Patent ID: 12230301

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG.7is a side sectional view of a PZT microactuator assembly114having a constraining layer130bonded thereto in accordance with an embodiment of present invention. In keeping with the orientation shown in the figure, the side of the PZT which is bonded to the suspension will be referred to as the bottom side129of PZT114, and the side of the PZT away from the side at which the PZT is bonded to the suspension will be referred to as the top side127. According to the invention, one or more constraining layers or constraining elements130is bonded to the top side127of microactuator PZT element120. The constraining layer130preferably comprises a stiff and resilient material such as stainless steel and is preferably bonded directly to the top surface127of the PZT element120including its top electrode126, or the SST material may itself serve as the top electrode thus making it unnecessary to separately metalize the top surface. The constraining layer130is stiff enough so as to significantly reduce, eliminate, or even reverse the bending of the PZT when actuated. The constraining layer130preferably has a layer131of gold or other contact metal in order to ensure a good electrical connection to the SST.

Alternatively, instead of the constraining layer130being stainless steel, it could be ceramic such as an unactivated (unpoled, or unpolarized) layer of the same ceramic material as forms the piezoelectric layer120, and could be integrated into the assembly by either bonding or by deposition. The ceramic material is unpolarized meaning that it exhibits substantially less piezoelectric behavior, such as less than 10% as much piezoelectric behavior, as the poled ceramic that defines piezoelectric layer120. Such an assembly, defining a stack consisting from the bottom up of electrode/poled PZT/electrode/unpoled PZT, may be easier to manufacture than a stack of electrode/PZT/electrode/SST.

In the discussion that follows, for simplicity of discussion top and bottom electrodes126,128are sometimes omitted from the figures and from the discussion, it being understood that PZT microactuators will almost always have at least some type of top and bottom electrode.

A layer of copper or nickel may be deposited onto the constraining layer130before gold layer131is applied in order to increase the adhesion of the gold to the SST, as discussed in U.S. Pat. No. 8,395,866 issued to Schreiber et al. which is owned by the assignee of the present application, and which is hereby incorporated by reference for its teaching of electrodepositing other metals onto stainless steel. Similarly, the electrodes126,128may comprise a combination of nickel and/or chromium, and gold (NiCr/Au).

124-167 (FIG.5). In one illustrative embodiment according to a simulation, the thicknesses of the various layers were:

130PZT3 μm126, 128, 131NiCr/Au0.5 μm

The thin film PZT had a length of 1.20 mm, the PZT bonding had a width of 0.15 mm at both ends, and the piezoelectric coefficient d31was 250 pm/V. In some embodiments, the SST layer may be at least 12 micrometers thick in order to provide adequate support.

In the above example the DSA suspension exhibited a stroke sensitivity of 26.1 nm/V according to a simulation. In contrast, a 45 μm thick bulk PZT (d31=320 pm/V) with the same geometry would typically exhibit a stroke sensitivity of only 7.2 nm/V.

The ratio of thicknesses of the SST layer to the PZT layer may be as high as 1:1, or even 1.25:1 or even higher. As the thickness ratio of the constraint layer to the PZT reaches approximately 1:25, the stroke sensitivity improvement due to the constraint layer may start to be negative, indicating the thickness limitation of the PZT constraint layer.

FIG.8Ais a side sectional view of a PZT microactuator114ofFIG.7when a voltage is applied to the PZT so as to expand it. The PZT stroke consists of two vectors, one that is the pure extension stroke δe, the other is the extension contribution δ1due to the constraining layer causing the PZT's right tip to bend upward, instead of bending downward as would be the case without the restraining layer. The total stroke length is δe+δ1. In expansion mode therefore, the PZT assumes a slightly concave shape when viewed from the top, i.e. the PZT top surface assumes a slightly concave shape, which is in a bending direction that is the opposite from the bending of the prior art PZT ofFIG.4. That bending according to the invention therefore adds to the effective stroke length rather than subtracting from it.

FIG.8Bis a side sectional view of a PZT microactuator ofFIG.7when a voltage is applied to the PZT114so as to contract it. The PZT stroke consists of two vectors, one that is the pure contraction stroke −|δc|, the other is the contraction contribution δ2due to the constraining layer causing the PZT's right tip to bend downward, instead of bending upward as would be the case without the restraining layer. The total stroke length is −[δc+δ2]. In contraction mode therefore, the PZT assumes a slightly convex shape when viewed from the top, i.e., the PZT top surface assumes a slightly convex shape, which is in a bending direction that is the opposite from the bending of the prior art PZT ofFIG.4. That bending according to the invention therefore adds to the effective stroke length rather than subtracting from it.

Adding the constraining layer130to a PZT microactuator114has no appreciate affect on the stroke length for the otherwise unrestrained and unbonded PZT114. When that PZT114is bonded to a suspension18at its bottom ends such as shown inFIG.4, however, the effect of the constraining layer is actually to slightly increase the stroke length. Stainless steel has a Young's modulus of around 190-210 GPa. Preferably the material for the constraining layer has a Young's modulus of greater than 50 GPa, and more preferably greater than 100 GPa, and more preferably still greater than 150 GPa.

FIG.9is a graph of stroke length per unit input voltage in units of nm/V verses constraining layer thickness, for a PZT114that is 130 μm thick and has a constraining layer130of stainless steel bonded thereto, according to a simulation. Adding an SST restraining layer of 20 μm, 40 μm, and 60 μm thick onto the PZT top surface each result in an increased total stroke length. Adding a constraining layer therefore actually increased the total stroke length.

One could also hold constant the total combined thickness of the PZT and the constraining layer, and determine an optimal thickness for the constraining layer.FIG.10is a side elevation view of a combined PZT and constraining layer bonded thereto according to the invention, in which the total thickness is kept constant at 130 μm.FIG.11is a graph of stroke length vs. PZT thickness for the PZT ofFIG.10where the combined thickness of the PZT and the restraining layer is kept constant at 130 μm according to a simulation. With no constraining layer, the 130 μm thick PZT has a stroke length of approximately 14.5 nm/V. With a constraining layer130thickness of 65 μm and a PZT thickness of 65 μm, the PZT has a stroke length of approximately 20 nm/V. Adding the constraining layer therefore actually increased the effective stroke length by approximately 35%.

FIG.12is a graph of GDA stroke sensitivity versus constraining layer thickness for a GDA suspension having the microactuator ofFIG.7for a PZT element that is 45 μm thick and a stainless steel constraining layer on top of varying thicknesses, according to a simulation. As seen in the graph, a constraining layer that is 30 μm thick increased the GDA stroke sensitivity from 9 nm/V to slightly more than 14.5 μm, which represents an increase in stroke length of greater than 50%.

FIGS.13(a)-13(h)illustrate one manufacturing process by which a PZT microactuator assembly having a constraining layer according to the invention can be produced. This method is an example of an additive method in which the PZT material is deposited onto a substrate that will be the constraint layer. The process begins with a first substrate140as shown inFIG.13(a). InFIG.13(b)a first UV/thermal tape142is applied to the substrate. InFIG.13(c)a pre-formed constraining layer130is added onto the tape. InFIG.13(d)an electrode layer126is deposited onto the SST such as by sputtering or other well known deposition processes. InFIG.13(e)a PZT layer120is formed on the electrode layer by the sol-gel method or other known methods. InFIG.13(f)a second electrode128is deposited onto the exposed side of the PZT such as by sputtering. InFIG.13(g)the constraining layer130is separated from the tape, and the product is flipped over onto a second tape143and a second substrate141. InFIG.13(h)the product is then diced such as by mechanical sawing or laser cutting in order to singulate the individual microactuators114. This process produces a microactuator114in which the PZT element120including its electrodes is bonded directly to the constraining layer130without any other material, such as an organic material such as polyimide that would render the restraining effect of the restraining layer less effective, therebetween. The electrode layers may be of materials such as Au, Ni, Cr, and/or Cu. Au has a Young's modulus of about 79 GPA, Cu has a Young's modulus of about 117 GPa, Ni has a Young's modulus of about 200, and Cr has a Young's modulus of about 278. Preferably, there is no intermediary layer between the constraining layer130and the PZT element120that has a Young's modulus that is less than 20 GPa, or which has a Young's modulus that is substantially less than the Young's modulus of the restraining layer, or a Young's modulus that is less than half the Young's modulus of the restraining layer.

Although other methods could be used to produce the product such as by bonding the restraining layer directly to the PZT surface by adhesive such as epoxy, the method shown inFIGS.13(a)-13(g)is currently anticipated to be the preferred method.

The constraining layer130acts as a substrate for the PZT layer120both during the additive manufacturing process as well as in the finished product. The constraining layer130is therefore sometimes referred to as a substrate.

FIGS.14(a) and14(b)are oblique views of a gimbal mounted dual stage actuated (GSA) suspension150being assembled with thin film PZT microactuator motors114according to the invention. In a GSA suspension the PZTs are mounted on the trace gimbal which includes a gimbal assembly, and act directly on the gimbaled area of the suspension that holds the read/write head slider164.FIG.14(a)shows the suspension150before PZT microactuator assemblies114are attached. Each of two microactuators114will be bonded to, and will span the gap170between, tongue154to which the distal end of microactuator114will be bonded, and portion156of the trace gimbal to which a proximal end of microactuator114will be bonded.FIG.14(b)shows the suspension150after PZT microactuators114are attached. When microactuator assembly114is activated, it expands or contracts and thus changes the length of the gap170between the tongue154and portion156of the trace gimbal, thus effecting fine positioning movements of head slider164which carries the read/write transducer.

FIG.15is cross-sectional view ofFIG.14(b)taken along section line B-B″. GSA suspension150includes a trace gimbal152, which includes layers of stainless steel, an insulator157such as polyimide, and a layer of signal conducting traces158such as Cu covered by a protective metal159such as Au or a combination of Ni/Au. Microactuators114are attached at their distal ends to stainless steel tongues154extending from the gimbal area by conductive adhesive162such as epoxy containing Ag particles to make it electrically conductive, and at their proximal ends to a mounting area156of the stainless steel by non-conductive adhesive161such as non-conductive epoxy. The driving voltage electrical connection is made by a spot of conductive adhesive160that extends from the gold plated copper contact pad158to the top surface of PZT microactuator114, and more particularly in this case to the constraining layer130which constitutes the top electrode of the microactuator.

The SST substrate thickness may be varied to some degree without compromising the benefits of the disclosed thin film PZT structure.FIG.16is a graph of stroke sensitivity versus SST constraining layer thickness for the microactuator ofFIG.15according to a simulation. A thin film PZT having a 40 μm thick SST constraining layer exhibited a stroke sensitivity of 20 nm/V according to a simulation, which is almost 3 times the stroke sensitivity of the aforementioned 45 μm thick bulk PZT. A 45 μm thick SST constraining layer, however, would provide better protection to the thin film PZT microactuator.

FIGS.17(a)-17(f)illustrates an alternative process for manufacturing a thin film PZT structure having an SST constraining layer according to the invention. InFIG.17(a)the process begins with a silicon substrate144instead of a substrate140and tape142as inFIG.18(b). InFIG.17(b)an constraining layer (130) is bonded to the silicon. The process otherwise proceeds in essentially the same way as the process ofFIGS.13(c)-13(h), including the flipping of the assembly over and removal of the silicon substrate inFIG.17(e). Additionally, these figures explicitly show the addition of final NiCr/Au layer131, which was not explicitly shown inFIG.13(e).

As mentioned above, different types of constraint layers may be used in different implementations. Other rigid materials, either conductive or non-conductive, can also be used as the constraint layer or substrate. Silicon, for example, could be used as the constraint layer material.FIG.18is a top plan view of a thin film PZT structure having a silicon constraint layer according to an embodiment of the invention.FIG.19a cross sectional view of the microactuator ofFIG.18taken along section line A-A′. Because the silicon constraint layer230is non-conductive, a via232is provided in order to conduct the PZT driving voltage from a conductive top layer234such as Au over silicon230through to the metalized electrode126on the PZT element120. The via may be formed and filled with conductive metal as disclosed in U.S. Pat. No. 7,781,679 issued to Schreiber et al. which is owned by the assignee of the present invention and which is incorporated herein by reference for its teachings of conductive vias and methods of forming conductive vias.

FIG.20is a graph of stroke sensitivity versus silicon substrate thickness for the microactuator ofFIG.19according to a simulation. As shown in the graph, a thin film PZT having a thickness of 3 μm and a 20 μm thick silicon substrate may exhibit a stroke sensitivity of 31.5 nm/V. This is more than 4 times as high as the stroke sensitivity of a design with a 45 μm thick bulk PZT. The silicon substrate also helps to improve the reliability of the thin film PZT.

FIGS.21(a)-21(e)illustrate a process for manufacturing the thin film PZT structure ofFIG.18. The process begins inFIGS.21(a) and21(b)with a silicon substrate having a hole or via232that has been formed in it such as by laser drilling. InFIG.21(c)the NiCr/Au layer is added to silicon substrate230to form top electrode126. The NiCr/Au also fills the hole to make it an electrical via232. More generally, other conductive material may be used to fill the via. InFIG.22(d)a PZT thin film120is deposited such as by the sol-gel method, and another layer of NiCr/Au is added to form the bottom electrode128. InFIG.22(e)the material is flipped over, and a final NiCr/Au layer131is added. Layers131and126are electrically connected by via232so that a voltage (or ground potential) applied to the conductive gold layer131will be transmitted to the PZT element126. This manufacturing process for a thin film PZT microactuator having a silicon substrate may be less complicated than the manufacturing process for the thin film PZT having an SST substrate.

In an alternative embodiment, the middle via on the silicon substrate can be replaced by one or more vias at the end the silicon. Therefore, after the final dicing, a half-circle will be formed at each end of the silicon.FIG.22is a top plan view of a thin film PZT microactuator having a silicon or other non-conductive constraining layer330with conductive top layer231such as a metallization layer thereon, and having side vias234,236that electrically connect top layer231to top electrode126.FIG.23is a sectional view of the PZT ofFIG.22taken along section line A-A′. The manufacturing process for this embodiment can be otherwise identical with that ofFIGS.21(a)-21(e).

The constraining layer may be larger (of greater surface area) than the PZT element, the same size as the PZT element, or may be smaller (of lesser surface area) than the PZT element.FIG.24is a side sectional view of a PZT microactuator assembly414in which the constraint layer430is smaller than the PZT element420, giving the microactuator a step-like structure having a step434and an exposed shelf422that is uncovered by the restraining layer430, and with the shelf422being where the electrical connection is made to the PZT element420. One benefit of such a construction including a step where the electrical connection is made is that the completed assembly including the electrical connection has a lower profile than if the restraining layer430covers the entire PZT420. A lower profile is advantageous because it means that more hard drive platters and their suspensions can be stacked together within a given platter stack height, thus increasing the data storage capacity within a given volume of disk drive assembly. It is anticipated that the constraint layer430would cover more than 50% but less than 95% of the top surface of PZT element420in order to accommodate the electrical connection on shelf422.

Simulations have shown that microactuators constructed according to the invention exhibit enhanced stroke sensitivity, and also exhibited reduced sway mode gain and torsion mode gain. These are advantageous in increasing head positioning control loop bandwidth, which translates into both lower data seek times and lower susceptibility to vibrations.

FIG.25is an oblique view of a GSA suspension having a pair of the PZT microactuators414ofFIG.24.

FIG.26is a sectional view of the GSA suspension ofFIG.25taken along section line A-A′. In this embodiment conductive adhesive460such as conductive epoxy does not extend over the restraint layer430. Rather, conductive epoxy460extends onto shelf422on top of PZT element420and establishes electrical connection to the PZT420and to the overall microactuator assembly414by that surface. As depicted, the electrical connection defined by conductive epoxy460has an uppermost extent that is lower than the top surface of the SST restraint layer430. More generally, regardless of whether the electrical connection is made by conductive adhesive, a wire that is bonded such as by thermosonic bonding, soldering, or other techniques, the electrical connection461to the microactuator assembly414can be made to be no higher than, or even lower than, the uppermost extent of microactuator414. This allows the microactuator assembly414including its electrical connection to be as thin as possible, which in turn allows for a denser stack of data storage disk platters within the platter stack of a disk drive assembly.

The figure also explicitly shows gold layer469over the stainless steel tongue154of the trace gimbal to which microactuator414is mounted. Gold layer469provides corrosion resistance and enhanced conductivity to the SST.

In this embodiment as with all of the other embodiments, the restraining layer and more generally the top surface of the PZT microactuator assembly, will normally have nothing bonded to it other than an electrical connection.

FIG.27is a graph of the frequency response of the PZT frequency response function of the suspension ofFIG.26, according to a simulation. The suspension exhibited reduced sway mode gain and torsion mode gain as compared to a simulation without the constraint layer430. These are advantageous in increasing head positioning control loop bandwidth, which translates into both lower data seek times and lower susceptibility to vibrations.

FIGS.28(a)-28(j)illustrate a process for manufacturing the PZT assembly114ofFIG.24. InFIG.28(a)a bulk PZT wafer420is placed onto a transfer tape422. InFIG.28(b)the top electrode layer426is formed such as by sputtering and/or electrodeposition. InFIG.28(c)a mask436is placed over parts of top electrode426. InFIG.28(d)conductive epoxy432is applied. InFIG.28(e)a stainless steel layer that will be constraint layer430is applied to the epoxy, which is then cured. InFIG.28(f)the mask436is removed. InFIG.28(g)the assembly is flipped over and placed down onto a second transfer tape443. InFIG.28(h)the bottom electrode layer428is formed such as by sputtering and/or electrodeposition. The PZT element420is then polarized. InFIG.28(i)the assembly is then flipped over once more to a third transfer tape444. InFIG.28(j)then assembly is singulated by cutting, to produce finished PZT microactuator assemblies414.

FIG.29is a side sectional view of a multi-layer PZT assembly514according to an additional embodiment of the invention. The assembly includes multi-layer PZT element520, a first electrode526that wraps around the device, and a second electrode528, and a constraint layer530that is bonded to the PZT element520by conductive epoxy532. The figure shows a 2-layer PZT device. More generally, the device could be an n-layer PZT device.

FIG.30is a side sectional view of a multi-layer PZT microactuator assembly614according to an additional embodiment of the invention in which an extra thick electrode acts as the restraint layer. In this embodiment, PZT element620has a top electrode626and bottom electrode628. Top electrode626includes a thinner first part622defining a shelf, and a thicker second part630which performs the majority of the restraining function. Step634lies at the transition from the thinner first part622to the thicker second part630. Second electrode626could be applied to PZT element620by a deposition process including masking to create step634, or by a deposition process with selective removal of material to create the step. Alternatively, second electrode626could be a piece of conductive material such as SST that is formed separately and then bonded to PZT element620. Top electrode626could therefore be of the same material, or of a different material, than bottom electrode628. Thicker second part630could be at least 50% thicker than thinner part622and/or second electrode628, or thicker second party630could be at least twice as thick as thinner part622and/or second electrode628. As with the embodiment ofFIGS.24-26, the electrical connection could be made to the shelf defined by thinner part622, with the electrical connection not extending as high as, or higher than, the top surface of the thicker part630that defines the restraint layer.

The scope of the invention is not limited to the exact embodiments shown. Variations will be obvious to those skilled in the art after receiving the teachings herein. For example, the restraining layer need not be stainless steel, but can be some other relatively stiff and resilient material. The restraining layer need not be a single layer of one material, but could be composed of different layers of different materials. Although the restraining layer can cover the entire surface or substantially the entire top surface, the restraining could cover less than the entire surface, e.g., more than 90% of the top surface area, more than 75% of the top surface area, more than 50% of the top surface area, or even more than 25% of the top surface area. In embodiments having the step feature, the restraint layer is anticipated to cover less than 95% of the top surface of the microactuator. The constraining layer need not be a single integral layer, but could comprise multiple pieces such as a plurality of constraining strips arranged side by side on the top surface of the PZT, with the strips extending either in the direction of expansion/contraction or perpendicular to it. In one embodiment, the constraining layer could comprise two constraining pieces of stainless steel or other material bonded onto the top surface of the PZT, with the size and location of the two constraining pieces and their bonding generally mirroring the mounting area of two mounting shelves to which the PZT is bonded on its bottom surface. When the overall stiffness added by the restraining layer on the top of the device generally matches the overall stiffness added to the bottom of the device by being bonded to the suspension, and the bonded areas generally mirror each other, the net bending produced should be zero or close to zero. The result will be a PZT microactuator that, as mounted and deployed in a suspension, exhibits virtually no bending upon actuation.

In any and all of the embodiments discussed herein or suggested thereby, the constraining layer could be chosen so as to reduce the PZT bending that would otherwise occur during actuation, or it could be chosen so as to eliminate as much as possible any PZT bending, or it could be chosen so as to reverse the sign of the PZT bending. In applications in which the PZT(s) will be used as hard disk drive microactuator(s), it is envisioned that using a constraining layer to reverse the sign of the bending as shown and described in the illustrative examples above will be desirable in most cases because that increases the effective stroke length. In other applications for PZTs, however, it might not be desirable to reverse the sign. Thus, the invention can be used in general to control both the direction and the amount of the bending of a PZT, regardless of how the PZT is mounted or otherwise affixed to other components within any particular application. Depending on the application and the parameters chosen, the constraining layer can be used to decrease the PZT bending to less than 50% of what it otherwise would be, or to less than 25% of what it otherwise would be, or to reverse the sign of the bending. When the sign is reversed, a PZT that is bonded at or near its ends on its bottom surface and which has a restraining layer on top will bend such that its top surface assumes a concave shape when the PZT is in expansion or extension mode, rather than assuming a convex shape as would a similar PZT that does not have a restraining layer. Similarly, the PZT will assume a convex shape when the PZT is in contraction mode, rather than assuming a concave shape as would a similar PZT that does not have a restraining layer.

For various reasons, PZT elements are sometimes pre-stressed in an application such that when the PZT is not actuated by any voltage it is already bent in one direction or another, i.e., it is already either concave or convex. Of course, such pre-stressed PZTs could be used as microactuators in the present invention. In such a case, the PZT might not bend into a net or absolute concave shape or a net or absolute convex shape. For example, if the PZT is pre-stressed so that it already has a concave shape, upon activation with a positive activation voltage the device might bend into a more concave shape, and upon activation with a negative activation voltage the device might bend into a less concave shape which might be a nominally flat shape or it might be a convex shape. Unless specifically delineated therefore, the terms “concave” and “convex” should be understood in relative terms rather than in absolute terms.

FIG.31is a cross sectional view of an embodiment of a multi-layer microactuator PZT assembly3100in which the restraining layer(s) of the microactuator assembly comprises one or more active PZT layers3130,3140tending to act in the opposite direction as the main active PZT layer3120which is adjacent the surface of the suspension to which the microactuator3100is bonded. The PZT restraining layers3130,3140thus constrain and actively oppose the action of main PZT layer3120, and thus can be referred to as “constraining layers” or “opposing layers.”

The PZT layers3120,3130, and3140are arranged in stacked planar relationships to one another. The main PZT layer3120comprises active PZT area3121which was subject to an electric field during poling and hence was poled, and which is subject to an electric field during device activation and hence will expand or contract, and also includes inactive PZT areas3122and3123which are not subjected to significant electric fields during either poling or activation and hence are not significantly piezoelectrically active. The device includes: a first or bottom electrode3124; a second and top electrode3126for the active PZT area; a third electrode3132including end3128such that electrode3132both extends between the first active constraining layer3130and the second active constraining layer3140and wraps around the end of the PZT; and fourth electrode3142on top of the second active constraining layer3140including wrap-around portion3143which wraps around both the side and the bottom of the device. The device may be bonded to the suspension using conductive adhesive such as conductive epoxy3160mechanically and electrically bonding electrode3142to drive voltage electrical contact pad158which provides the microactuator driving voltage, and using conductive epoxy3162which mechanically and electrically bonds electrodes3124and3128of the device to grounded part154of the suspension.

To understand the operation of the device, one must understand how the device has been poled.FIG.32shows the poling of the device ofFIG.31, including the resulting poling directions of the various layers of active PZT material. Three voltages are applied: a positive voltage (Vp+) is applied to electrode3124; a negative voltage (Vp−) is applied to electrode3128; and ground is applied to electrode3142. The arrows in the figure show the resulting poling directions for active PZT layers3120,3130, and3140.

Returning toFIG.31, the figure shows how the device3100is connected in this illustrative embodiment. Conductive epoxy3162bridges and thus electrically gangs electrodes3124and3132, essentially taking what had been a 3-pole device during poling and changing it to a 2-pole device in operation. The ganging of electrodes could be accomplished by other well known means for making electrical connections other than conductive epoxy3162, but using the same conductive epoxy3162as is used to bond the device to the suspension assembly accomplishes the ganging function without requiring a separate ganging step.

When a voltage is applied to electrode3142that causes main PZT layer3120to expand in the x-direction (from left to right) as seen in the figure due to the expansion of active area3121, the active PZT constraining layers3130and3140will contract in the x-direction. That is, the two constraining layers3130,3140tend to counteract, or act in the opposite direction, as the main PZT layer3120.

Explained in greater detail, when the device is poled as shown inFIG.32, and the device is electrically connected as shown inFIG.31, the device operates as follows. A positive device activation voltage applied at electrical contact pad158and electrode3142, together with electrode3124being grounded, causes the following reaction. The activation voltage applied to main PZT layer3120is the opposite of the polarity during poling. The main PZT layer3120therefore contracts in the z-direction and thus expands in the x-direction. At the same time, the activation voltage is of the same polarity as was applied to the two constraining layers3130,3140during poling. Those PZT layers therefore expand in the z-direction and thus contact in the x-direction. The two constraining layers3130,3140therefore are tending to contract while the main PZT layer3120is tending to expand in the relevant direction.

The effect of the constraining layers acting in the opposite direction as the main PZT layer is similar to that described earlier with respect to a passive constraining layer such as constraining layer130inFIG.10, and similar restraining layers230,330,430,530, and630in other embodiments discussed above. The action of the active PZT restraining layers reduces the bending that would otherwise occur due to the main PZT layer and its mounting (binding) to the suspension, and can even reverse the sign of the bending, in either case increasing the net displacement caused by the microactuator as mounted.

FIG.33is an exploded view of the microactuator assembly ofFIG.31, showing conceptually the electrical connections. Optional features that were not visible inFIG.31andFIG.32but are visible inFIG.33include patterning3133on electrode3132and a voltage reducer3144associated with electrode3142whose functions are described below.

A thinner microactuator assembly is desired for a number of reasons including: (1) less mass on the suspension, particularly at or near the gimbal in a gimbal-based DSA suspension which is sometimes referred to as a GSA suspension, which in turns means a greater lift-off force as measured in g-forces, i.e., a greater resistance to shock; (2) reduced windage; and (3) greater stack density within the head stack assembly which means that more data can be stored in the same volume of disk drive stack assembly space. It would thus be desirable to make the PZT constraining layers to be quite thin. However, the thinner the PZT constraining layers are, the higher the electric field strengths are across those layers during operation, and hence the more prone they are to being depoled during operation due to too high an electric field strength. Nominally, therefore, the main PZT layer and the constraining PZT layers should have the same thickness.

One solution to making the constraining PZT layers thinner without subjecting them to depoling is to reduce the strength of the electric field(s) across the constraining layer(s) using one or more of various possible means without significantly reducing the electric field across the main PZT layer. A first means for accomplishing that objective is to pattern one or more of the electrodes that is operationally associated with one of the active PZT constraining layers but is not operationally associated with the main PZT layer, such as by adding holes3133in electrode3132or other electrical voids. The patterning could also take the form of a mesh pattern such as a grid of parallel or intersecting conductors with electrical voids between them. By reducing the percentage of area of the electrical conductor within the planar electrode3132, the electric field strength across constraining layers3130and3140are effectively reduced without reducing the electric field strength across main PZT layer3120.

A second solution is to increase the coercive electric field strength of the constraining layer(s) so that the constraining layers are more resistant to depoling. The coercive electric field strength, or simply “coercivity” when referring to a piezoelectric material, is a measure of how great an electric field strength is required in order to depole the piezoelectric material. Making the constraining layer(s)3130,3140have a higher coercivity than the main PZT layer3120allows those constraining layers to be made thinner without risk of depoling when subjected to the same activation voltage as the main PZT layer. The constraining layers3130,3140can be made to have higher coercivities, possibly at the price of some loss of d31stroke length or other desirable characteristics, by using a different or slightly different piezoelectric material, or by other processing.

Another solution is to reduce the effective voltage that is applied to the driven electrode associated with the constraining layer(s) by using some kind of a voltage reducer such as a voltage divider resistor network, a diode, a voltage regulator, or any one of various functionally similar devices which will occur to one of skill in the field. In the figure, generalized voltage reducer3144reduces the voltage received by electrode3142, thus reducing the electric field strength experienced by constraining layer3140but not by main PZT layer3120. The voltage divider can be integrally formed and thus disposed between the adjacent piezoelectric layers, such as by applying the metallization that forms the electrode layer in such as way as to form a voltage divider resistor network on the surface of the PZT material. A simple resistive voltage divider would require a ground which could be made available on the same layer. Many constructions are possible as will be apparent to designers of such devices.

Patterning3133and voltage reducer3144both decrease the strength of the electric field across constraining layer3140, thus allowing constraining layer3140to be made thinner without unacceptable exposing it to depoling during operation. Either electrode patterning and/or a voltage reducer, and/or some other means for reducing the electric field strength across constraining layers3130and/or3140, could be used. The patterning3133is integrally formed with electrode3132and thus is integrally formed with, and integrated into the microactuator assembly. A voltage reducer for one of the electrodes could be either integrally formed with and integrated into the assembly, or could possibly be provided external to the assembly provided that the associated electrode has its own electrical lead and is not ganged with the other electrodes.

All three of those solutions discussed above may be applied to piezoelectric microactuators having a single active constraining layer, two active constraining layers such as shown inFIGS.31-33, or more generally n active restraining layers such as illustrated inFIG.35.

FIG.34is a graph showing the stroke sensitivity (in nm/V) of microactuators having one or more active restraining layers according to simulations for various constraining layer constructions (CLC), with a main PZT layer of 45 μm thick, without any patterning3133or voltage reducer3144to decrease the electric field strength, for three different constructions:a) one inactive restraining layer (“passive CLC,” the diamond shaped data points);b) one active restraining layer (“single layer,” the square data points); andc) two active restraining layers (“double layer,” the triangular data points).

The data indicates that, at least for the parameters that were studied, a PZT microactuator having one active restraining layer acting in the opposite direction as the main PZT layer always produces higher stroke sensitivity than one in which the restraining layer is inactive material. The highest stroke sensitivity is achieved using multiple active thin layers of PZT acting as restraining layers (i.e., acting in the opposite direction as the main PZT layer). Specifically, the highest stroke sensitivity was achieved using two restraining layers that were each 5 μm thick, or approximately 11% the thickness of the main PZT layer. Thus, the constraining layer is preferably less than 50% as thick as the main PZT layer, or more preferably less than 20% as thick as the main PZT layer, or more preferably still within the range of 5-15% as thick as the main PZT layer.

For two active restraining layers, the stroke sensitivity decreases dramatically with increasing thickness of the restraining layers, with the highest stroke sensitivity for the case of two active constraining layers each of about 5 μm thick. Thus, the microactuator preferably has two or more restraining layers of a combined thickness that is less than the thickness of the main PZT layer, and more preferably their combined thickness is less than 50% the thickness of the main PZT layer, and more preferably still each constraining layer is less than half as thick as the main PZT layer, and more preferably still each constraining layer is less than 20% as thick as the main PZT layer, and more preferably still each constraining layer is within the range of 5-15% as thick as the main PZT layer.

For a microactuator assembly having a single active restraining layer, the loss in stroke sensitivity as the restraining layer thickness increases was not nearly as dramatic as for the case of two active restraining layers. A local maxima occurs at about 10 μm thickness for the single active restraining layer. Thus, for a microactuator assembly having a single active restraining layer, the thickness of that layer is preferably in the range of 10-40% as thick as the main PZT layer, and more preferably in the range of about 10-20% as thick as the main PZT layer.

FIG.35is cross sectional view of another embodiment in which the microactuator assembly comprises multiple active PZT layers, and conceptually showing the poling process and the resulting poling directions. When the device ofFIG.35is electrically and mechanically bonded to a suspension such as shown inFIG.31with electrodes3524and3528ganged by conductive epoxy, the result is one main active PZT layer, and three active PZT layers acting as restraining layers because they tend to act in the opposite direction as the main active PZT layer. That is, the bottom PZT layer expands while the top three PZT layers contract, or vice versa.

The construction of the microactuator assembly can be easily extended from a device having one active main PZT layer and two active PZT restraining layers as shown inFIGS.31-33, and one active main PZT layer and three active PZT restraining layers shown inFIG.35, to any number of active main layers and active restraining layers. The electric field strength across one or more of the constraining layers can be reduced by various means including electrode patterning and/or a voltage reducer. Experimentation will reveal optimal numbers of constraining layers and optimal thicknesses for different applications.

The PZT microactuators disclosed herein can be used as actuators in fields other than disk drive suspensions. Such microactuators and their construction details therefore constitute inventive devices regardless of what environment they are used it, be that environment the disk drive suspension environment or any other environment.

It will be understood that the terms “generally,” “approximately,” “about,” “substantially,” and “coplanar” as used within the specification and the claims herein allow for a certain amount of variation from any exact dimensions, measurements, and arrangements, and that those terms should be understood within the context of the description and operation of the invention as disclosed herein.

It will further be understood that terms such as “top,” “bottom,” “above,” and “below” as used within the specification and the claims herein are terms of convenience that denote the spatial relationships of parts relative to each other rather than to any specific spatial or gravitational orientation. Thus, the terms are intended to encompass an assembly of component parts regardless of whether the assembly is oriented in the particular orientation shown in the drawings and described in the specification, upside down from that orientation, or any other rotational variation.

All features disclosed in the specification, including the claims, abstract, and drawings, and all the steps in any method or process disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. Each feature disclosed in the specification, including the claims, abstract, and drawings, can be replaced by alternative features serving the same, equivalent, or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

It will be appreciated that the term “present invention” as used herein should not be construed to mean that only a single invention having a single essential element or group of elements is presented. Similarly, it will also be appreciated that the term “present invention” encompasses a number of separate innovations which can each be considered separate inventions. Although the present invention has thus been described in detail with regard to the preferred embodiments and drawings thereof, it should be apparent to those skilled in the art that various adaptations and modifications of the present invention may be accomplished without departing from the spirit and the scope of the invention. Accordingly, it is to be understood that the detailed description and the accompanying drawings as set forth hereinabove are not intended to limit the breadth of the present invention, which should be inferred only from the following claims and their appropriately construed legal equivalents.