MEMS for high-density data storage
Thin Films and Nanosynthesis Laboratory,
Department of Mechanical and Aerospace Engineering,
State University of New York at Buffalo, Buffalo, NY 14260
Abstract – MEMs for high-density data storage usage are studied under three main classifications. First of all, several approaches have been utilized to modify conventional data storage system, to achieve positioning more precisely and faster. Different type of design and techniques are employed or introduced to manufacture such MEMs. Secondly, scanning probe microscopy (SPM) method is studied for even higher density data storage. MEMs are critical in this new technique for achieving read and write data, miniaturizing and integrating the whole systems. Thirdly, MEMs mirror for 3D holographic data storage is also analyzed. Finally, in MEMs point of view, some new considerations towards high-density data storage are reviewed in this study.
I. MEMs for conventional data storage system.
Conventional dual-stage servo architecture for data storage system is shown in the Fig.1.
Fig.1 The servo positioning mechanism of a conventional magnetic disk drives.
Read and write elements, which transfer data to and from the disk, are affixed to a ceramic slider, which is bonded to a gimbal at the end of the stainless steel suspension.
An electromagnetic voice-coil motor (VCM) attached to the opposite end of the
suspension is used to move the slider radially across the disk. Increased tracking accuracy can be provided by a dual-stage control system, using the VCM for coarse positioning
and a micro-actuator mounted between the slider and suspension for fine positioning.
(1) Piezoelectric micro-actuators for conventional disk drives (actuated suspension)
Increasing the track density revealed many kinds of disturbance. One of them is the nonlinear response of actuator’s ball-bearing friction. One of the answers is to introduce a piezoelectric micro-actuator into the conventional micro-actuator. A planar-type piezoelectric micro-actuator is placed on the bottom of head suspension, which need lower driving voltage and has higher resonance frequency. In this approach, conventional assembly and machining techniques are used to integrate an electromagnetic or piezoelectric actuator into a conventional steel suspension.
A planar piezoelectric micro-actuator on a head-arm assembly is shown in Fig.2. The micro-actuators are built into the stainless steel head-mounting block and placed between the head suspension and the arm. The integrated micro-actuator is only 300？m thick and
185mg in weight. This micro-actuator has four piezoelectric elements to generate displacement. Each piezoelectric element is 90？m thick and 5.3mm in length.
Fig.2 Head-arm assembly with piezoelectric micro-actuator.
Fig.3 Planar piezoelectric micro-actuator for conventional disk drive.
Fig.3 shows a close-up of the micro-actuator. There are three sections separated by slits: a fixed section, connected to the head arm; a movable section, connected to the head
suspension, and two elastic sections sandwiched by piezoelectric elements. Two
piezoelectric elements were connected to both sides of an elastic section and polarized in advance. When the same voltage is applied to the pairs of the piezoelectric elements, one pair expands and the other contracts longitudinally. A movable section connects to the head suspension and swings by the movement of the two pairs of piezoelectric elements. The displacement generated by the piezoelectric elements is amplified eight times at the
Fig. 4 The relationship between applied voltage and displacement.
A figure of applied voltage and head displacement is shown in Fig.4. By applying only ?12V the micro-actuator generates 2.3？m displacement. Hysteresis of the figure makes
constant phase delay of about -5？ at low-frequency region in the micro-actuator transfer function.
Fig.5 The tracking errors of VCM and VCM with micro-actuator.
Fig.5 shows the tracking errors of the VCM controller and for VCM plus micro-
actuator. The maximum tracking error of the previous one is ，0.43？m, but for VCM
plus micro-actuator, the error is ，0.26？m. (~50% decreasing) A disadvantage to these
designs is that the actuator is located far from the read/write elements and therefore have a limited bandwidth due to suspension vibration.
(2) Angular electrostatic micro-actuator
Angular electrostatic micro-actuators suitable for use in a two-stage servo system for
magnetic disk drives have been fabricated from molded chemical vapor deposited (CVD)
poly-silicon using the so called HexSil process. A 2.6mm-diameter device has been
shown to be capable of positioning the read/write elements of a 30% pico-slider over a
，1？m range, with a bandwidth of 2kHz. The structures are formed by depositing poly-
silicon via CVD into deep trenches etched into a silicon mold wafer. Upon release, the actuators are assembled onto a target wafer using a solder bond. The solder-bonding
process will provide easy integration of mechanical structures with integrated circuits, allowing separate optimization of the circuit and structure fabrication processes. An
advantage of HexSil is that once the mold wafer has undergone the initial plasma etching, it may be reused for subsequent poly-silicon depositions, amortizing the cost of the deep-trench etching over many structural runs and thereby significantly reducing the cost of finished actuators. Furthermore, 100？m-high structures may be made from a 3？m
deposition of poly-silicon, increasing overall fabrication speed.
Fig.6 100？m-high 2.6mm dia. rotary electrostatic actuator made via the HexSil process
from a 3？m poly-silicon film of low-pressure chemical vapor deposition (LPCVD)
As shown in Fig.6, the actuator consists of a fixed outer ring, or stator, and a mobile
inner ring, or rotor, which is connected to an anchored central column via narrow poly-
silicon flexures. Actuation is accomplished via capacitive parallel plates, which are attached to the rotor and stator in opposing pairs. A voltage applied across these plates results in an electrostatic force which rotates the central rotor. The stator is made up of four separate electrically isolated quadrants. In Fig.7, the isolation is achieved with thin breakaway beams connecting the stator quadrants.
An electrostatic design was chosen for ease of fabrication-the structural material of the device need only be conductive, rather than ferromagnetic or piezoelectric. Furthermore,
electrostatic actuators allow high accuracy, capacitive measurement of displacement, and are capable of high-bandwidth operation.
Fig.7 A view of one quadrant of the rotary micro-actuator, showing the flexural
suspension, electrostatic plates and breakaway tethers.
Fig.8 Simplified HexSil process flow.
A simplified fabrication process flow is shown in Fig.8. It includes several steps as following:
(1) Use reactive etching (RIE) to etch deep trenches into a silicon mold wafer, (2) 2-level sacrificial oxide layer is deposited,
(3) The surface layer is patterned,
(4) Removal of the sacrificial layer,
(5) Actuator alignment,
(6) Actuator is bonded to a target substrate,
(7) Retraction of the mold, a new fabrication process begins again.
Fig.9 shows a 100？m high structure from the first run, which was released and transferred onto an adhesive pad. The 3？m wide flexures connecting the rotor to the anchored central column is shown also in Fig.10.
Fig.9 SEM of an actuator which has been solder bonded to a target die.
Testing result and theoretical calculation show that the actuator will guarantee an operating range of ，1？m up to a maximum frequency of 2.5KHz. Additionally, the
devices were batch fabricated with a low-cost process using 3？m LPCVD deposition of
poly-silicon to create structures. This allows the integration of micro-mechanical structures with standard CMOS at low cost and minimize additional processing complexity.
Fig.10 Flexures connecting the rotor to the anchored central column.
(3) MEMS micro-actuator for hard-disk-drive (HDD) tracking servo
Several requirements for MEMs in HDD tracking servo are shown as following:
(1) The temperature stability of dimension.
Typical HDD products guarantee an operational ambient temperature range of 5-55？C,
while the typical micro-electrostatic actuator has small inter-electrode gaps that must be strictly kept. Thus a materials with lower TCE is preferred for the structure. For instance,
invar elecro-deposited layer can meet such purpose.
(2) The micro-actuator must be flexible in the operational direction and very stiff in other
A high-aspect-ratio structure and large structural height are necessary to enhance the stiffness in Z-direction to withstand the force, which is applied from the suspension beam to the slider to press the slider down to the disk surface. In addition, the resonant frequencies of the operational direction would be much lower than that of other directions. (3) The force output and the manufacturing cost.
Since the actuator must drive a slider weighted with a few milligrams and the maximum usable area and voltage are limited, and efficient actuator design is required. Generally, this needs a high-aspcet-ratio interelectrode gap in MEMs.
To meet above requirements, a fabrication technology is developed i.e., so-called high-aspect-ratio polymer dry etching technology.
Fig.11 High-aspect-ratio etching tool.
Fig.11 shows a schematic of the high-aspcet-ratio polymer etching tool. Two RF power
sources are attached to this system. The bottom RF source is connected to the sample,
and capacitively couples to the plasma. This power mainly controls the sample's self-bias voltage, which has large effect on the etching anisotropy. The top RF source is
connected to the coil attached at the top of the chamber, and inductively couples to the plasma. This power controls the plasma density. Thus, two important etching parameters,
anisotropy and etching rate, can be controlled by adjusting the powers of two RF sources.
The micro-actuator is basically an electrostatic comb-drive actuator. The driving force is generated by electrostatic comb-drive actuators attached at the top and bottom of the moving stage. The driving force could be expressed as:
2hnV；F (1) ;g
where, ； is the permittivity of air, h is the structure height, n is the number of the
electrode pairs, and V is the driving voltage, g is the gap width. Note that the driving
force is proportional to the aspect ratio of the inter-electrode gap h/g. Therefore, the
aspect ratio of both line and spaces are very important.
A simplified process was used as shown in Fig.12. A sacrificial layer (PSG) and a seed
layer are deposited on a silicon substrate. Then a thick polymer layer (40？m) is coated by
a single-RPM spin-coating method. The polymer layer is patterned with a high-aspect ratio by above described plasma etching, followed by electro-deposition of the invar layer. After the polymer layer and the seed layer are removed, the sacrificial layer is etched with buffered HF. This time-controlled etching release the narrow beam structure, while wider structures remain anchored to the substrate.
Fig.12 Fabrication process of micro-actuator.
Fig.13 shows a SEM photograph of this kind of micro-actuator. The width of the comb electrode is 4？m, and the gap in between is also 4？m.