3 N.sub.4), silicon dioxide (SiO.sub.2),
 aluminum oxide (Al.sub.2 O.sub.3), tantalum pentoxide (Ta.sub.2 O.sub.5)
 and titanium oxide (TiO.sub.2). Alternatively, one or more layers are
 formed on top of dielectric region 22, e.g., for prevention of impurity
 migration. According to embodiments of the invention, dielectric region 22
 has a thickness within the range, e.g., of approximately 1000 to
 approximately 10,000.ANG..
 In another embodiment of the invention, scratch resistant region 14
 comprises one or more sensing elements, e.g., an array of sensing elements
 26 and interconnects or interconnect wires 27 as shown in FIG. 2a. In one
 arrangement of this embodiment of the invention, sensing elements 26 and
 interconnects 27 are formed between substrate 12 and dielectric region 22,
 as shown, and coupled to sensing circuitry (not shown) via coupling 28.
 Yet, another suitable arrangement of this embodiment includes forming
 sensing elements 26 and interconnects 27 within dielectric region 22, as
 shown generally in FIG. 2b.
 For scratch resistant regions within sensors such as capacitive sensors,
 the desired characteristics include mechanical strength, low thermal
 expansion coefficient, and resistance to corrosion. Conventional sensors
 use aluminum and aluminum alloys, e.g., as sensing elements, contacts and
 interconnects, typically because of the characteristic low resistivity
 associated with aluminum. For example, aluminum has a resistivity of
 approximately 1-2 .mu.-ohm-cm. However, the mechanical hardness of
 aluminum is less than approximately 150 kg/MM.sup.2 (Knoop hardness).
 There are various methods to measure hardness and various units to quantify
 the hardness or mechanical strength of a material. For example, Knoop
 hardness is expressed in units of kg/mm .sup.2. Young modulus is another
 measure of hardness. Young modulus is described in units of dynes/cm.sup.2
 or Newtons/m.sup.2 or Pascals (Pa) or Gigapascals (GPa), where 1
 GPa=10.sup.9 Pa=10.sup.9 Newtons/m.sup.2 =10.sup.10 dynes/cm.sup.2 For
 purposes of discussion herein, the mechanical hardness for various
 materials will be expressed as the Knoop hardness of the material in bulk
 form. It is known that the corresponding thin film hardness is
 proportional, but much less than the bulk hardness of a given material.
 By comparison, titanium has a mechanical hardness of approximately 200-250
 kg/mm.sup.2, tungsten has a mechanical hardness of approximately 400-500
 kg/mm.sup.2, and titanium nitride has a mechanical hardness of
 approximately 1500-2500 kg/mm.sup.2. But, the resistivity of these
 materials is slightly greater than that of aluminum and many of the
 aluminum alloys typically used in such applications. For example, the
 resistivity titanium is approximately 40-50 .mu.-ohm-cm, the resistivity
 of tungsten is approximately 5-10 .mu.ohm-cm, and the resistivity of
 titanium nitride is approximately 60-100 .mu.-ohm-cm.
 For capacitive applications such as in fingerprint sensors, sensing
 elements 26 and interconnects 27 do not carry relatively large currents
 and therefore lower resistivities do not necessarily enhance the
 electrical performance. Also, in other low power electronic devices and
 integrated circuit applications, large currents are not being carried by
 sensing elements 26 and interconnects 27 and therefore appropriate circuit
 design changes such as increasing the widths of one or more interconnects
 is possible with materials according to embodiments of the invention with
 relatively high mechanical hardness, e.g., greater than approximately 200
 kg/mm .
 In conventional electronic devices including integrated circuits, materials
 such as titanium nitride are used over aluminum regions to prevent
 reflection and over and under aluminum regions to prevent migration.
 However, conventional devices have not contemplated using materials such
 as titanium nitride for purposes of scratch and mechanical resistance.
 Typically, the top or last level of the metal layer or layers is important
 for scratch and mechanical resistance. However, in conventional electronic
 devices, materials such as titanium nitride are used on top of or beneath
 the aluminum regions for various metal levels to prevent reflection and
 migration.
 According to embodiments of the invention, the top or last metal layer or
 layers, e.g., scratch resistant region 14 as shown in FIG. 1 and sensing
 elements 26 and interconnects 27 as shown in FIGS. 2a-b, are not made of
 aluminum or aluminum alloys or other soft, low resistivity materials
 typically used in such applications. Rather, scratch resistant region 14
 and sensing elements 26 and interconnects 27 are made of materials having
 a mechanical hardness much greater than that of aluminum or other
 conventional sensing element and interconnect materials. In this manner,
 devices according to embodiments of the invention enjoy a region that,
 both alone and with a dielectric region formed thereon, enjoys improved
 scratch and mechanical resistance.
 As discussed earlier, scratches, stress, and other defects inflicted upon
 sensors by external elements are exacerbated by the use of sensing
 elements and interconnects that are made of aluminum, aluminum alloys and
 other relatively soft materials. For example, as illustrated in FIGS.
 3a-c, when using a dielectric region 62 (e.g., Al.sub.2 O.sub.3, or
 Si.sub.3 N.sub.4) formed on an aluminum region 64 (e.g., in a capacitive
 sensor 66), local stress applied to the surface of the dielectric region
 tends to deform or displace the aluminum underneath, creating a gap and/or
 bridge (as shown generally in FIG. 3b). Scratches are caused, e.g., by
 fingernails (e.g., in association with a capacitive or other type of
 fingerprint sensor). Scratches also are caused, e.g., by the point of a
 writing instrument. Also, as shown in FIG. 4, in sensors and other
 electronic devices having top metal layers without a dielectric region
 thereon, external stress from, e.g., fingernails, writing instruments, or
 other relatively sharp objects cause undesirable displacement of sensing
 elements 26 and interconnects 27.
 Embodiments of the invention provide improved scratch resistance by
 replacing the aluminum and other soft metal layers with a material that
 has a much greater mechanical hardness. Furthermore, according to
 embodiments of the invention, it has been observed that, in general, the
 scratch resistance of a dielectric material formed on a sensing region
 formed by soft materials is inversely proportional to the thickness of the
 sensing region. Therefore, for further improved scratch resistance, the
 sensing region under the dielectric material should be formed as thinly as
 practicable for the given application.
 The materials used for scratch resistant region 14 according to embodiments
 of the invention, e.g., titanium nitride (TiN), metals such as tungsten
 (W), tantalum (Ta), titanium (Ti) and alloys of these metals, are useful
 with or without the additional dielectric region formed thereon. However,
 with respect to the use of tungsten (W) or other metals that have a
 relatively high level of surface roughness after deposition, the tungsten
 region often is used in conjunction with a subsequent polishing technique,
 e.g., a chemical-mechanical polishing (CMP), to reduce the surface
 roughness and increase the scratch resistance.
 As mentioned previously herein, materials used for scratch and mechanical
 resistance according to embodiments of the invention also have relatively
 low thermal expansion coefficients. Therefore, in embodiments where
 sensing elements and interconnect wires exist beneath dielectric regions,
 such materials reduce the mismatch of thermal expansion between the
 sensing elements and interconnect wires and the dielectric region
 thereabove, thus providing high thermal and mechanical stability.
 For example, a dielectric material of Al.sub.2 O.sub.3 has a thermal
 expansion coefficient of approximately 4-6 (.times.10.sup.31 6 per degree
 Celsius). By comparison, the thermal expansion coefficient of aluminum is
 greater than 25 (.times.10.sup.-6 per .degree. C.), while the thermal
 expansion coefficient of tungsten is less than 5 (.times.10.sup.-6 per
 .degree. C.),
 Also, some hard metals, e.g. TiN, have very high corrosion resistance to
 chlorides such as salt (NaCl). As discussed previously herein, resistance
 to corrosion is a desirable characteristics of materials used in
 embodiments of the invention.
 Furthermore, materials discussed herein for use in embodiments of the
 invention also are useful for applications where open ICs and sensor
 surfaces need to be cleaned. The cleaning may create severe scratches and
 mechanical stress.
 Although embodiments of the invention have been described for use with
 capacitive, semiconductor sensors such as fingerprint sensors, embodiments
 of the invention are suitable for use with other kinds of sensors, e.g.,
 pressure sensors, in which scratch resistant regions are employed with or
 without additional regions of dielectric materials or other materials.
 Also, other applications for which embodiments of the invention are
 suitable for use include, e.g., smart cards, radio frequency (RF) cards,
 micromachines, microelectro-mechanical sensors (MEMs) and other open
 integrated circuit applications in which scratch and/or mechanical
 resistance is desirable. Embodiment of the invention also are useful to
 enhance product yields during assembly, e.g., in the packaging of
 integrated circuits, and the assembly of displays, 3-dimensional (3D)
 memory and integrated circuit stacks. Such applications include the use of
 non-semiconductor devices such as piezoelectric devices and/or pressure
 sensors.
 It will be apparent to those skilled in the art that many changes and
 substitutions can be made to the embodiments of the scratch resistant
 devices herein described without departing from the spirit and scope of
 the invention as defined by the appended claims and their full scope of
 equivalents.