It is projected that by the year 2001, the disk drive industry will be shipping products with track density on the order of 25,000 tracks-per-inch, which would require a servo bandwidth of at least 3 kHz. This paper presents initial fabrication results of an industry and government supported research project at Caltech and UCLA to develop piggyback electromagnetically driven microactuators for such applications, which are fabricated using the state-of-the-art silicon micromachining techniques

Desai, Amish

Miller, Raanan

Miu, Denny K.

Tai, Yu-Chong

Tang, Weilong

Temesvary, Viktoria

Caltech Authors - Main

2964 IEEE TRANSACTIONS ON MAGNETICS, VOL. 31, NO. 6, NOVEMBER 1995 
Silicon Micromachined Electromagnetic Microactuators 
for Rigid Disk Drives 
Weilong Tang 1 , Viktoria Temesvary 1 , Raanan Miller 2 , Amish Desai 2 , 
Yu-Chong Tai 2 and Denny K. Miu 1 
1 Mechanical Engineering, UCLA, Los Angeles, CA 90024-1597 
2 Electrical Engineering, Caltech, 116-81, Pasadena, CA 91125 
Abstract 
It is projected that by the year 2001, disk drive in-
dustry will be shipping products with track density on 
the order of 25,000 tracks-per-inch, which would require 
a servo bandwidth of at least 3 kHz. This paper presents 
initial fabrication results of an industry and government 
supported research project at Caltech and UCLA to de-
velop piggyback electromagnetically driven microactu-
ators for such applications, which· are fabricated using 
the state-of-the-art silicon micromachining techniques. 
Introduction 
Since 1991, track density of magnetic recording rigid 
disk drives has gone up approximately 30% every year. 
Together with a similar increase in bit density, they 
enable the industry to maintain an annual growth of 
areal recording density on the order of 60%. Currently, 
the industry is shipping products with track density in 
excess of 5,000 tracks-per-inch (TPI) which is expected 
to increase to 12,000 by the year 1998 and to 25,000 by 
2001. At a track density of 25,000, the track-to-track 
spacing is 1 µm and the allowable off-track error is less 
than 120 nm. 
There are many sources of off-track errors in rigid 
disk drives which include contributing factors such as 
mechanical resonances, spindle runouts, temperature 
drifts, humidity variations, external shocks/vibrations, 
bearing hysteresis, cable bias, servo pattern errors, and 
residual vibrations, etc. Obviously, the higher the servo 
bandwdith, the easier it is to correct these errors; and 
the higher the track density, the more important it is 
to have high-bandwidth servo system. Unfortunately, 
servo bandwidth of computer rigid disk drives for the 
last ten years has been limited to less than 600 Hz, 
mainly by unavoidable inherent in-the-loop mechanical 
resonances. For 25,000 TPI, it is projected that servo 
bandwidth must be higher than 3 kHz; furthermore, 
with spindle speed increased to 7 ,200 RPM, the required 
bandwidth will increase to 4 kHz. 
It is well recognized throughout the industry that 
this level of performance simply cannot be achieved by 
using or improving upon the existing hardware. One 
solution is to add a dual-stage piggyback microactuator 
system similar to that of optical drives (see References 
[1] and [2]). 
At UCLA and Caltech, for the last three years, we 
have been involved in a collaborative effort to develop 
micromachined electromagnetcially driven microactua-
tors for future high-track-density computer disk drives. 
Results and Discussions 
Figure 1 shows a previously documented silicon mi-
crogimbal which is now used as the structural platform 
for a planar electromagnetically driven piggyback mi-
croactuator (see Reference [3]). This active device is 
fabricated within the center coupon of the silicon mi-
crogimbal such that the R/W slider can be moved rela-
tive to the gimbal/suspension assembly. This approach 
has the advantage that any structural resonances in the 
suspension arm is completely eliminated from the servo 
loop, allowing a substantial increase in servo bandwidth. 
In addition, this design can be very low-profile such that 
it is consistent with the disk-to-disk spacing require-
ment of future products. 
Figure 2 shows a detail schematic of the planar vari-
able reluctance microactuator which uses wrap-around 
copper coils (the entire device fits inside the center 
coupon of the microgimbal which measures less than 1.5 
mm square). Instead of a solid platform, a set of hair-
pin-like planar microsprings are micromachined into the 
silicon microgimbal to allow in-plane motion relative to 
the outside frame. Unlike the serpentine springs of the 
microgimbal, these planar microsprings are designed to 
have maximum in-plane compliance while maintaining 
adequate out-of-plane, pitch and roll stiffnesses. 
As shown in Figure 2, a pair of thin-film variable-
reluctance microactuators are used to move the 
slider/recording head in the in-plane direction, which 
is attached to the center movable permalloy core. The 
two stationary enclosing permalloy cores of the microac-
0018-9464/95$04.00 © 1995 IEEE 
tuator are mounted on the outside frame of the center 
coupon of the microgimbal and are constructed of one 
layer of permalloy with wrapped-around helical flat cop-
per coils. When a voltage is applied, magnetic force will 
be exerted on the slider, pulling it towards the stator 
and resulting in in-plane/cross-track micron-level fine 
motion. This design has the important advantage that 
the required fabrication technology is very similar to 
that of thin-film inductive heads, leveraging the exper-
tise of the magnetic recording industry. 
Figure 3 summaries the fabrication process for the 
electromagnetic microactuator with wrap-around cop-
per coils which requires six masks, in addition to those 
necessary for the microgimbals. After oxidation and 
back-etch, the first step involves thermal evaporation of 
the titanium/copper seed layer and patterning of the 
photoresist electroplating mold for the bottom copper 
traces. Subsequent to electroplating and removal of the 
seed layer, a thin layer of photoresist will be patterned 
and hot baked to form an insulation layer. Then an-
other seed layer will be evaporated and permalloy will 
be electroplated using the frame-plating technique typ-
ical of the thin-film head industry. 
Finally another photoresist insulation layer and the 
top copper will be deposited which wraps around the 
permalloy core and makes contact with the bottom cop-
per layer to form complete copper coils. At the end 
of the microactuator fabrication, thick photoresist is 
patterned as the RJE mask and both the serpentine 
microsprings of the microgimbal and the hair-pin pla-
nar microsprings of the microactuators will be plasma 
etched until free-standing. Figure 4 shows one of our 
first working prototypes. 
The advantage of the microactuator design as shown 
in Figures 2 and 4 is that it is very space efficient such 
that a large number of turns can be packaged in a given 
real estate, maximizing the available reluctance force. 
The disadvantages, however, are mc;i.ny. Since there are 
many contact vias between the two copper layers, as is 
well known in the thin-film head industry, the electrical 
resistance could be quite high and more importantly, 
production yield could be a potential problem. A better 
design would be that of a planar copper coil similar to 
that of the thin-film head. 
Furthermore, since the in-plane dimensions are 
rather limited due to the size of the microgimbal (which 
has to be consistent with that of the 30% form factor 
pico-sliders), the only way to increase force, in addition 
to increasing the number of turns, is to increase the 
thickness of the permalloy cores. Specifically, for the 
variable-reluctance microactuator, the resulting force is 
(1) 
where N = number of turns, I = current, w = width, 
t = thickness of permalloy and g = gap. If we assume 
that in the worst case the microactuator has to move a 
pico-slider (with mass M = 2 mg) at a displacement of 
2965 
8 = 0.5 µm (since the current VCM design is capable of 
5,000 TPI) at w = 3 kHz, the force required is 
F = M8w2 = 0.3mN (2) 
then for N = 40, I = 85 mA, w = .25 mm, g = 10 
µm, the required permalloy thickness t is 20 µm. In 
the current design, increasing the permalloy thickness 
is a potential problem for the step coverage of the top 
copper layer. 
Finally and most importantly, as can be seen from 
Figures 2 and 4, the contact area between the movable 
platform and the slider is very small, potentially de-
grading the mechanical integrity of the entire system. 
Figure 5 shows an alternative design where instead of 
a copper coil wrapped around a single layer of permal-
loy, two layers of planar coils are sandwiched between 
two layers of permalloy. In this design, the top half of 
the center coupon is devoted to the variable reluctance 
microactuators and the bottom half to the planar mi-
crosprings, significantly increases the contact area be-
tween the movable platform and the slider. 
The fabrication process for the electromagnetic mi-
croactuator with planar coils requires four additional 
masks compared to the earlier design. In this new de-
sign, cavities are first plasma-etched into the silicon sub-
strate and the bottom permalloy layer is electroplated 
into the cavities which are subsequently planarized us-
ing photoresist insulation layer. After the deposition of 
two layers of planar copper coils and insulations, the top 
layer of permalloy is electroplated which makes contact 
with the bottom layer, completing the magnetic circuit. 
Since the permalloy layers are either buried into the sil-
icon or is fabricated in the last photolithography step 
before RIE, step coverage is less of a problem resulting 
in a thicker permalloy (at least 10 µm). Figure 6 shows 
a fabricated electromagnetic microactuator with planar 
copper coils. 
More results related to design, analysis and testing 
of these micromachined devices will be presented in a 
later paper (see [4]). 
References 
[1] Mori, K., ... , "A Dual-Stage Magnetic Disk Drive 
Actuator Using A Piezoelectric Device For A High 
Track Density,'' IEEE Trans. on Magn., Vol. 27(6), 
1991, pp. 5298-5300. 
[2] Fan, L.-S., ... , "Magnetic Recording Head 
Positioning at Very High Track Densities Using a 
Microactuator-Based, Two-Stage Servo System,'' to ap-
pear in the IEEE Trans. on Indust. Elect., 1995. 
[3] Temesvary, V., ... , "Design, Fabrication and 
Testing of Silicon Microgimbals for Super-Compact 
Rigid Disk Drives," IEEE/ASME J. of MEMS, Vol. 4, 
No. 1, March 1995, pp. 18-27. 
[4] Tang, W., ... , "Design, Fabrication and Testing 
of Micromachined Electromagnetic Microactuators for 
Rigid Disk Drives,'' in preparation. 
2966 
Stainless-Steel 
Loadbeam 
30% Pico Slider 
Planar 
Piggyback 
Micro actuator 
Figure 1: Design concept for a silicon micromachined elec-
tromagnetic piggyback planar microactuator for 
high TPI applications. 
Wrap-Around 
Copper Coils 
(only top layer 
is shown) 
Movable 
Permalloy 
Cores 
Hair-Pin 
Planar 
Springs 
Figure 2: Schematic drawing of the silicon microactuators 
with wrap-around copper coils. 
Si02 * evaporate Ti/Cu seed layer 
~ * electroplate bottom Cu conductor 
•••• * spin PR as insulator 
* evaporate Ti/Cu seed layer 
* electroplate permalloy core 
* remove excess permalloy 
* spin PR as insulator 
····· * evaporate Ti/Cu seed layer 
* electroplate top Cu conductor 
Figure 3: Processing steps for the silicon microactuators 
with wrap-around copper coils. 
Figure 4: SEM micrograph of a fabricated electromag-
netic microactuator with wrap-around copper 
coils (the length of the center coupon is about 
1.25 mm). 
Stationary 
Permalloy 
Cores (bottom) 
Planar Copper Coils 
(only one layer is shown) 
Movable 
Permalloy 
Cores 
Stationary 
Permalloy 
Cores (top) 
Hair-Pin 
Planar 
Springs 
Figure 5: Schematic drawing of the silicon microactuators 
with planar copper coils. 
Figure 6: SEM micrograph of a fabricated electromagnetic 
microactuator with planar copper coils. 


Silicon micromachined electromagnetic microactuators for rigid disk drives

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Silicon micromachined electromagnetic microactuators for rigid disk drives

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