A MICRO-SCALE ACTUATOR
Technical Field
[001] The present invention relates to a micro-scale
actuator with a linear vibration resistant resonator.
Background of the invention
[002] Micro-electromagnetic actuators or resonators with
high amplitudes and reliability are important requirements for the rapidly growing gyroscopic applications. Conventional micro resonators that are produced generally with a wafer bonding process result in exposed electrical connections, thereby causing reliability and low performance issues during functional operations.
[003] In a micro fabrication by a wafer bonding process,
two similar or non-similar materials are attached with or without
influence of electrical and mechanical forces. When two non-
similar materials are attached together in the wafer bonding
process, a mismatch between both material properties usually
occurs, resulting in building up of residual stresses at the joint
interfaces thereby affecting the mechanical response of elements
of the actuator or resonator. This anomaly of increase in residual
stresses, is attributed to the thermal coefficient of expansion
occurring during the wafer bonding process.
[004] In conventional micro resonator fabrication, using
anodic bonding process, structural elements are formed in silicon material with electrical connections arranged on it and is further bonded to a glass substrate, which has a close match of thermal expansion coefficient. In such anodic bonding processes, a high electrical field for joining silicon and glass materials is involved,
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causing physical damage to the electrode material that is deposited on the silicon structure. Further, the mismatch of thermal coefficient of expansion also causes an increase in residual stresses, leading to undesired deformations in suspended structural elements. In some of the high temperature environment applications, the bonding interface stresses tend to modify thereby leading to the reliability issues of micro actuator or resonator.
[005] The presence of exposed electrical connection with
narrow branches over the structure elements at high currents, develop high current density regions. In semiconductors, high current density in narrow electrical lines causes physical damage of lines due to a phenomenon of electromigration. Since, the micro resonator or actuator usually operates with an electromagnetic actuation, it requires a high current for achieving a better performance. At high currents, the narrow electrical lines over the silicon structure develops an electromigration phenomenon by damaging the electrical lines. This phenomenon also limits the high performance and causes reliability issues.
Objects of the present invention
[006] The primary object of the present invention is to
provide a micro-scale actuator with a linear vibration resistant
resonator, for high current applications.
[007] An object of the present invention is to provide a
micro-scale actuator with a linear vibration resistant resonator
having movable flexible bridges, a ring portion and a stationary
anchor portion to render an improved vibration sensitivity ratio.
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[008] An object of the present invention is to provide a
micro-scale actuator with an electromigration preventive member to prevent the influence of high currents on the actuator.
[009] Another object of the present invention is to provide
a method for fabricating micro-scale actuator with a linear vibration resistant resonator and an electro migration preventive layer, for high current applications.
[010] Yet another object of the present invention is to
provide process for fabricating a micro-scale actuator made from a single wafer to prevent an influence of residual stresses on the micro-scale actuator.
Summary of the present invention
[011] Accordingly, a micro-scale actuator of the present
invention includes a primary electrical insulator with an opening is disposed on a substrate. At least a linear vibration resistant resonator with movable flexible bridges is connected to a movable ring portion and to a stationary anchor portion, is disposed on the primary electrical insulator. The linear vibration resistant resonator is disposed in between the primary electrical insulator and at least a secondary electrical insulator. At least an electrode member with a groove pattern corresponding to the flexible bridges and the movable ring portion and with a plurality of electrodes, is coupled to at least the secondary electrical insulator. At least an electromigration preventive member with a groove pattern corresponding to the flexible bridges and the movable ring portion, is disposed on at least the electrode member. A driving member operably connected to at least the linear vibration resistant resonator. The present invention also
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provides a process for the fabrication of the micro-scale actuator from a single silicon on insulator (SOI) wafer.
Brief description of the drawings
[012] FIG.1 is an exemplary perspective view of micro-
scale actuator of the present invention.
[013] FIG.2 is an exploded view of the micro-scale
actuator as shown in FIG.1.
[014] FIG.3(a) and FIG.3(b) are partial cut views of
micro-scale actuator of the present invention, where thickness of the movable flexible bridges is equal the thickness of the ring portion.
[015] FIG.4(a) and FIG.4(b)are partial cut views of
micro-scale actuator of the present invention, where thickness of
the movable flexible bridges is less than the thickness of the ring
portion.
[016] FIG.5(a) and FIG.5(b)are partial cut views of
micro-scale actuator of the present invention, where thickness of
the movable flexible bridges is more than the thickness of the
ring portion.
[017] FIG.6(a) is a perspective view of the electrode
member of the micro-scale actuator of the present invention and
FIG.6(b) is an enlarged partial view of the electrode member.
[018] FIG.7 is a cross-sectional view of the micro-scale
actuator of the present invention.
[019] FIG.8 is an isometric view of the micro-scale
actuator of the present invention with a magnet assembly as a
driving member.
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[020] FIG.9(a) and (b) is a side and cross-sectional views
of the micro-scale actuator of the present invention with a
magnet assembly as a driving member.
[021] FIG.10 is a schematic expression of amplitude of
vibrations of the micro-scale actuator of the present invention.
[022] FIG.11 is a schematic expression of amplitude of
vibrations of the micro-scale actuator of the present invention as
used in a gyroscopic application.
[023] FIG.12 is an exploded view of the micro-scale
actuator of the present invention with a plurality of secondary
electrical insulators and electromigration preventive members.
[024] FIG.13 is an exploded view of the micro-scale
actuator of the present invention with a piezo electric driving member.
[025] FIG.14 a broad schematic depiction of the process
steps for the fabrication of the micro-scale actuator of the present invention.
[026] FIG.15 a broad schematic depiction of the process
steps for the fabrication of the driving assembly for the micro-scale actuator of the present invention.
[027] FIG.16 is an exemplary resonance measurement
setup with lock-in amplifier along with the micro-scale actuator of the present invention.
[028] FIG.17(a) is an exemplary graphical depiction of
frequency response of the micro-scale actuator of the present invention used for gyroscopic applications.
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[029] FIG.17(b) is an exemplary graphical depiction of
measurement of gyroscope rate of the micro-scale actuator of the present invention when used for gyroscopic applications.
Detailed description of the invention
[030] The embodiments of the invention will now be
described in further detail. It is to be understood that limiting the description to the preferred exemplary of the invention is to only facilitate description of the present invention and it is envisaged that those skilled in the art may incorporate modifications and equivalents, without departing from the scope of the defined claims.
[031] The embodiments of the micro-scale actuator, which
acts as a ring resonator or a gyroscope, will now be described, with reference to FIGs.1-13, either individually or in combination thereof.
[032] FIG.1 illustrates a broad structural elements of a
micro-scale actuator 100 of the present invention that is provided with a substrate 101. The substrate 101 is advantageously a handle silicon layer with a monocrystalline silicon as preferable base material with a desired thickness and size. The thickness of the substrate 101 is preferably in the range of about 100 to 800 µm and the preferred size is in the range of about 1 to 18" (inch) diameter. The substrate 101 of the present invention, is advantageously selected to possess an electrical resistivity this is either high or low and is preferably in the range of 0.001to 100 ohms-cm. The electronic properties of the substrate 101 is modified with the addition of a suitable dopant of the type n-type or p-type. Exemplary p-type dopants
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include boron, aluminium, nitrogen, gallium, and indium and n-type dopants include phosphorus, arsenic, antimony, bismuth, and lithium. The substrate 101 is provided with different crystallographic substrate orientations, such as (100), (110) and (111). The substrate 101 of the present invention is arranged as a support and opening in it acts as an amplitude enhancer by decreasing air damping effects for the micro-scale actuator 100. The shape of the substrate 101 is exemplarily shown as non-circular in FIG.1. It is understood here that other configurations such as circular can be suitably adapted for use. The substrate
101 is selectively etched with a hollow pattern and the opening
is concentrically aligned with linear vibration resistant resonator
103, to release the vibration resistant resonator 103, provided
an opening as illustratively shown in FIG.2.
[033] A primary electrical insulator 102 with the
corresponding hollow pattern of the substrate 101, is adhered to the substrate 101, which is preferably made of silicon dioxide material having a thickness in the range of 50 nm to 10µm. The primary electrical insulator 102 is disposed on the substrate 101 to isolate the substrate 101 electrically. The primary electrical insulator 102 also acts as a passivation layer having a material with a higher selectivity so that it acts as an etch stopper and also as a masking layer for a linear vibration resistant resonator as hereinafter described. This arrangement of the primary electrical insulator 102 prevents the etching of the linear vibration resistant resonator. The primary electrical insulator
102 is preferably formed by implantation of oxygen (SIMOX)
process or by a smart cut process. The electrical resistivity of the
primary electrical insulator 102 is about 1017ohms-cm. The
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primary electrical insulator 102 is also reciprocally etched to provide a reciprocal opening as that of the substrate 101.
[034] A linear vibration resistant resonator 103 is adhered
to the primary electrical insulator 102 as shown in FIGs.1 and 2. The linear vibration resistant resonator 103 is configured to be of same shape as the substrate 101 and the primary electrical insulator 102 and is provided with a central opening that is co-axial to the openings of the substrate 101 and the primary electrical insulator 102. The linear vibration resistant resonator 103 is advantageously made of monocrystalline silicon that is disposed. An anchor portion 103a of the linear vibration resistant resonator 103, which is movable, acts as a supporting member for adhering to the primary electrical insulator 102. The thickness of the linear vibration resistant resonator 103 is preferably in the range of 50 to 100 µm and the size in the range of from 1 to 18" (inch) diameter. The electrical resistivity of the linear vibration resistant resonator 103 is maintained at a higher or lower levels considering the incorporation of secondary electrical insulator as hereinafter described, which is preferably greater than 100 ohms-cm.
[035] The anchor portion 103a is also defined by an
opening that is reciprocal and co-axial to the openings of the substrate 101 and the primary electrical insulator 102. A plurality of movable flexible bridges 103b with their one ends are integrally connected to the inner periphery of the opening of the linear vibration resistant resonator 103 and with their other ends are integrally connected to the movable ring portion 103c as particularly shown in FIGs.1 and 2. In this arrangement the anchor portion 103a is stationary and whereas the plurality of
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movable flexible bridges 103b and the ring portion 103c are movable in x-y plane as particularly shown in FIG.3. The reduction in the linear vibration of the linear vibration resistant resonator 103 enhances signal to noise ratio, by not responding to undesired vibrations, thus provides better amplitude ratio for given input energy to the actuator.
[036] In an aspect of the present invention, the micro-
scale actuator 100 as shown in FIG.3(a) and (b), is provided with movable flexible bridges 103b and the ring portion 103c having substantially same thickness. Therefore, the thickness T(flexible bridge) is equal to T(ring portion), as illustrated in FIGs.3(a) and 3(b). Preference to equal thickness of movable flexible bridges 103b and the ring portion 103c eases the fabrication of the micro-scale actuator and helps in achieving moderate operating frequencies.
[037] In another aspect of the present invention, the
micro-scale actuator 100 as shown in FIG.4(a) and FIG.4(b),
is provided with the movable flexible bridges 103b and the ring
portion 103c, where the thickness of the movable flexible
bridges 103b is less than the thickness of the ring portion 103c.
This arrangement of the micro-scale actuator 100 assists in
achieving higher frequencies with lesser amplitudes.
[038] In another aspect of the present invention, the
micro-scale actuator 100 as shown in FIG.5(a) and FIG.5(b), is provided with the movable flexible bridges 103b and the ring portion 103c where the thickness of the movable flexible bridges 103b is greater than the thickness of the ring portion 103c. In this advantageous arrangement, the micro-scale actuator 100
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renders lower operating frequencies with higher amplitudes,
thereby enabling an enhanced performance.
[039] A secondary electrical insulator 104, which also acts
as a secondary passivation layer, is preferably formed from silicon dioxide or silicon nitride and is adhered to the linear vibration resistant resonator 103, as shown in FIG.2. The thickness of the secondary insulator 104 isin the range of 50 to 10 µm. The secondary electrical insulator 104 is disposed between the electrode member 105 and the linear vibration resistant resonator 103 to form electrical isolation. The arrangement of the secondary electrical insulator 104 prevents any electrical short circuits among the electrical connections of electrode member 105. The thickness of the secondary electrical insulator 104 determines the electrical breakdown voltage, since with an increase in the thickness the breakdown voltage also increases. The enhanced thickness also decreases the leakage current or noise level in device operation. The secondary electrical insulator 104 can be formed by various techniques such as deposition and growth process. Exemplary deposition methods are, sputtering, chemical vapour deposition, electron beam evaporation, and Plasma enhanced chemical vapour deposition. Exemplary growth methods are wet oxidation and dry oxidation. The electrical resistivity of the secondary electrical insulator 104 is preferably about 1017 ohms-cm. The secondary electrical insulator 104 can also be in the form of multiple layers formed with different electrical insulation materials. Accordingly, the secondary electrical insulator 104 is provided with a groove pattern corresponding to the flexible bridges 103b and the movable ring portion 103c.
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[040] FIG.6(a) and 6(b) show an arrangement of the
electrode member 105 over the ring portion as well as the
stationary anchor portion 103a. Electrode lines 105a formed on
the ring portion are utilized for actuating the linear vibration
resistant resonator 103 and some of the electrode lines 105a
are used for measuring electrical signals arising from the
movement of the linear vibration resistant resonator 103 under
the influence of external stimuli, such as acceleration or angular
rotation. Electrode ports 105b are formed on the stationary
anchor portion 103a to access all electrical connections formed
in electrode member 105 for electrical connectivity.
[041] Electrical connections for the micro-scale actuator
100 are drawn from the electrode member 105. Electrical conducting openings or points 107 in the electromigration preventive member 106 provides access to the bond pads in the electrode member 105. For electrical connections, preferably, gold or aluminium wires are used for wire bonding. An alternative voltage source is connected to the electrodes to actuate the linear vibration resistant resonator 103. The alternative voltages can be preferably in the range 1 Vac to 5 Vac amplitudes.
[042] The preferred embodiments of the electrode member
105 of the micro-scale actuator of the present invention are now described by particularly referring to FIGs.1 and 2. The electrode member 105 is formed by an electrical conductive material, such as aluminum or gold with a thickness in the range of 0.5 to 2 µm. The electrode member 105 is disposed in between the secondary electrical insulator 104 and an electromigration preventive member 106. The thickness of the
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electrode member 105 defines the electrical resistance of the
electrical connections on the device, as the thickness of the
electrode member 105 is increased the corresponding electrical
resistance decreases. The electrode member 105 is formed by
sputtering and evaporation methods. The electrode arrangement
of the micro-scale actuator 100 of the present invention further
includes electrode lines 105a and electrode ports 105b. Part of
electrode lines 105a are separated by an electrical grounding, to
enhance the electrical signal readout by suppressing the signal
interference or crosstalk between input and output electrical
signals. The electrode ports 105b are arranged on the stationary
anchor portion 103a of the micro-scale actuator 100. The
electrode lines 105a on the movable ring portion are connected
to the electrode ports 105b running over movable flexible
bridges 103b. The electrode arrangement of the electrode
member 105 provides an electrical connectivity to the linear
vibration resistant resonator 103. Accordingly, the electrode
member 105 is provided with a groove pattern corresponding to
the flexible bridges 103b and the movable ring portion 103c.
[043] As shown in FIGs.1 and 2, the electromigration
preventive member 106 is formed from an electrically insulating material, such as silicon dioxide or silicon nitride having a thickness in the range of 2 to 4 µm. The electromigration preventive member 106 is disposed on top of the electrode member 105. This arrangement of the electromigration preventive member 106 prevents the electrical short circuits among electrical connections of the electrode member 105 and the electromigration preventive member 106, during device operation. Thickness of the electromigration preventive member
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106 is determined by the thickness of the electrode member
105. Accordingly, the thickness of the electromigration
preventive member 106 is preferably at least equal to the
thickness of the electrode member 105 so as to render a
conformal protective profile to the electrode member 105. The
electromigration preventive member 106 facilitates decrease in
the leakage of current and electrical interference between the
electrical connections during the device operation. The
electromigration preventive member 106 is formed by processes
such as deposition or growth processes. Exemplary deposition
methods include sputtering, chemical vapour deposition, electron
beam evaporation and plasma enhanced chemical vapour
deposition. Exemplary growth methods for the electromigration
preventive member 106 include wet and dry oxidation. The
electrical resistivity for the electromigration preventive member
106 is preferably about 1017 ohms-cm. The electromigration
preventive member 106 is provided with openings 108 to
expose the electrical terminals of the electrode member 105.
[044] The conformal profile of the electromigration
preventive member 106 over the electrode member 105 is as shown in FIG.7. The conformal profile of the electromigration preventive member 106 provides coverage of all the sides of electrode lines of the electrode member 105 and prevents physical damage of the electrode lines due to electromigration phenomenon. The conformal feature of the electromigration preventive member 106 not only physically constraints the electrode member 105 from any movement caused by electromigration at high current densities but also maintains an electrical isolation between electrode lines 105a.
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[045] The electromigration preventive member 106 is
judiciously placed over the electrode member and includes dedicated openings 107 for connecting electrical means. The electromigration preventive member 106 allows high current through narrow electrical lines, by physically constraining the flow of electrodes and hence higher amplitudes can be achieved thus enhances performance of actuator/resonator. The inner boundaries of the dedicated openings 107 are smaller than the outer boundaries of the electrodes. Accordingly, the electromigration preventive member 106 is provided with a groove pattern corresponding to the flexible bridges 103b and the movable ring portion 103c.
[046] In yet another aspect of the present invention as
shown in FIG.8 and FIG.9, the micro-scale actuator 100 of the present invention is provided with a driving member 108. In this exemplary aspect, a magnetic assembly is adapted for use as the driving member 108 and the magnetic assembly includes a brass carrier, a bottom and top pole of the magnet. The linear vibration resistant resonator 103 is disposed to overlay the bottom pole of the magnet. With precision assembly, the magnet is mounted inside the ring portion 103c of the linear vibration resistant resonator 103 and connected to the bottom pole. The top pole with cavity or groove is mounted co-axially on the magnet to cover the ring portion of linear vibration resistant resonator 103. In this arrangement, the magnetic field is formed between the top and bottom pole. The magnetic field is essentially normal to the plane of electrode member 105. When an oscillating voltage is supplied through the electrodes lines
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105a on ring portion 103c forms a magnetic field around the electrical lines 105a.
[047] As particularly shown in FIG.9(a) and (b) a brass
carrier supports the magnet 108 at a bottom pole 108a of the
micro-scale actuator 100. The bottom pole 108a is a nickel-iron
alloy INVAR material, which is a magnetic material and its
properties do not change with the change in temperature. The
bottom pole 108a extends the pole of magnet 108, which is
larger than the linear vibration resistant resonator 103. The
magnet 108 is placed co-axially on bottom pole and it is
Samarium cobalt–rare earth magnet. The micro-scale actuator
100 is coaxially mounted on the brass carrier. A top pole cap
108b is made of INVAR material is placed over the magnet 108
to extend the magnetic pole to cover the movable portion of the
linear vibration resistant resonator 103. The arrangement of top,
bottom poles and ring portion is coaxial so as to ensure that the
ring portion 103c is placed within the magnetic field. The
closeness of the top pole to the ring portion 103c enhances the
amplitude of oscillations by increased Lorentz forces. In this
embodiment, the magnet can be of any shape, while maintaining
a uniform shapes of the top and bottom poles same as the ring
portion 103c of the linear vibration resistant resonator 103 with
larger boundary ensures magnetic field coverage over ring
portion 103c of the linear vibration resistant resonator 103.
[048] In a further aspect of the present invention, the
variable thickness and width of the movable flexible bridges 103b and the ring portion 103c along with the stationary anchor portion 103a imparts a resistance to the movement to the linear vibration resistant resonator 103, along the z-axis of the micro-
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scale actuator 100 and permits the movement only along the x-y plane in order to enhance the vibration sensitivity of the linear vibration resistant resonator 103. The thickness and size of the movable flexible bridges 103b and the size of the ring portion 103c are the critical design parameters of micro-actuator, and combination of these parameters determines the operating frequency at desired deformation shape (mode shape). The micro–scale actuator 100 is preferred to operate in deformation state, where the ring portion 103c deforms, preferably in elliptical shapes, as shown in FIG.10 and FIG.11. In this particular embodiment the shape of the linear vibration resistant resonator 103 achieves a perfect symmetry shape, by which the micro-actuator 100 maintains mechanical balancing of forces, while avoiding the effects of working temperatures on micro-scale actuator 100.
[049] The effect of variation in thickness of the movable
flexible bridges and the ring portion of the micro-scale actuator on the amplitude of vibrations is illustrated in FIG.10. The elliptical shapes represent amplitude of vibrations of the linear vibration resistant resonator with the movable flexible bridges and the ring portion. The amplitude (a) of vibrations of the linear vibration resistant resonator 103, where the thickness of the movable flexible bridges 103b is greater than thickness of the ring portion 103c i.e., T(Bridge) > T(ring), the amplitude of vibration of the micro-scale actuator 100 is enhanced substantially. Whereas, in case of the micro-scale actuator 100 with thickness parameters viz., T(Bridge) = T(ring) and T(Bridge) < T(ring), the reduction in the amplitude (a) of vibrations is observed. Therefore, by varying the thickness and
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width of the movable flexible bridges 103b and the ring portion
103c of the linear vibration resistant resonator 103 of the
present invention, the variation in the frequency of mode
operation can be achieved. By maintaining the same mode shape
of oscillations, its operational frequency is achieved at lower
values. Particularly, at lower frequency operation, the maximum
input energy is utilized for actuation. Hence, by variable
thickness and width of the ring and movable flexible bridges an
efficient micro-scale actuator 100 is produced.
[050] The presence of the effect of variable thickness and
width of the ring portion 103c and movable flexible bridges 103b provide for lower operating frequencies. The operation of the micro-scale actuator 100 at lower operating frequencies results in the gain of actuation amplitude and a resultant radial motion, which is a Coriolis force induced motion, due to applied external rotation rate.
[051] In yet another aspect of the present invention a
gyroscopic application of the micro-scale actuator 100 of the present invention, is illustrated by particularly referring to FIG.11. As shown in FIG.11, variations in the thickness of the movable flexible bridges 103b and the ring portion 103c of the micro-scale actuator influences the amplitude of vibration of the micro-scale actuator 100. Hence, for a gyroscopic application, the micro-scale actuator 100 provides a better sensitivity, where the thickness of the movable flexible bridges 103b is preferably greater than the thickness of the movable ring portion 103c, thereby enabling the operations at lower frequencies. For the gyroscopic application, the micro-scale actuator 100 requires a drive frequency oscillation, which is generally a normal frequency
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oscillation. The micro-scale actuator 100 also requires and a sense frequency oscillation, which is a displacement, caused due to the Coriolis effect. Accordingly, in a gyroscopic application of the micro-scale actuator 100, the interaction between the permanent magnetic field and current-induced magnetic field forms a mechanical force (Lorentz force), resulting in the movement (oscillation) of the linear vibration resistant resonator 103. When an external angular rotation (Ω) is applied to the linear vibration resistant resonator 103, it experiences a Coriolis force (F) in the sense direction, which is orthogonal to the movement of the linear vibration resistant resonator 103. The resultant of the Coriolis force(F), causes motion as shown as “resultant radial motion” in FIG.11. This motion causes movement of the linear vibration resistant resonator 103 thereby resulting in the interruption of magnetic field that induces motional electromagnetic force in corresponding output electrical lines 105a. The induced Lorentz force causes deformations or displacements of ring portion 103c with correlated frequency.
[052] In yet another aspect of the present invention, as
shown in FIG.12, a plurality of secondary electrical insulators 104 and 104a and electromigration preventive members 106 and 106a are provided to the micro-scale actuator 100. The plurality of secondary electrical insulators 104 and 104a offer an enhanced electrical isolation of the electrode lines from the linear vibration resistant resonator 103, resulting in the prevention of leakage electrical inputs and outputs from the micro-scale actuator 100. The electrical insulators 104a ensure filling of voids formed in initial secondary electrical insulator 104 and
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thus creates better electrical isolation at a higher breakdown
voltage. Whereas, advantageous incorporation of the additional
electromigration preventive members 106a and 106b offer an
enhanced conformal profile, particularly whenever the thickness
of the electrode member 105 is much higher. The additional
electromigration preventive members 106a and 106b also
envelope voids that appear during formation of the initial
electromigration preventive layer. This feature also enables the
redundancy of protecting electrode lines from electromigration,
resulting in better reliability of the micro-scale actuator 100. The
feature of plurality of secondary electrical insulators 104, 104a
and the electromigration preventive member 106, adapts the
micro-scale actuator 100 for high current applications by
preventing electrical break down and electromigration.
[053] In further aspect of the present invention as shown
in FIG.13, the driving member 108 is made of a piezo electric material. The exemplary piezo electric materials include lead zirconate titanate (PZT) or aluminum nitride (AlN) or ZnO can be used as piezo electric material. The thickness of the driving member is in the range of 100 nm to 2µm. The driving member 108 is disposed between the electrode members 105a and 105b. The electromigration preventive member 106 with dedicated openings is placed over top electrode member 105b. The dedicate openings are place such way that top electrode member 105b and bottom electrode member 105a are accessible to electrical means. By applying appropriate voltage signals to the electrode members 105a and 105b, the driving member 108 actuates the linear vibration resistant resonator 103. Since, the piezo electric actuation is a reversible action,
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due to the presence of deformation in the linear vibration
resistant resonator 103, under external stimuli, generates the
equivalent potential between electrode members 105a and
105b. The driving member 108 is formed by processes such as
spin coating, sputtering, evaporation or by metal oxide chemical
vapor deposition. The piezoelectric coefficients of the driving
member 108 are preferably D31 and D33 and these coefficients
defines the direction of motion on the application of a voltage
signal. For instance, with a piezoelectric coefficient of D31, when
a voltage is applied along one axis, the displacement or
deformation occurs in 3-axis direction. The values of these
coefficients are in the range of 100pm/V to 500 pm/V. The
piezoelectric coefficients for the driving member 108 are
preferably set at high values. For instance, the micro-actuator
100 as shown in the FIG.13 when used for gyroscopic
applications, the portions of electrode lines with combination of
top electrode member 105b and bottom electrode member
105a are applied with voltage signal to provide piezoelectric
actuation in drive direction. The other portions of electrode lines
with combination of top electrode member 105b and bottom
electrode member 105a read out the output voltage signal,
resulting from the reverse piezo electric effect due to Coriolis
force and the deformation of the movable flexible bridges 103b
and the ring portion 103c, for an applied external rotation rate.
[054] In yet another aspect of the present invention the
thickness of the movable flexible bridges 103b and the ring portion 103c of the micro-scale actuator 100 is varied to study its impact on the oscillations (in-plane oscillations). The
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mechanically simulated results of the micro-scale actuator 100 are tabulated and provided in the following Table 1.
Table 1
FLEXIBLE RING BRIDGE
W

[055] It can be seen from the Table 1 that the variable
thickness the movable flexible bridges 103b and the ring portion 103c plays a role in increasing or decreasing the sensitivity of the micro-scale actuator 100. Where, the thickness of the movable flexible bridges 103b and the ring portion 103c is same, the recorded oscillation frequency is 14.532 kHz and where the thickness of the movable flexible bridges 103b is higher than the thickness of the ring portion 103c, it is observed that sensitivity of micro-scale actuator 100 is increased due to the reduction in the frequency of interest i.e., 14.379 kHz. Further, whenever the thickness of the ring portion 103c is higher than the thickness of the movable flexible bridges 103b, a decrease in the sensitivity of micro-scale actuator 100 is observed due to the increase in frequency of interest (14.550
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kHz). Similarly, in case of variable widths, as the width of the movable flexible bridges 103b and ring portion 103c is reduced the frequency of interest decreases thereby increasing the sensitivity of the actuator. As shown in Table 1, when the widths of movable flexible bridges and the ring portion are 60,220 microns the recorded frequency is 26.135 kHz and whereas, whenever the widths are varied to 40, 120 microns, respectively, the recorded frequency is reduced to 13.881 kHz, thereby establishing an increase in the sensitivity of the micro-scale actuator 100.
[056] In a similar way, the effect of thickness of the of the
movable flexible bridges 103b and the ring portion 103c of the micro-scale actuator 100 on the out-of-plane oscillation frequencies, is also studied (mechanical simulation studies )and the results are tabulated in Table 2.
Table 2

Thickness of movable flexible bridges & ring portion of the micro-scale actuator Out-of-plane frequencies
Thickness 60 (µm) 4390 Hz
Thickness 100 (µm) 5857 Hz
Thickness 150 (µm) 6883 Hz
[057] It can seen from the Table 2, that the out-of-plane
mode frequency of the micro-scale actuator 100 increases as the thickness of the movable flexible bridges 103b and the ring portion 103c is increased. Therefore, higher frequencies are
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preferred to minimize the influence of linear vibrations of the linear vibration resistant resonator 103 in out-of-plane directions. For instance, out-of-plane oscillation frequency of 4390 Hz at 60 µm thickness is comparatively lesser than and the out-of-plane oscillation frequency is 6883 Hz at 150 µm thickness, thereby highlighting the preference of a higher thickness the movable flexible bridges 103b and the ring portion 103c of the micro-scale actuator 100.
[058] Therefore, with the linear vibration resistant
resonator 103 having movable flexible bridges 103b, the ring
portion 103c and the stationary anchor portion 103a, the
micro-scale actuator 100 renders an improved vibration
sensitivity ratio and also suitable for high current applications.
[059] Accordingly, the micro-scale actuator 100 of the
present invention comprises, a primary electrical insulator 102 with an opening is disposed on a substrate 101. At least a linear vibration resistant resonator 103 with movable flexible bridges 103b that are radially connected to a movable ring portion 103c and to a stationary anchor portion 103a is disposed on the primary electrical insulator 102. The linear vibration resistant resonator 103 is disposed in between the primary electrical insulator 102 and at least a secondary electrical insulator 104. At least an electrode member 105 with a groove pattern corresponding to the flexible bridges 103b and the movable ring portion 103c and with a plurality of electrodes 105a, is coupled to at least the secondary electrical insulator 104. At least an electromigration preventive member 106 with a groove pattern corresponding to the flexible bridges 103b and the movable ring portion 103c, is disposed on at least the electrode member 105;
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and a driving member 108 operably connected to at least the
linear vibration resistant resonator 103.
[060] In yet another aspect of the present invention, a
plurality of linear vibration resistant resonators, secondary
electrical insulators, electrode members and electromigration
preventive members are disposed on the substrate 101.
[061] In a further aspect of the present invention, the
thickness of the movable flexible bridges 103b is equal to,
greater or less than the thickness of the ring portion 103c.
[062] In another aspect of the present invention, the
thickness of the movable flexible bridges 103b and the ring
portion 103c is greater than the width of the movable flexible
bridges 103b and the ring portion 103c.
[063] In yet another aspect of the present invention, the
linear vibration resistant resonator 103, the secondary electrical
insulator 104, the electrode member 105 and the
electromigration preventive member 106 are coaxial.
[064] In a further aspect of the present invention, the
driving member 108 is a magnet or a piezo-electric device.
[065] It is also an aspect of the present invention wherein
a plurality of electrical conducting points 107 are disposed on
the electromigration preventive layer 106.
[066] In yet another aspect of the present invention, the
inner boundaries of the electrical conducting points 107 are
smaller than outer boundaries of the electrodes 105a.
[067] It is also an aspect of the present invention wherein
the arrangement of the electromigration preventive layer 106 on
the plurality of electrodes 105a is conformal.
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[068] In yet another aspect of the present invention, the
process steps for the fabrication of the micro-scale actuator 100 of the present invention are now described. In the instant process the fabrication of the micro-scale actuator is performed by using single silicon on insulator wafer (SOI wafer) as basis material for substrate involving preferably four masking stages, where the preferred masks are an electrode mask, a structure mask, an opening mask and a dimple mask. This process flow includes a selective etching of the substrate to form the linear vibration resistant resonator with movable flexible bridges and the movable ring portion. The primary electrical insulator acts as an etch stopper in the preferred dry etching of the substrate. In this process the linear vibration resistant resonator and these elements are preferably not doped.
[069] Initially, the SOI wafer as shown in FIG.14(a)
includes a primary electrical insulator of SiO2 arranged in between the device silicon and the substrate. The SOI wafer is subjected to cleaning, in a known manner, in order remove organic and inorganic impurities from the SOI wafer. The cleaned SOI wafer is placed in a plasma-enhanced chemical vapour deposition (PECVD) chamber at a temperature in the range of 100-3500C and under the pressure in the range of 900-1000 mTorr. Silane, nitrous oxide and nitrogen gases are permitted into the PECVD chamber to form SiO2 on the SOI wafer and to obtain the product as shown in FIG.14(b), where the SOI wafer is deposited with a secondary electrical insulator on the top of substrate. The secondary electrical insulator is deposited with PECVD method or is grown by a thermal oxidation process. The secondary electrical insulator is used as a passivation layer. The
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surface of the secondary electrical insulator is cleaned in the presence of an acid such as sulphuric acid and a solvent such as acetone and isopropyl alcohol, to remove the floating residue particles.
[070] An electrode member is formed preferably of
aluminum, on the secondary electrical insulator as shown in
FIG.14(c). Alternate materials such as gold, copper, titanium,
platinum can also suitably used for the formation of the electrode
member. This electrode member is preferably non-magnetic
material. The electrode member is deposited by sputtering or
through an electron-beam evaporation method.
[071] In a further step, a required pattern of electrodes is
formed initially by photolithography and the formed pattern is etched by a dry etching process on the secondary electrical insulator, where the pattern is a combination of electrode ports and lines, as shown in FIG.14(d).
[072] The electrode member thus formed is etched
selectively to form the electrode tracks and a central opening as shown in FIG.14(e). In this step, a dry selective etching of the electrode member is performed in an anisotropic manner, in the presence of hydro bromide (HBr) (10 standard cubic centimetre per minute (sccm) and Cl2 (40sccm). The etching is performed with an inductively coupled plasma (ICP) (800W) at a radio frequency of 250W, a temperature of about 60oC and under a chamber pressure 5 mTorr. Alternately, the electrode layer can also be etched using a wet chemical process for an isotropic etch process in order to reduce the width of the electrical tracks of the electrode member.
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[073] Thereafter, the electromigration preventive layer
with openings, is deposited on the electrode member, through
PECVD (Plasma enhanced chemical vapour deposition) method.
The electromigration preventive layer assists in prevention of
electro-migration of electrode tracks during the operation of the
actuator, as shown in FIG.14(f). The electromigration
preventive layer is deposited in a conformal manner so that the
electrode layer is covered completely and electrode lines are not
exposed. However, the electrode ports are exposed to the
openings of the he electromigration preventive layer, where the
sizes of the openings are lesser than the size of electrode ports.
[074] A mask layer of SiO2 is deposited on the SOI wafer
for the dry etching of the electromigration preventive layer, as shown in FIG.14(g).
[075] The selective etching of the mask layer is performed
to form a structure mask, as shown in FIG.14(h), which is the linear vibration resistant resonator with movable flexible bridges that are radially connected to the movable ring portion and the stationary anchor portion. The mask layer is preferably etched using a dry etch process in an inductively coupled plasma (ICP) (1500W) and in the presence of fluoroform (CHF3) flow=40sccm, RF-50W and at 0oC temperature.
[076] The etched SOI wafer is flipped or turned over and
deposited with a silicon dioxide mask layer on the bottom surface of the substrate, as shown in FIG.14(i). This mask layer is deposited with PECVD method or grown by thermal oxidation process. This layer is used as a masking layer for dry etching of the substrate. The thickness of mask layer is determined by the
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selectivity between the substrate and the silicon dioxide, in a deep reactive ion etching process.
[077] A selective etching of the mask layer is performed to
form a dimple mask pattern, as shown in FIG.14(j). The etching
is performed by hydrofluoric acid (wet chemical etching) or a
reactive ion etching (dry etching). The wet chemical etching is
isotropic in nature leading to an extended/reduction in structural
features, whereas with etching method it is possible to achieve
greater anisotropic features. Subsequent to the etching of the
mask layer the SOI wafer is subjected to dicing operation.
[078] A cavity is formed in substrate as shown
FIG.14(k),by a deep reactive ion etching (DRIE), in order to form high aspect ratio structures with greater depth that are anisotropic in nature. Cavity features in the mask layer are transferred on to the substrate by etching silicon using DRIE process. Etching of substrate continues it reaches primary electrical insulator. The primary electrical insulator primary insulator acts as an etch stop in DRIE process. The depth of the etching and time of etching can be minimized by choosing a substrate with a lesser thickness.
[079] The SOI wafer is flipped again to expose the
structural pattern and the pattern is etched using DRIE process as shown in FIG.14(l). Etching of linear vibration resistant resonator is continued till it reaches the primary electrical insulator. The primary electrical insulator acts as an etch stopper in DRIE of the silicon etching process.
[080] As shown in FIG.14(m) and (n), the primary
insulator, and the mask layers are etched using dry etching process in an inductively coupled plasma (ICP) (1500W) and in
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the presence of fluoroform (CHF3) flow=40sccm, RF-50W and at
a temperature 0oC temperature. The etching process is
controlled with time of etching based on the etch rate. As shown
in FIG.14(m), the linear vibration resistant resonator forms the
functional elements by deep reactive ion etching (DRIE). This
process forms anisotropic high aspect ratio structures. The
functional elements in the mask layer are transferred on to
substrate by etching the Si using DRIE process. Etching of
substrate continues till reaching primary electrical insulator. The
primary electrical insulator acts as a etch stop in DRIE of silicon
process. The depth of the etching and time of etching can be
minimized by choosing substrate of lesser thickness.
[081] In yet another aspect of the present invention, the
embodiments pertaining to the assembly of the exemplary driving member 108 on the micro-scale actuator 100 of the present invention are now described by referring to FIG.15(a-f), where FIG.15(a) is a brass carrier with cavity and FIG.15(b) is an Invar disk of nickel-iron alloy, which is mounted as bottom pole, FIG.15(c) is a mount of a magnet, FIG.15(d) is the linear vibration resistant resonator 103, FIG.15(e) is a coaxial mount of the linear vibration resistant resonator 103 on the brass carrier and FIG.15(f) is a mount of an Invar cap for extending top pole of magnet. A brass carrier with a cavity is selected for the support of entire assembly and the bottom pole as shown in FIG.15(a). An Invar disk is used as a bottom pole and is mounted in the cavity of the brass carrier as shown in FIG.15(b). A samarium–cobalt (SmCo) magnet, which is a type of rare earth strong permanent magnet, is mounted on bottom pole. The micro machinedlinear vibration resistant resonator 103
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is mounted on brass carrier and arranged coaxial to the magnet
as shown in FIG.15(e). Finally, a Invar top pole is mounted
over the magnet, extending top side of magnetic pole over the
ring portion of the linear vibration resistant resonator 103. The
magnetic poles are extended to cover the electrode member for
magnetic actuation and inductive sense. The gap between the
top pole cap and the surface of the linear vibration resistant
resonator 103 is a key parameter for Lorentz force and motional
electromotive force. Electrical connections are formed using wire
bonding from the micro-actuator 100 to the electrical means.
[082] Accordingly, the process for the fabrication of micro-
scale actuator 100, comprises the steps of selecting a SOI wafer
with a substrate, a primary insulator and a device silicon,
followed by depositing of at least a secondary electrical insulator
on the SOI wafer. An electrode member with a plurality of
electrodes, electrode ports and electrode lines is deposited on
the secondary electrical insulator. At least an electromigration
preventive layer with a plurality of electrical conducting points is
deposited on the electrode member. A linear vibration resistant
resonator is formed by etching the substrate till the primary
electrical insulator to make the linear vibration resistant
resonator hollow. Finally, etching the device silicon, the primary
insulator, the secondary insulator and the electrode member to
obtain the linear vibration resistant resonator with movable
flexible bridges, a movable ring portion and a stationary anchor
portion and to form groove patterns corresponding to the
movable flexible bridges and the ring portion.
[083] The process steps of the present invention can be
suitably adapted for depositing a plurality of linear vibration
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resistant resonators, secondary electrical insulators, electrode
members and electromigration preventive members on the SOI
wafer.
[084] In the process steps of the present invention the
formation of the electromigration preventive member is
conformal.
[085] The functional aspects of the micro-scale actuator of
present invention are now illustrated with a non-limiting
example.
Example 1
[086] The micro-scale actuator of the present invention,
while used for gyroscope applications, is subjected to rate measurements. Initially, resonant frequencies of the micro-scale actuator are measured by an lock-in amplifier as shown in FIG.16, which is also used to actuate the actuator. In a frequency sweep mode, a span of frequencies with a voltage, actuate the linear vibration resistant resonator, as result of interaction between induced magnetic field around electrical lines on the ring portion and the permanent magnetic field, thereby causing elliptical motion. The frequency response of the micro-scale actuator is as shown in FIG.17(a), which are peak frequencies at 11.9 kHz and 12.6 kHz, under atmospheric conditions, which are within permissible fabrication tolerances. With the similar electrical connections as shown in FIG.16, where the connections 14-15 and 6-7 are used for actuation terminals and the connections 4-5 and 12-13 are used as sense terminals. A harmonically actuated ring structure at 11.9 kHz in elliptical mode, when an external rotation rate is applied, the micro-scale actuator experiences Coriolis force orthogonal to
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drive direction and rotation axis. The resultant force deforms the ring portion along sense axis causing oscillations as shown in FIG.11. Sense oscillations are amplitude modulated signal with drive frequency as a carrier frequency and rate as an envelope. The lock-in amplifier demodulates sense oscillations at drive frequency and provides rate information of gyroscope. FIG.17(b) shows gyroscope rate measurements at rate frequency of 0.3 Hz preliminary rate measurements with rate sensitivity of 0.04 µV/deg/s at atmospheric conditions with a rate oscillation frequency of 0.3 Hz.
[087] It is also understood that the following claims are
intended to cover all the generic and specific features of the invention herein described and all statements of the scope of the invention, which as a matter of language might be said to fall there between.
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We claim:
1. A micro-scale actuator 100, comprising:
- a primary electrical insulator 102 with an opening is disposed on a substrate 101;
- at least a linear vibration resistant resonator 103 with movable flexible bridges 103b that are radially connected to a movable ring portion 103c and to a stationary anchor portion 103a, is disposed on the primary electrical insulator 102;
- the linear vibration resistant resonator 103 is disposed in between the primary electrical insulator 102 and at least a secondary electrical insulator 104;
- at least an electrode member 105 with a groove pattern corresponding to the flexible bridges 103b and the movable ring portion 103c and with a plurality of electrodes 105a, is disposed on at least the secondary electrical insulator 104;
- at least an electromigration preventive member 106 with a groove pattern corresponding to the flexible bridges 103b and the movable ring portion 103c, is disposed on at least the electrode member 105; and
- a driving member 108 operably coupled to at least the linear vibration resistant resonator 103.
2. The micro-scale actuator 100 as claimed in claim 1, wherein
a plurality of linear vibration resistant resonators, secondary
electrical insulators, electrode members and
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electromigration preventive members are disposed on the substrate 101.
3. The micro-scale actuator as claimed in claim 1, wherein the thickness of the movable flexible bridges 103b is equal to, greater or less than the thickness of the ring portion 103c.
4. The micro-scale actuator as claimed in claim 1, wherein the thickness of the movable flexible bridges 103b and the ring portion 103c is greater than the width of the movable flexible bridges 103b and the ring portion 103c.
5. The micro-scale actuator as claimed in claim 1, wherein the linear vibration resistant resonator 103, the secondary electrical insulator 104, the electrode member 105 and the electromigration preventive member 106 are coaxial.
6. The micro-scale actuator as claimed in claim 1, wherein the driving member 108 is a magnet or a piezo-electric device.
7. The micro-scale actuator as claimed in claim 1, wherein a plurality of electrical conducting points 107 are disposed on the electromigration preventive layer 106.
8. The micro-scale actuator as claimed in claim 1, wherein inner boundaries of the electrical conducting points 107 are smaller than outer boundaries of the electrodes 105a.
9. The micro-scale actuator as claimed in claim 1, wherein the arrangement of the electromigration preventive layer 106 on the plurality of electrodes 105a is conformal.
10. The process for the fabrication of micro-scale actuator 100, comprising the steps of:
- selecting a SOI wafer with a substrate, a primary insulator and a device silicon;
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- depositing at least a secondary electrical insulator on the SOI wafer;
- depositing an electrode member with a plurality of electrodes, electrode ports and electrode lines, on the secondary electrical insulator;
- forming at least an electromigration preventive layer with a plurality of electrical conducting points on the electrode member;
- forming a linear vibration resistant resonator and etching the substrate till the primary electrical insulator to make the linear vibration resistant resonator hollow; and
- etching the device silicon, the primary insulator, the secondary insulator and the electrode member to obtain the linear vibration resistant resonator with movable flexible bridges, a movable ring portion and a stationary anchor portion and to form groove patterns corresponding to the movable flexible bridges and the ring portion.

11. The process as claimed in claim 10, wherein a plurality of linear vibration resistant resonators, secondary electrical insulators, electrode members and electromigration preventive members are deposited on the SOI wafer.
12. The process as claimed in claim 10, wherein the formation of the electromigration preventive member is conformal.

Dated : 14th day of June 2017.
J SURESH PATENT AGENT FOR THE APPLICANT
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