INVAR MEMS MILLIACTUATOR FOR HARD DISK DRIVE APPLICATION

Toshiki HiranoT Long-Sheng Fan, and Jenny Q. Gao International Business Machines, Research Division, Almaden Research Center

650 Harry Road, San Jose, CA 95120-6099 E-mail:hirano@almaden.ibm.com, lsfan@almaden.ibm.com, jengao@almaden.ibm.com

ABSTRACT

High-bandwidth servo actuation of the recording head is necessary for the h t w e high areal recording density hard- disk drives. The "piggy-back" actuation scheme, in which MEMS milliactuator is used as the secondary actuator, is one of the promising solutions. There are several require- ments for the milliactuator in this application, including thermal stability to ensure wide-range operation temper- ature, and a verticalihorizontal directional selectivity of the stiffness, which needs high-aspect-ratio structures. To meet the thermal stability requirement, low-thermal- expansion-alloy (Invar) electro-deposition process was developed. High-aspect-ratio etching technology was also established to define a thick and high-aspect-ratio structure. The milliactuator was designed, fabricated and characterized, and the measured frequency response indicated that the device is suitable for use as a servo actuator.

1 INTRODUCTION

The areal recording density of a hard-disk-dnves (HDDs) continues to increase by almost 60% every year. To maintain this pace, the recording track width must be reduced further so that more tracks can be packed onto the recording disk with the same radius as now. This requires high-bandwidth servo position control of the recording head, because the head must follow the narrow data track with high accuracy. One promising approach is the piggy- back actuation scheme[ 11, which uses the conventional voice-coil motor (VCM) as a coarse, low-bandwidth actuator, and the microfabricated MEMS milliactuator as a fine, high-bandwidth actuator. Fig. 1 shows a schematic of apiggy-back system in which a milliactuator sandwiched between the suspension beam and the slider moves the slider's position relative to the suspension beam. Since the mass of the slider is very small, and the suspension vibrational modes can be compensated by the

'Lon assignment from IBM Japan, Tokyo Research Laboratory

milliactuator, a high servo-control bandwidth is feasible.

SUDER

f RECORDING HEAD

Fig. 1 Piggy-back actuation scheme

There are several requirements for this milliactua- tor. The first is the temperature stability of the dimen- sion. Typical HDD products guarantee an operational temperature range of 5 to 55"C, while the typical micro- electrostatic actuator has small inter-electrode gap that must be strictly kept. The gap width change can cause a shift of actuation characteristics shift or, in the worst case, the short-circuit failure.

Secondly, the milliactuator must be flexible in the operational direction, and very stiff in other directions (Z and all rotational directions), because the resonant frequencies of the other directions must be much higher than that of the operational direction. In addition, the Z- directional stiffness of the actuator must be high enough to sustain the force which is applied from the suspension beam to the slider to press the slider down to the disk

0-7803-3744-1/97/$5.00 0 1997 IEEE 378

surface. A high-aspect-ratio structure and large structural height are necessary for this requirement.

The last requirement is low manufacturing cost, since the milliactuator will be used for HDDs in which the market is very competitive.

To fulfill these requirements, two new fabrication technologies were developed: an Invar electro-deposition technology and a high-aspect-ratio polymer dry etching technology. A prototype milliactuator was successfully fabricated by using the technologies, and its operational characteristics were measured.

2 NEW FABRICATION TECHNOLOGIES

2.1 Invar electro-deposition process Invar (64% iron/36% nickel alloy) is known for its low thermal coefficient of expansion (TCE). It is very im- portant to use low TCE material for the structure of the actuator to ensure wide range of operational temperatures. Fig. 2 shows an example of an electrostatic comb-drive actuator. When the temperature changes, the suspension beam expands or shrinks due to the thermal expansion mismatch between the structure and the substrate, and the gap between the stationary electrode and the suspended electrode changes. In this case, the gap change Ag is expressed by the following equation:

Ag = At x ( c y 5 ~ 5 - c y 5 ~ b ) x 1, (1)

where At is the temperature change, cy,,, and cy,,!, are the TCE of the suspension beam material and substrate material, respectively, and I is the suspension length. Assuming an operational temperature range of 5 to 55OC (At = 5OoC), the suspension is made of copper (a,,, = 16.5 x 1OP6/K), the substrate is silicon CY,,^ = 2.6 x 10-6/K), and the suspension length is 1 mm, the gap change Ag = 0.7 pm. Since the typical gap size of the comb-drive actuator is around 2 pm, this large gap change is not acceptable. Thus, a material with low TCE is preferred for the structure.

Full film electro-deposition of Invar has been re- ported by other groups[4-8], but additional considera- tions are necessary for MEMS application, such as film stress, thick film deposition, vertical and lateral uni- formity. Thus, we developed Invar electro-deposition process for MEMS applicationp].

Fig. 3 illustrates the electroplating bath set-up. This is a type of “paddle cell” which is originally developed by L. Romankiw’s group[lO]. The name “paddle cell” comes from the paddle which moves back and forth between the cathode and the anode. This paddle agitates the electrolyte above the sample surface. This design is know to give a uniform plating result, and this type of cell is widely used in the thin-film magnetic head industry. The temperature is monitored by a temperature

Structure

Substrate

Fig. 2 Electrostatic comb-drive actuator

probe, and kept constant by a controller and a heater. An outer electrode surrounds the sample to adjust the current distribution on the sample surface. The currents applied to the sample and the outer electrode are independently controlled by two current sources.

-1 Temperature 1 Controller

Current Current Source1 Source2

Paddle Outer Electrode

Fig. 3 Paddle cell

The electrolyte is based on the low-stress nickel sulfamate bath. Table 1 shows the composition of the electrolyte.

Table. 1 Electrolvte comDosition Nickel Sulfamate (MA) Ni Br2 (MA) Fe SO4 (M/l)

Sodium Saccharin (g/l) Ascorbic acid (g/l) Wetting Agent (g/l) PH Temperature (“ C ) Current Density (mA/cm2)

H3J303 (g/l)

0.9 0.1

0-0.15 25 5 1

0.01 2.5-3.5 20-50 15-40

Nickel sulfamate (Ni(NH2S03)2) is used as the nickel ion source, and nickel bromide (NiBr2) is added as an anode activator. Ferrous sulfate (FeS04) is the iron ion

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source. Sodium saccharin is used as a stress reducer. The purpose of the ascorbic acid is to prevent the oxidation of ferrous ion.

Figure 4 shows the iron composition of the plated film vs. the TCE. The thermal expansion reaches a minimum at around 64% iron composition (Invar compo- sition), and the minimum value is 6.3 x 10-6/K, which is almost a half of pure nickel.

0 ' 1 2 1 1 ' ' 1 1 1 ' '

0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 Fe wt%

Fig. 4 Thermal coefficient of expansion vs. iron composition

2.2 High-aspect-ratio polymer etching A special plasma etchmg apparatus was set up for high- aspect-ratio polymer etching, which is used to pattern the plating mold. This etching machine is capable of etching up to 50 pm thick polymer film with 3 pm-line width definition and an etch rate of more than 2 pm/min. The details will be reported in another publication.

3 MILLIACTUATOR DESIGN AND PROCESS

3.1 Design Fig. 5 shows the design of the prototype milliactuator. The overall size including binding pads is 3 mm by 4 mm. The central block consisting of thin beams is the moving stage on which the slider is attached. The moving stage and the moving electrodes are suspended by flexure beams. There are a number of beams to sustain the vertical force applied to the slider. The stiffness ratio between the vertical direction and the lateral direction (the operational direction) must be maximized, because the structure needs to be very stiff in the vertical direction so that it does not deform a lot due to the vertical load, while the stiffness in the operational direction should be very soft. In this type of straight beam suspension design, the vertical stiffness of a suspension beam IC, is expressed by

(2) Eh3w I C , = ___

13 7

Fig. 5 Milliactuator design

where E is the Young's modulus, h is the structural height, w is the beam width, and 1 is the suspension length. On the other hand, the lateral stiffness I C , is expressed by

Ehw3 I C , = - 13 .

Thus, the stiffness ratio is

k Z / k = ( h / ~ ) ~ = (aspectratio)2. (4)

Consequently, the aspect ratio of the suspension beam must be maximized to increase the stiffness ratio.

3.2 Process In this prototype, a typical one-mask process [ l l] was used for the process simplicity. Fig. 6 shows the fabri- cation sequence. A sacrificial layer (PSG), a seed layer, and a thick polymer layer (40 pm) are depositedlcoated on a silicon substrate (a). The polymer layer is patterned with a high aspect ratio by plasma etching (b), followed by the electro-deposition of W A R layer (c). ARer the polymer layer and the seed layer are removed (d), the sacrificial layer is etched by buffered HF (e). This time- controlled etching releases the structure that consists of narrow beams, while large parts remain anchored to the substrate.

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(e)

Silicon Substrate

Sacrificial Layer

@ Seed layer (Metal)

Thick Polymer

Plated lnvar

Fig. 6 Fabrication Process of actuator

4 RESULTS

Fig. 7 shows an SEM photograph of the comb-electrode part of the Invar milliactuator, which is made from 40- pm-thick Invar film. The moving part which consists of meshed structure was released from the substrate. The tip of the suspended comb electrode was lifted a little because of the non-uniform residual stress distribution in the film.

Fig. 8 Milliactuator with slider block

doppler vibrometer (LDV). Fig. 9(a) shows the frequency response of the actuator without a dummy slider. The first resonant frequency was 2.7 kHz. After the dummy slider had been attached, the response of the same actuator was measured. Fig. 9(b) shows the result. The first resonant frequency dropped to 1.3 kHz because of the mass of the slider. The second resonance was around 23 kHz, and the slope between the first and second resonances was fairly smooth. This indicates that the device is suitable for use as a servo actuator.

MFREO RESP

(a) without slider M FR

-60.0

dB

-220

F X U X

(b) with slider Fig. 9 Frequency response of the actuator

Fig. 7 SEM photograph of Invar actuator

5 CONCLUSION

A microfabricated milliactuator for hard-disk drive servo actuator application was developed and tested. This ap- plication needs several requirements including dimension stability over a wide temperature range and large horizon- talAatera1 stiffness ratio. To meet those requirements, two

Fig. 8 shows an SEM photograph of an NiFe mil- liactuator which was used for operational characteristics measurement. A ceramic block whose size is equal to the size of the slider, was attached on the actuator.

An ac voltage with high dc bias voltage was applied to the actuator, and the amplitude was measured by a laser

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new fabrication technologies - Invar electro-deposition process and high-aspect-ration polymer etclllng - were developed. Invar films with thermal coefficients of ex- pansion as low as 6.3 x 10-6/K were electroplated. A plasma etching apparatus was set up, and a high-aspect- ratio mold was etched into up to 50 pm polymer with more than 2 p d m i n etch rate. A prototype milliactuator was designed, fabricated, and tested. The fist resonant frequency of 2.7 kHz was measured for the milliactuator without slider, and the frequency dropped to 1.3 kHz when the slider was attached. The frequency response curve indicates that this actuator is suitable for use as a servo actuator.

ACKNOWLEDGEMENTS

This research was supported by DAPRA contract number DABT63-95-C-0026. The authors wish to thank Dr. Neil Robertson and Dr. Mike Armstrong of IBM Almaden Research Center for helpful discussions throughout the course of this research.

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