Patent Application
20090190254
The present invention relates to movers used for information storage devices. More particularly, the present invention relates to movers which provide relative movement between substrates of information storage systems.
Disc based storage systems are well known and have been used to store information. In such systems, a disc storage medium is rotated while a transducing head is positioned radially across the disc surface. This allows the areas of the medium surface to be accessed for writing and reading information.
Another type of storage system uses substrates in which one substrate provides a storage medium and another substrate carries a transducing head. The substrates are microelectromechanical structures (MEMS) formed using micromachining techniques. Data can be read from, or written to, different areas of the medium substrate by providing relative movement between the two substrates. Various techniques are known to provide such movement and are shown, for example, in Walmsley et al., U.S. Pat. No. 6,882,019 titled “MOVABLE MICRO-ELECTROMECHANICAL DEVICE”; Ives, U.S. Pat. No. 6,925,047 titled “HIGH DENSITY DATA STORAGE MODULE”; Hartwell et al., U.S. Pat. No. 6,930,368 titled “MEMS HAVING A THREE-WAFER STRUCTURE”; Haeberle et al., U.S. Pat. No. 6,369,400 titled “MAGNETIC SCANNING OR POSITIONING SYSTEM WITH AT LEAST TWO DEGREES OF FREEDOM”; Fasen, U.S. Pat. No. 6,737,863 titled “ELECTROSTATIC DRIVE”; Brandt, U.S. Pat. No. 6,583,524 titled “MICRO-MOVER WITH BALANCED DYNAMICS”.
A micromachined mover includes a rotor substrate and a stator substrate. A suspension is configured to couple the rotor substrate to the stator substrate and allow relative movement therebetween in a plane of the substrates. The suspension is positioned on an interior portion of the substrates.
A MEMS-based mover consists of actuator and suspension structure. The suspension structure is typically compliant in-plane (x- and y-direction) and stiff out-of-plane (z-direction), while offering good decoupling of x- and y-axis motion. The actuator for the mover can be based on electrostatic, electro-thermal, or electromagnetic transduction. MEMS-based movers have been successfully built, with the majority having limited travel, normally about 1-30 μm. Examples of electrostatically actuated movers in a probe device are shown in
A cross-sectional view of a probe device 10 shown in
In some instances a larger stroke is required than that provided by the design of
Design of a MEMS mover for large in-plane stroke and large recording area is challenging for several reasons. Large strokes require long springs to meet stress and linearity requirements. For manufacturability, folded springs with a gimbal can provide uncoupled x and y in-plane motion. To meet the large stroke requirement and to move the large mass of the rotor (against operating vibration and shock), the electrostatic actuator must also generate large in-plane force. To compensate for undesirable parasitic z-force and to maintain z-stability while achieving large stroke, a large out-of-plane-to-in-plane stiffness ratio Kz/Kx of more than 400 is desirable. Out-of-plane stiffness is particularly important for probe storage because undesirable motion in z-direction would affect read/write physics. A large rigid gimbal structure would improve Kz/Kx but would reduce the media area. Given large folded springs and gimbal structure, a relatively small media area to chip area ratio is achieved.
The present invention provides a mover configuration to address the above challenges. The designs offer a significantly higher area efficiency.
Unlike the mover configuration discussed above where the moving platform is surrounded by the springs and gimbal, with the present invention the mover has its springs and gimbal in a central area and the springs and gimbal are surrounded by the moving platform. This “inverse” mechanical design leads to a significantly smaller gimbal, and thus more area for the media movable platform. The mover has a similar cross-section to that described above and is composed of three layers: a rotor substrate that has a moving platform with integrated media and actuation/position sensing electrodes suspended by springs and gimbal, which is sandwiched between a stator substrate that has a mating actuation (and sensing) electrodes, and a head substrate, with arrays of read/write heads and pre-amp electronics (and sensing electronics). Different configurations of capacitive sensing and actuation electrode placement and designs are possible.
A second example configuration is illustrated in the cross-sectional view of
The rotor substrate 50 as shown in
While the mover designs illustrated here are electrostatically actuated, the present invention is not limited to electrostatic actuation. Furthermore, position sensing techniques other than capacitance sensing may be used. The mover design may be hermetically sealed depending on specific application requirements or method of implementation.
The above designs offer significant improvement in the area efficiency of the probe device. The first configuration may provide twice as much area as the prior art. The improved area efficiency has two-fold benefits. It offers more media area and more area for the electrostatic actuator.
Additional media area reduces the required areal density for the specific capacity for the device, thereby reducing development time for the probe product (or making it twice as competitive). Efficient use of silicon area also enables compact storage devices, thus allowing application of this storage technology in a larger variety of product markets.
The increased area provided by the present invention for the electrostatic actuator provides a linear improvement in the force that can be obtained from the actuator. This gain in force can be used to: reduce the voltage requirement, which affects the overall power consumption; reduce the risk of the voltage breakdown; provide additional force margin; or reduce the magnitude of the generated out-of-plane force, thereby providing a more robust device.
For the design shown in
For the mover 70, shown in
The area efficiency of the invention increases when larger form factors are considered. A comparison of the area efficiency of the mover 70 and a prior art mover is shown in
Mover 70 may be in an array of 2×2 devices. Such a configuration is less susceptible to out-of-plane modes of vibration. One configuration includes 2×1, 2×1×2, etc.
Mover 70 provides the same in-plane stiffness and a very similar out-of-plane stiffness as the prior art movers. Although the gimbal structures are shorter and thus stiffer in mover 70, that benefit is diminished due to the specific boundary condition established through anchoring of the mover only at the central region. As a consequence, the rotor wafer 50 behaves as a fixed-free plate.
It should also be noted that the area efficiency of mover 70 is reduced if the mover is sealed. The sealing area occupies a region along the circumference. The overall area efficiency could be reduced by 15% or more.
The configuration of mover 100 shown in
An additional benefit of mover 100 is that the area efficiency of the media is independent of the layout of the suspension structure. The layout of the suspension structure affects only the area efficiency of the electrostatic actuator. Again, the higher the area efficiency of the electrostatic actuator the more force is available. The additional force can be traded for a larger anchoring area, which would then affect the dynamic properties of the device.
In addition to the spring design of
The suspension structure can be based on the folded-beams structure as shown in
The mover of the present invention can use different arrangements of the beams. For example, the folded-beam structure could use only two beams, instead of four; it could use more than four beams; the folded-beam structures could be arranged in such way that they are not symmetrically arranged along both x- and y-axis; or, the suspension structure could utilize another beam arrangement, different from the folded-beams structure.
Because the mover 70 and mover 100 both have support spring structures in the center, the rotational stiffness is reduced. However, several design optimizations are possible to achieve greater rotational stiffness. One example design includes moving the folded springs slightly away from the center to the outer edges of the mover. The spring/gimbal structure in this case would occupy a larger area. However, for the mover 100, there is no impact on the media area efficiency, although the actuator area efficiency is reduced. The area of the larger spring/gimbal structure may not be completely lost as some of the area may be used for actuation (or capacitive position sensing).
In addition, two or more smaller MEMS movers may be joined into a large MEMS mover. In this case, the large mover structure will be anchored in multiple points (distributed as needed) to yield improved dynamic performance. However, tradeoffs with system architecture must be carefully considered.
One fabrication challenge for the design of mover 70 is electrical routing to the capacitive sensing electrodes and the actuation electrodes on the rotor 50. This challenge may not necessarily apply to all probe storage applications or micro-positioner or actuator applications.
For the case where capacitive sensors are positioned between the media rotor surface 50 and head array surface 74 (
One method is to create through-vias 91 or side-wall interconnects on the fixed center anchor of the rotor substrate 50 shown in
A third option is to use a circuit implementation in which the capacitive electrodes on the rotor may be grounded. With such a configuration, the electrodes can comprise the bulk rotor substrate 50 which is made conductive through doping, thereby avoiding any metal routing or through-vias. In this case, the capacitive sensor may be located between the head and media surfaces as in
Fabrication challenges for the mover 100 of
As shown in
Hermetic sealing of the MEMS mover may be important in some configurations. Large out-of-plane damping is desirable for the mover to withstand operating shock and vibration. Squeeze-film damping between the layers of the MEMS structure may offer needed out-of-plane damping and thus a stable volume of gas is required inside the MEMS system. Secondly, protection of the media interface by maintaining a clean environment (free of undesirable debris, chemicals, and moisture) is important to read/write physics and reliability. For ease of manufacturing and assembly, a sealed MEMS device enables simple handling during the wafer dicing process and packaging assembly process. The requirement for hermeticity is not essential for all applications.
For hermetic sealing of the MEMS mover, many approaches are possible. As shown in
The two rotor parts as described can be fabricated from a single wafer. In this case, media may be first deposited on a silicon wafer A large gap is then etched between the media rotor and the fixed frame area. This gap must be adequately large to accommodate the in-plane stroke with some margin. The media rotor may remain to be attached to the frame prior to bonding to the stator substrate via tethers to be eliminated later. Otherwise, a carrier wafer may be used to hold the media rotor and the fixed frame together during the silicon gap etch and wafer bonding. Alternatively, the media wafer is bonded to the stator wafer prior to the gap etch/frame formation. Fabricating both parts of the rotor substrate using a single wafer as described helps to ensure that the relative position of the frame and the media rotor is precise.
Another approach is to create the fixed frame and the moving media separately. In this case, the frame and the media substrate would be attached to the stator substrate 72 in two separate steps. One benefit of this approach is that the bonding process for the fixed frame may be independently optimized from the bonding process for the media substrate. Further, fabrication of the media substrate would be simplified because the media would not need to be protected against harsh silicon etching chemicals or conditions. Media “chips” may be bonded individually to the center anchor on the rotor or multiple media “chips” can be bonded simultaneously. This approach would reduce media production cost because all of the media wafer surface may be dedicated to media material (instead of partially to gaps or springs or fixed frame). However, a total of four separate wafers are required for this approach.
Note that the frame of the rotor 50 need not be first bonded to the stator 72 and then bonded to the head substrate 74. One alternative is to attach the frame first to the head substrate 74. Then the “stacked” head substrate with the frame is bonded to the stator substrate 72.
Alternatively, the fixed spacer frame can be fabricated as part of the stator substrate 72 (
A method is also needed to seal the bottom of the MEMS wafer stack. Because the springs of mover 100 are located on the stator substrate 72, gaps in the spring structures allow airflow into the MEMS mover cavity through the stator wafer 72. Sealing of this opening may occur in many points of the fabrication or assembly process. Tradeoffs of reliability, yield, cost, and development time must be considered to evaluate many possible schemes. The scheme may also be influenced by how the springs are fabricated on the stator, how the rotor substrate is processed, and how the different wafers are bonded together.
There are many possible methods to create a sealed cavity despite the spring structure. One method is to create a stator substrate with a sealing layer 200 shown in
Alternatively, instead of using an SOI wafer as the stator substrate starting material, a silicon wafer, as a fourth layer, may be bonded to the stator substrate to provide sealing. This approach may likely added extra thickness to the stack but may be mitigated via wafer grinding or thinning technology.
Alternatively, the stator substrate opening may be sealed using a “membrane”. The bottom sealing “layer” may be made of a single or multiple layers of material as needed to achieve desired rigidity, robustness, filtration, and degree of hermedicity. This layer may be deposited via microfabrication techniques or traditional manufacturing techniques. This bulk layer may be an adhesive material to attach the MEMS module to a circuit substrate.
An example method to create a cavity covered by a membrane is as follows: a shallow cavity may be etched in a Si wafer as the starting material of the stator substrate. The cavity is then refilled with a sacrificial material, such as oxide, photoresist, metal, etc. If needed, the surface may be polished flat. A layer serving as the sealing layer would be deposited. Then metallization/insulation and the DRIE processing of the spring/gimbal structure would be carried out on the opposite side of the wafer. The rotor may then be bonded to the stator, followed by the removal of the sacrificial layer through a wet or dry chemical etch. If necessary, an etch hole/pattern may be added to the rotor to facilitate sacrificial etching. Alternatively, the sacrificial material is removed prior to rotor bonding. However, a special process is required to bond the rotor to the freely suspended pedestal structure.
As shown, the stator substrate 72 for multiple movers are joined as one to simplify routing or assembly. The media rotor and head chip may be bonded to a large stator wafer, which is then diced into 2×2 arrays. Alternatively, the wafer may be diced into separate individual MEMS modules or 1×2 modules or other combinations.
Capacitive sensors may be located on the same surface of the media substrate as shown, with matching electrodes on the heads chip. Alternatively, the media area may be maximized by placing the capacitance sensors on the same surfaces as the actuation electrodes, as discussed in previous paragraphs.
The “ring” for sealing the media rotor as shown is part of the head chip 74. The seal ring may also be part of the stator substrate (bonded or deposited or integrated) and may be created in a number of ways as described in previous paragraphs.
As shown the MEMS chips and the support electronics, which may include the System On a Chip (SOC) and high-voltage supply and drivers, are mounted on a circuit substrate 210 (with wire traces and other passive components (not shown)). The support electronics are stacked and are located on the side of the 2×2 MEMS arrays. Other possible configuration may include, but are not limited to 1×2 arrays of MEMS chips on either side of the support electronics.
As shown, the head chip is directly mounted on the circuit substrate. Alternatively, the layer stack of the MEMS module may be reversed so that the stator substrate is directly mounted on the circuit substrate instead. The assembly array is sealed in a housing 212.
The present invention provides a number of features including:
Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. As used herein, the term “suspension” includes any combination of springs and gimbals. “Center” and “interior” portions refer to portions of the mover in which the moving portion (such as the media) is positioned around the suspension or in which the moving portion is above or below the suspension. “Moving substrate” includes the portion(s) of a substrate which move, such as the rotor, media, or other portion. “Suspension” includes an apparatus which couples two substrates together but allows relative movement therebetween.
