INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF MICROMECHANICS AND MICROENGINEERING
J. Micromech. Microeng. 11 (2001) 1 6 www.iop.org/Journals/jm PII: S0960-1317(01)12880-9
Fabrication of an electrostatic
track-following micro actuator for hard
disk drives using SOI wafer
Bong-Hwan Kim and Kukjin Chun
Inter-university Semiconductor Research Center (ISRC) and
School of Electrical Engineering, Seoul National University, SOEE #038,
Kwanak PO Box 34, Seoul 151-742, Korea
E-mail: [e-mail]
Received 28 March 2000
Abstract
This paper presents track-following control using an electrostatic
microactuator for super-high density hard disk drives (HDDs). The
electrostatic microactuator, a high aspect ratio track-following microactuator
(TFMA) which is capable of driving 0.3 µg magnetic head for HDDs, is
designed and fabricated by a microelectromechanical systems process. It
was fabricated on a silicon on insulator wafer with a 20 µm thick active
silicon layer and a 2 µm thick thermally grown silicon dioxide layer; a
piggyback electrostatic principle was used for driving the TFMA. The first
vibration mode frequency of the TFMA was 18.5 kHz, which is enough for a
recording density of higher than 10 Gb in-2. Its displacement was 1.4 µm
when a 15 V dc bias plus a 15 V ac sinusoidal driving input was applied and
its electrostatic force was 50.4 µN when the input voltage was 30.7 V. A
track-following feedback controller is designed using a feedback nonlinear
compensator, which is derived from the feedforward nonlinear compensator.
The fabricated actuator shows 7.51 dB of gain margin and 50.98ć% of phase
margin for a 2.21 kHz servo bandwidth.
(Some figures in this article are in colour only in the electronic version; seewww.iop.org)
thermal and shape memory principles. Among these, the
1. Introduction
electrostatic, electromagnetic and piezoelectric types are
Conventionally, most hard disk drives (HDDs) have the frequently used as microactuators for HDDs. Most recently
characteristics of a recording density of a few gigabits per Naniwa et al [9] reported the comparisons of piggyback
square inch, a track density of 5 kTPI (track per inch) actuators. A disadvantage of the electromagnetic and
and a servo bandwidth of 500 600 Hz. These are realized piezoelectric types is that they locate the actuator far from the
by the technology of a 4 µm track width and Ä…0.5 µm read/write elements and therefore have limited bandwidth due
tracking accuracy. HDDs commonly utilize a voice coil motor to suspension vibration. The electrostatic actuator proposed in
(VCM) for track seeking and track following to satisfy these this paper can be assembled very near to the head, increasing
characteristics. In the future, however, the specifications for the bandwidth for servo control. However, almost all
HDDs will be a recording density of 10 Gb in-2, a track density electrostatic microactuators require higher driving voltage than
of 25 kTPI and the servo bandwidth of 2 kHz, which requires electromagnetic microactuators and are troublesome because
a 1µm track width, Ä…0.1 µm tracking accuracy and a greater of additional fabrication processes of the head. However, the
than 2 kHz servo bandwidth [1 4]. proposed microactuator only needs an easy fabrication process
Since conventional servo actuators cannot provide this using a silicon on insulator (SOI) wafer. The actuator presented
level of track accuracy, a new microactuator to realize a in this paper differs from other electrostatic actuators which use
system with the recording density of 10 Gb in-2 is needed. copper [2] or nickel [5, 10] as the structural layer.
Several types of microactuator are being developed based on In order to achieve fast tracking for a track pitch below
electrostatic [2, 3, 5], electromagnetic [6, 7], piezoelectric [8], 1 µm, an additional microactuator is needed for a conventional
0960-1317/01/010001+06$30.00 © 2001 IOP Publishing Ltd Printed in the UK 1
B-H Kim and K Chun
l
b
VCM : Track-Seeking
Microactuator : Track-Following Track-Seeking
Suspension
VCM
g
Disk
2
g
1
Slider
y
g
3
Microactuator & R/W Head
(a)
l
f
Slider
Figure 2. The schematic of the TFMA.
R/W Head Plate
Microactuator
method (FEM) using ANSYS software and the structure was
R/W Head
designed to be symmetric to reduce any residual stress. The
flexure stiffness and natural frequency of the TFMA were
calculated from the following equations:
Protection
Cover
K " EI (1)
(b)
1
I = (width)3(thickness) (2)
Figure 1. Design concept of dual-stage servo system and
12
head/slider/TFMA assembly. (a) Dual-stage servo system.

(b) Head-assembly.
K
wn = (3)
M
microactuator such as VCM. Therefore, a dual-stage servo
where K is the flexure stiffness, E is Young s modulus of
system [1] is essential for the fast tracking of the suspension,
silicon and M is the mass of the moving part of a TFMA.
which is composed of a VCM and a microactuator. In
To drive a 1 µm displacement within a 20 V applied voltage,
the dual-stage servo system, a track-following microactuator
the flexure and finger gaps were defined using a MATLAB
(TFMA) is used as a fine and high-bandwidth actuator while
simulation. As shown in figure 2, we derived an equation to
a conventional VCM is used as a coarse and low-bandwidth
calculate electrostatic force of the TFMA as follows:
actuator. Figure 1(a) shows the design concept of the dual-
1 t 1 tlf
2 2
stage servo system and the head/slider/TFMA assembly. The
Fes = n1µ0V + n2µ0V
2 g1 2 (g2 - y)2
assembly consists of a slider, a TFMA, a head and a protection
1 tlb
cover. Fusion bonding is applied to attach the TFMA on the top
2
- n3µ0V (4)
of the slider. Then, a magnetic read/write head is integrated on 2 (g3 + y)2
the head plate of the fabricated TFMA. After processing for a
where V is the applied voltage, µ0 is the permittivity of free
slider, a TFMA and a read/write head is ended, fusion bonding
space, t is the thickness of the TFMA, n1, n2 and n3 are the
is also used between the slider and the protection cover as
number of finger pairs, lf and lb are the length of the finger
shown in figure 1(b).
and finger bar, y is the displacement, g1, g2 and g3 are lengths
(see figure 2 for details). The values of g1, g2, g3, lb and lf ,
which are results of the MATLAB simulation, are 2.2 3 µm,
2. Design and fabrication
7 µm, 8 µm, 135 µm and 3 µm, respectively.
2.1. Design
2.2. Fabrication
In order to implement high bandwidth, high flexure stiffness
and high driving force, a SOI wafer and a comb-drive-type
A TFMA with a metal signal line to access the recording was
principle were used. The parameters that should be considered
fabricated with the following procedure, as shown in figure 3.
in the design are the natural frequency, easy fabrication,
The starting material is a 100 mm diameter SOI wafer with
operating stability, cost and the protection from data damage.
a 20 µm thick active silicon layer and a 2 µm thick silicon
The specifications of the TFMA are defined as follows.
dioxide intermediate layer on a 525 µm thick silicon substrate.
In order to simplify the fabrication process, we used only
" Size of microactuator: 1000 µm×300 µm.
three masks: a silicon nitride mask, a metal mask and a CVD
" Area of head plate: 300 µm×100 µm.
ć%
oxide mask. First, we doped the active silicon at 1000 C
" Applied voltage for 1 µm stroke: less than 20 V.
ć%
for 60 min after a pre-deposition with POCl3 at 1000 C for
" First natural frequency of microactuator: greater than
30 min. The junction depth was 2 µm and the sheet resistance
10 kHz.
was 2.6 / Silicon nitride, 2500 Å thick, was deposited by
.
For a 2 kHz servo bandwidth, the natural frequency of the low-pressure CVD and was patterned and etched by reactive
system should be greater than 15 kHz, resulting in a comb drive ion etching for the contact between metal and silicon. A 1 µm
of a 2µm flexure width, 3 µm comb width and 2.2 µm finger thick molybdenum (Mo) layer was sputtered and patterned.
gap. The flexure stiffness was calculated by a finite-element A 6 µm thick tetraethylorthosilicate (TEOS) deposited by
2
T
ra
c
k
-F
o
llo
w
in
g
Fabrication of an electrostatic TFMA
(a)
+ y
spring stator stator spring
R/W
head
plate
Figure 3. Process flow of the TFMA. mover
signal
plasma-enhanced CVD was used as a hard mask layer to deep
line
etch the silicon deep. This process used only one mask to
etch 1 µm Mo and 20 µm silicon. Finally, sacrificial oxide
spring stator spring
was etched in 7:1 BHF. Figure 4(a) shows a SEM image of
the fabricated TFMA with 2 µm wide flexures, 3 µm combs
(b)
and 2.2 µm gap. The aspect ratio is 10:1 for the silicon depth
l
f
versus width and the anisotropy is 0.997, which is adequate for
many applications.
g
3
mover
To fabricate the integrated signal line of a TFMA,
l
b
sequential etching is an important fabrication issue.
g
g
2
1
We developed the process to etch a 1 µm thick Mo and
20 µm thick silicon membrane using the only the TEOS hard
mask. Another significant issue is the stiction in the actuator.
t
stator
We developed a new anti-stiction coating procedure and used
it for this structure [11]. Figures 4(b) and (c) shows SEM
(c)
images of the TFMA with an integrated Mo signal line. This
figure indicates that an integrated Mo signal line of TFMA was
Figure 4. Fabricated microactuator. (a) SEM image of the
successfully made. fabricated TFMA with 2 µm wide flexures, 3 µm wide combs and
2.2 µm comb gaps. (b) SEM image of the microactuator. (c) Design
parameters of the microactuator.
3. Results and discussion
Vibrometer Controller Dynamic Signal Analyzer
system
3.1. Experiment
output system source
input out
The characteristics of the TFMA were measured using the Monitor
Digital
Digital Signal
Oscilloscope
measurement apparatus as shown in figure 5. The apparatus
Processor
G
consisted of a dynamic signal analyzer (HP 35670A), a
Voltage Amplifier
voltage amplifier (HP 6826A), a digital oscilloscope (Tektronix
D
B
TDS 754A), a fiber interferometer (Polytec OFV 512), a micro
amplified voltage
F
to F
C
spot head (Polytec OFV 130) and a vibrometer controller
: Microactuator Wafer
E
(Polytec OFV 3001). A
A : Fiber Interferometer
B : Micro Spot Head
The first natural frequency of the TFMA is 18.5 kHz
Optical Table
C : Laser Vibrometer Stand
and its displacement is 1.4 µm when a 15 V dc bias plus
D : Microscope
a 15 V ac sinusoidal driving input is applied. In particular,
E : XY Stage
its electrostatic force is 50.4 µN when the input voltage is
F : Probe
G : CCD Camera
30.7 V. The electrostatic force is calculated using equation (4)
and the measured parameters in table 1. Figure 6 shows the
Figure 5. Experimental apparatus for microactuator measurements.
frequency response of the TFMA with 2.2 µm finger gaps,
2.2 µm wide flexures and 3 µm wide combs. Figure 7
shows the driving voltage against the electrostatic force and an electrostatic force results in the measured nonlinear part of
the displacement characteristics for the same dimensions as in the microactuator. The modeling of the nonlinear part where
figure 6. When the voltage is applied to the microactuator, the displacement (y) is measured by a laser vibrometer and
3
B-H Kim and K Chun
Table 1. Measured parameter values.
One is SIMOX (separation by implanted oxygen) [15]. A
buried oxide layer is formed by high dose (<"1018 cm-2)
µ0 8.854 × 10-12 Fm-1 g1 1.82 µm
ć%
oxygen implantation followed by high-temperature (1300 C)
t 20 µm g2 4.55 µm
n1 800 g3 7.01 µm annealing. Buried oxide and thin-film silicon have already
n2 832 lf 3.34 µm
been used as piezoresistors and high-temperature silicon
n3 30 lb 134 µm
sensors [16]. Another fabrication process is ZMR (zone
melting recrystallization). A ZMR structure is fabricated by
the deposition of a polysilicon film on an oxidized Si wafer
[17]. The final process is bonded SOI [18]: SDB (silicon direct
bonding), SFB (silicon fusion bonding) and DWB (direct wafer
bonding) all have the same meaning in the SOI techniques.
Wafer bonding using thermal oxide was initially proposed
by Lasky in 1986 [19]. The basic process sequence for the
fabrication of the bonded SOI wafer is as follows. First,
an active wafer is bonded to the handle wafer, which has a
layer of thermally grown silicon dioxide upon it. Then, the
bonded pair wafer is annealed at a high temperature of about
ć%
1100 C to increase the bonding strength and to remove the
interfacial voids. In this paper, we used bonded SOI wafers
for fabrication because the bonded SOI technique can freely
change the thickness of silicon dioxide.
Most of the SOI applications are CMOS and SOI MEMS
devices with thin active silicon layers and less than 1 µm thick
silicon dioxide layers. However, Moore et al [20] showed an
Figure 6. Frequency response of the TFMA: 2.2 µm finger gaps,
accelerometer with a 3 µm Si layer on a 4 µm SiO2 layer on a
2.2 µm wide flexures and 3 µm wide combs.
thick 400 µm Si substrate. The accelerometer was fabricated
by HF and KOH wet etching. Timothy et al [21] showed a
new technique for providing both electrical isolation and an
embedded interconnect to a SOI based inertial sensor. This
process is similar to the SCREAM process [22].
3.3. Sacrificial oxide etching with metal line
Since Nathanson et al [30] etched sacrificial oxide to fabricate
a free-standing gold microbridge, used as a resonant gate
transistor, silicon dioxide sacrificial layer etching has become
a major surface micromachining method of fabrication of
microsensors and microactuators, which are often made of
polysilicon. The various wet etchants, such as HF (typically
Figure 7. Characteristic curve of voltage against electrostatic force
50%), BHF, NH4F/HF solutions and HNO3/HF solutions have
and displacement: 2.2 µm finger gaps, 2.2 µm wide flexures and
3 µm wide combs. been used to etch silicon dioxide. In particular, high selectivity
between the Al and the sacrificial oxide is crucial for the
mechanical, electrical or optical properties of the MEMS
theflexure stiffness is calculated using FEM after measuring the
structures because Al is widely used as a metal signal line
real value by SEM. The modeling and experiment values of the
in the IC industry [31]. Gennissen [32] reported sacrificial
electrostatic force of the microactuator are almost the same.
oxide etching compatible with Al metallization using 73%
HF and several mixed HF solutions. For most of the CMOS
3.2. Micromachining using SOI
compatible processes, Al is a useful material because of easy
sputtering, good quality and appropriate resistivity. Therefore,
Recently, as surface micromachining has appeared to realize
the characteristics of the Al are reported in many papers [31
mechanical structures, commercially available SOI wafers
have been attractive for smart power sensors and actuators, 34], but Al is also easily attacked by HF solutions. Although
as the etch component is dielectrically isolated [12, 13]. some researchers showed that pad etch had a higher selectivity
In particular, moving elements such as the cantilever, to Al when compared to standard BHF [34], this etchant has
microbridge, micropump, microvalve and diaphragm can lower etch rate to sacrificial oxide than HF solutions.
be fabricated by SOI. Many various SOI techniques for To overcome this limitation, we had to find some kind
microelectromechanical systems (MEMS) are shown in of material for the metal line on the TFMA because our
table 2. The silicon dioxide layer of SOI for sensors and structure required longer etch times. In this paper, since it
actuators is used as the etch stop, for dielectric isolation and was also critical that metal signal line should be passivated
as a sacrificial layer. In general, SOI wafer can be classified during sacrificial oxide etching, several metals such as Au, Al,
into three groups concerning the fabrication process [14]. Mo and Ti were recommended. All of these were good as
4
Fabrication of an electrostatic TFMA
Table 2. Comparison of SOI techniques.
Thickness of SOI
SOI SiO2
Application type Active Si SiO2 etching Reference
Pressure sensor SIMOX 0.2 µm 0.4 µm [23]
Pressure sensor SIMOX 0.2 µm 0.37 0.4 µm [24]
Accelerometer SDB 3 µm 4 µm HF&H2O [20]
Inertial sensor SDB 45 µm 1 µm HF [21]
Microactive probe SDB 0.2 µm 1.5 µm BHF [25]
Resonator SDB Various 1 µm BHF [26]
thicknesses
Optical chopper No data 1 µm 1 µm 50% HF [27]
Tunneling probe No data 20 50 µm 1 µm HF [28]
Capacitive accelerometer SDB No data No data No data [29]
TFMA (Our work) SDB 20 µm 2 µm BHF
Table 3. Etch rates of several solutions [Å/min.].
Wet oxide TEOS PSG Nitride Al Mo
HF (49%) 17 625 39 690 47 784 148.8 383.3 1.5
7:1 BHF 1326 1068 10 242 10.2 30 5
10:1 HF 484 1572 9216 15 3200 1.5
Solution 1a 890 1861 13 746 7.8 9 3.3
Solution 2b 426 1520 39 624 28.8 412.5 0.7
Solution 3c 168 483 2833 2.7 65.5 0.2
a
Solution 1 NH4F(40%):HF(49%):glycerin(C3H8O3) = 4:1:2.
b
Solution 2 HNO3:HF(49%):glycerin = 1:5:10.
c
Solution 3 NH4F:CH3COOH:C2HO2:deionized water = 13.5:31.8:4.2:
50.5.
a mask for etching. Considering only resistivity, Au is very the Micromachine Technology Development Program. In
good, but Au is difficult to dry etch and fine pattern. Ti is addition, this work was partly funded by the Ministry of
attacked easily during sacrificial oxide etching by HF solutions. Education. The authors would like to thank Sangjun Park,
Compared with Al, Mo is a more durable material to HF and Hyeon-Cheol Kim, Jong-Won Lee, Seung-Han Kim, Hyo-
BHF solutions, and has the better adhesion to silicon or silicon Jung Lee, Woo-Kyeong Seong, as well as Professor Dong-Il
nitride. This is why we selected Mo as a metal signal line. (Dan) Cho for their invaluable help.
Table 3 shows the etch rates for several solutions in Ångstroms
per minute.
References
4. Conclusions
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6