Micro-motors having rotors with diameters between 60 and 120 μm have been fabricated and driven electrostatically to continuous rotation. These motors were built using processes derived from IC (integrated circuit) microcircuit fabrication techniques. Initial tests on the motors show that friction plays a dominant role in their dynamic behavior. Observed rotational speeds have thus far been limited to several hundred r.p.m., which is a small fraction of what would be achievable if only natural frequency were to limit the response. Experimental starting voltages are at least an order of magnitude larger than had been expected (60 V at minimum and above 100 V for some structures). Observations of asynchronous as well as synchronous rotation between the driving fields and the rotors can be explained in terms of the torque/rotor-angle characteristics for the motors

Fan, Long-Sheng

Muller, Richard S.

Tai, Yu-Chong

English

Caltech Authors - Main

IC-PROCESSED MICRO-MOTORS: DESIGN, TECHNOLOGY, and TESTING 
Yu-Chong Tai, Long-Sheng Fan, and Richard S. Muller 
An NSFlIndustrylUniversi~ Cooperative Research Center 
Dep’t of EECS and the Electronics Research Laboratory 
University of California, Berkeley CA 94720 
Berkeley Sensor and Actuator Center 
ABSTRACT 
Micro-motors having rotors with diameters between 60 and 
120 pm have been fabricated and driven electrostatically to con- 
tinuous rotation. These motors were built using processes 
derived from IC micro-circuit fabrication techniques. Initial tests 
on the motors show that friction plays a dominant role in their 
dynamic behavior. Observed rotational speeds have thus far 
been limited to several hundred rpm which is a small fraction of 
what should be achievable if only natural frequency were to limit 
the response. Experimental starting voltages are at least an 
order-of-magnitude larger than had been expected (60 volts at 
minimum and above 100 V for some structures). Observations 
of asynchronous as well as synchronous rotation between the 
driving fields and the rotors can be explained in terms of the 
torquelrotor-angle characteristics for the motors. 
I. INTRODUCTION 
Design considerations for micro-motors have been dis- 
cussed recently [ 1,2]. Successful implementation of both step- 
ping and synchronous motors and initial testing results were 
presented by our group at the 1988 Int. Electron Devices Meet- 
ing [3]. In this paper, we describe new aspects of these 
motors, including theoretical design, advanced polysilicon tech- 
nology, and phenomenological mechanisms in several micro- 
motors that have been produced using IC processing [4]. 
11. DESIGN 
Shown in Fig. 1 is an SEM photograph of a three-phase 
micro-motor with 12 stators and a 4-pole rotor that was fabri- 
cated using a three-layer-polysilicon process. The radius of the 
rotor is 60 pm and the air gap between the rotor and stator is 2 
pm. Fig. 2 shows a schematic cross section of the motor. 
Several design features can be seen from Fig. 2. First, the 
micro-motor is built on a composite layer composed of 300 nm 
of silicon dioxide and 1 pm of silicon nimde. This layer helps 
to prevent electrical breakdown between the motor and the sili- 
con substrate. The 1-pm nitride layer is also needed to protect 
the oxide from HF etching while the sacrificial layer is being 
dissolved. Experimentally, we have found that this layer can 
withstand more than 500 V before breakdown occurs. A 
second feature is a ground plane, formed of first-layer polysili- 
con, situated beneath both the rotor and the stator. The ground 
plane, which acts as an electrostatic shield between the rotor 
and the substrate, improves the performance of the motor by 
eliminating vertical Coulombic forces on the rotor which result 
in increased frictional drag. Another advantage of the ground 
plane is that it provides the possibility of biasing the rotor since 
it is in electrical contact with the rotor through its hub. This 
provides an extra freedom for control of the motor compared to 
a floating rotor design- a freedom that might be useful to 
increase the electrical drive on start-up, for example. Another 
effective friction-reducing feature in the motor shown in Fig. 2 
is the use of silicon-nitride spacers at the vertical walls of the 
rotor and the stators. Figure 3 magnifies the hub and stator 
parts to illustrate this feature. The nitride spacer between the 
rotor and the hub forms a lubricating as well as a wear- 
protecting material with a smaller coefficient of friction and 
better wear-withstanding characteristics than are found for 
polysilicon. 
III. FABRICATION 
The fabrication process is based on the pin-joint technol- 
ogy, reported in [5,6]. In this discussion, we emphasize tech- 
niques to fabricate the design features discussed in the last sec- 
tion. Figure 4 shows the major steps in the motor fabrication. 
In Fig. 4a formation of the 300-nm thick ground plane on top 
of the oxide-nitride composite layer is indicated. After this 
layer is deposited, the wafers are covered with 2.2 pm of phos- 
phosilicate glass(PSG) which acts as the sacrificial layer. After 
opening anchor spots for the stator, a 1.5 pm second-layer 
polysilicon is deposited and oxidized to have a 100 nm thermal 
TH0249-3~89~0000~0001$1.0001989 IEEE 1 
oxide on top as shown in Fig. 4b. The polysilicon-oxide com- 
posite layer then is plasma-etched to form the rotor and stators. 
A 340-nm LPCVD nitride is then deposited (Fig. 4c) and 
reactive-ion-etched(R1E) to form the nitride spacers(Fig. 4 d). 
The thin oxide layer on the polysilicon is used to protect the 
polysilicon during the nitride RIE etch. Once the nitride 
spacers are fomed, the process follows the outline already 
reported [5,6], to form the axle-bushing hub. The completed 
motor (prior to being freed from the sacrificial support layer) is 
shown in Fig. 4e. 
IV. EXPERIMENTAL PROCEDURE 
The setup used to drive the motors is shown in Fig. 5. Its 
function is as follows: a square-wave, extracted from a pulse 
generator, is sent to a 'ITL counter which is linked to a TTL 
decoder. The decoder sends the three-phase signals to the a 
high-voltage driver and also feeds back a signal to restart the 
counter after each three counts. 
Three voltage phases are therefore repeatedly generated at 
the driver circuit and sent through current-limiting resistors to 
the micro-motors. The frequency of the phase voltage is con- 
trolled by the square-wave generator and the magnitude of the 
phase voltage is controlled by the voltage supplied to the 
driver. 
The voltage phases supplied to the micro-motors are 
shown in Fig. 6. The twelve stator poles are energized in a 
three-phase pattern. During tests, the phase voltage is first set 
to a constant value and the phase frequency then is varied from 
high to low until rotational motion of the motor is observed 
(through a video attachment on the microscope). Motor move- 
ments are video-recorded to determine the speed of rotation and 
other dynamical properties. 
V. MOTOR DYNAMICS 
Two-dimensional numerical simulators are used to deter- 
mine the static-torquehotor-position characteristics. The simula- 
tion results of the torque/position characteristics associated with 
4 qui-positioned stators energized at 100 V (and the remaining 
8 grounded) is shown in Fig. 7. The torque on the rotor is 
obtained by multiplying the simulated torque density times the 
thickness of the rotor which neglects any three-dimensional 
effects. Figure 7 has a span of only 90 degrees because the 
torque/position characteristic is periodic for this 4-pole rotor. 
A natural frequency f N ,  which arises from energy 
exchange between the electric field and the moving rotor, is an 
important motor parameter. The frequency f N  can be calcu- 
lated by considering the rotor mechanics near an equilibrium 
(T = 0) point and assuming zero friction. Then 
f N = $ f l  
where I is the rotational inertia of the rotor and -T is the 
slope of the T vs. 0 curve at T = 0. 
For the motor of Fig. 1 ,  the rotational inertia of the rotor 
is 2.01~10-*~ kg-m2 and T is approximated as 1.83~10-" Nt- 
m/rad. Hence f N  = 1.5 kHz (excitation voltage = 100 v). Note 
that the T vs. 0 curve is roughly linear only between * 15". 
When the starting angle Os is outside the f 15" range, the 
oscillating frequency will differ from fN because T (0) depends 
on 0. In general, the oscillating frequency can be calculated 
from 
here q is a dummy variable and T ( q )  is the torque in Fig. 7. It 
is straightforward to show that f (0,) in Eq. ( 2 )  reduces to f~ 
in Eq. ( 1 )  inside the f 15" range where T ( 0 ) = - T '  x0.  Equa- 
tion (2) shows that the oscillating frequency is a function of the 
starting angle Os.  As seen in Fig. 7, IT x 0 I 2 IT (0) I for all 8 
in [0, e,]. Hence, "- "- 
Equation (3) shows that f N  (in our case 1.5 kHz for 100 V 
applied to a single phase of 4 stators) is the maximum oscillat- 
ing frequency. 
Since the electrostatic torque acting on the rotor is propor- 
tional to the square of the voltage, the natural frequency at a 
different phase voltage Vp is 
~ N ( V ~ )  = 1.5(vp)2 100 kHz (4) 
As an example, if Vp equals 200 V, f N  is 6.1 kHz. 
The natural frequency of the motor sets an upper bound 
for the rate at which the the rotor can respond to a switched 
field on the stators. We can argue, for example, that 1 is
the minimum time for the rotor to travel from a starting angle 
8, to 8 = 0 if the initial angular velocity is zero as is the case 
for start/stop-mode stepping motors. In terms of a maximum 
rotational speed w,, we have 
4 f N  
240f N 
a, = ~ x60=-  rpm 
1 x n  
4 f N  
where n is the number of steps per revolution. As an example, 
with Vp = 200 V and n = 12, the maximum speed of our s tep  
ping motor is predicted to be 120 krpm using Eq. (5). 
Neglected in the analysis presented thus far are loading 
effects (other than inertia) on the rotor. These effects, princi- 
pally friction, reduce the effective maximum frequency below 
the calculations in Eqs. (4) and (5). From our measurements, 
the frictional effects are completely dominant because the 
observed maximum rotational speed at 200 V is 150 rpm. 
With friction, the term T ( q )  in Eq. (2) should be replaced by 
Tm,(q), the net torque on the rotor which is smaller than T ( q ) .  
At this point insufficient information is available about the fric- 
tional mechanisms in our motors to go further with this 
analysis. 
The foregoing discussion refers to open-loop drive of the 
motors, which is the mode used for our experiments. The 
natural frequency limitation on rpm could be exceeded with 
closed-loop control, which would be possible with the addition 
of a rotor-position sensor. 
VI. SYNCHRONOUS AND ASYNCHRONOUS MOTION 
Another surprising result from dynamical studies of the 
motors in addition to the frictional effects discussed above is 
the observation of asynchronous as well as synchronous motion 
in the motors. Specifically, we observe that it is possible for 
the rotor to rotate in opposition to the rotational sense of the 
phasing field. This behavior, we believe, can be understood in 
terms of the dynamic balance between electrostatic-drive torque 
and that due to friction. It is easiest to introduce this concept 
by considering the motor driven by only one phase of the 
three-phase voltage drive. 
Figure 8 (a) shows the voltage connections for single- 
phase drive of the motor of Fig. 1 as well as a defining condi- 
tion for the initial position of the rotor (8 = 30 "). The angle 8 
in Fig. 8 (a) is measured from a rest (T = 0) position. Shown 
in Fig. 8 (b) (which is similar to Fig. 7) is the corresponding 
normalized torque/position characteristic. Indicated by straight 
lines on Fig. 8 (b) are assumed constant frictional-torque values 
which, of course, oppose the applied torque T .  
Consider that the horizontal stator (in Fig. 8 (a)) is ener- 
gized. Since T exceeds the frictional value, the rotor will 
begin to rotate in the clockwise direction and will accelerate 
until it reaches angle P at which the frictional torque equals the 
the driving torque. As 8 decreases toward zero, the rotor will 
decelerate due to friction. Whether the rotor passes through 8 = 
0 depends upon specific values for frictional and driving torque 
as well as on the rotor inertia. If it passes 8 = 0, it will con- 
tinue to decelerate between 8 = 0 and 8 = Q. The rotor may 
stop anywhere between angles P and Q, but if it can pass point 
Q, it will decelerate more strongly because the driving torque is 
reversed in this region. If the rotor stops between 8 = Q and e 
= -30°, it will accelerate again, but this time in a counter- 
clockwise direction. Once again, the rotor will repeat a similar 
acceleration-deceleration-stop cycle except that the direction of 
rotation is different. Several cycles may occur, but eventually 
the rotor will stop between 8 = P and 8 = Q. 
The final resting place of the rotor after the cyclic motion 
discussed above (when the motor is energized by a single 
phase) indicates whether synchronous or asynchronous rotation 
will occur in the motor when it is driven by three phases. 
a) Three-phase synchronous mode 
Normalized three-phase torque/position characteristics of 
the motor are shown in Fig. 9. In Fig. 9, the drive voltages are 
phased f30" apart because the motor has twelve stator poles. 
The starting position is again assumed at 8 = 0. In Fig. 9, 8 = 
P and 8 = Q are the angles at which frictional and driving 
torque are in balance for phase A and 8 = R is one of the 
corresponding angles for phase B. We conclude that if a static 
three-phase excitation of the stator field results in a final resting 
position for the rotor between 8 = P and 8 = R or any of the 
corresponding *=-shifted regions, the motor will operate 
6 
synchronously (where m is an integer). 
The explanation is in the following. Based on the discus- 
sion for single-phase excitation above, the rotor would tend to 
come to rest inside the frictional band, either between 8 = P 
and 8 = R, or between 8 = R and 8 = Q. If the former, then 
the rotor will rotate counter-clockwise because of the positive, 
larger-than-friction, driving torque from phase B. A similar 
behavior would be repeated with respect to phase C so that the 
rotor would rotate synchronously with the field following 
phases ... ABCABCABC .... 
6) Three-phase asynchronous mode 
The motor and field rotation will be in opposite directions, 
however, if the rotor rest position under the conditions 
described (in section (a) above) occurs between 8 = R and 8 = 
Q. or its fmK-shifted regions. The reason is as follows. If, 
after phase A, the rotor rests between 8 = R and 8 = Q, the 
next phase (B) will not be able to rotate the rotor because 
inside that region the driving torque produced by this phase is 
smaller than the maximum frictional torque. However, phase C 
will apply a negative torque, which can overcome the friction 
on the rotor and make it rotate in a clockwise direction, oppo- 
site to the rotation of the stator fields. For continuous motion 
phase C must leave the rotor at rest between 8 = R" and 8 = 
Q". Then, next phase A will not be able to turn the rotor 
because of the friction. However, the next cycle of phase B will 
again cause clockwise rotation. A feature of this motion is that 
the rotor rotates at only one-half the frequency of the driving 
6 
3 
fields because it is driven only during a half cycle of one 
phase; for an applied signal ... ABCABC ..., the rotor rotates 
. . . ACB ACB .... 
Experimentally, both modes have been observed. In some 
cases, it appears that both synchronous and asynchronous 
behavior alternates spontaneously. It is believed this behavior 
can be explained by irregular friction in real motors, since fric- 
tion determines the critical angle values for the rotational varia- 
tions described above. 
VII. CONCLUSIONS 
Initial tests of electrostatically-driven IC-processed micro- 
motors show that frictional effects play a dominant role in their 
dynamic behavior. Rotational speeds in these motors are only 
a small fraction of what should be achievable if only natural 
frequency were to limit the response of the rotor. The experi- 
mental starting voltages are at least an order-of-magnitude 
larger than had been expected (60 volts at minium and above 
100 V for some structures). 
Observations of asynchronous as well as synchronous rota- 
tion between the driving fields and the rotors are explained in 
terms of the torquehotor-angle characteristics. 
Acknowledgements We thank Profs. D.J. Angelakos, R.M. 
White, D. Giandomenico, and Paul T. Yang for valuable dis- 
cussion, and K. Voros, R. Hamilton, and the staff of the Berke- 
ley Microfabrication Laboratory for assistance in processing. 
REFERENCES 
W.S. Trimmer and K.J. Gabriel, "Design Considera- 
tions for a Practical Elecuostatic Micromotor," Sen- 
sors and Acruators, 11, 189-206, (March, 1987). 
S.F. Bart, T.A. Lober, R.T. Howe, J. H. Lang, and 
M.F. Schlecht, "Design Considerations for Microfabri- 
cated Electric Actuators," Sensors and Actuators, 14, 
269-292 (July, 1988). 
L.S. Fan, Y.C. Tai, and R.S. Muller,"IC-processed 
Electrostatic Micro-motors," 1988 IEEE Int. Elecrr. 
Devices Meeting, 666-669, San Francisco, CA, 
December 11-14, 1988. 
Patent Pending. 
R.S. Muller, L.S. Fan, and Y.C. Tai, "Micromechani- 
cal Elements and Methods for Their Fabrication," 
U.S. Patent 4,740,410, issued to the Regents of the 
University of California, April 26, 1988. 
[6] L.S. Fan, Y.C. Tai, and R.S. Muller,"Integrated mov- 
able micromechanical structures for sensors and 
actuators," IEEE Trans. Electro. Devices, vol. ED- 
35, pp.724-730, June 1988. 
Figure 1. SEM photograph of a fabricated micro- 
motor; rotor diameter = 120 flm. 
hub ,3rd plysilicon 
TI 1 2nd plysilicon 
I 1  I Si3N4 lubricant 
ground plane 
si02 Si3N4 
Figure 2. Schematic cross section of the micro-motor 
in Fig. 1. 
4 
lHub Stator 
( 4  
I < Substrate > 
I 
I 
silicon nitride 
E A  : polysilicon 
: silicon nitride 
: silicon dioxide 
Figure 3. Cross section showing construction of the 
hub, rotor, and stator for the micro-motor of Fig. 1. 
l,,,,,,,,,,-,,,,,,,,~ I 
I 
I 
silicon- nitride spacers 
(4 
- - 
counter -decoder driver 
4 .  
- 
resistors 
reset 
Figure 5. Setup for 3-phase drive of the micro-motors. 
83 . -  
Figure 6. Electrical connections for 3-phase operation 
of the motor. 
Figure 4. Major fabrication steps to construct the 
micro-motors. 
5 
-45 -30 -15 0 .  15 3O 45 
Angular position (degree) 
Figure 7. Torque/position characteristics of the 
micro-motor as obtained f?om two-dimensional simu- 
lation. 
I 
0 . 5  
3 
a 
P g 0. 
-1 
-45 -30 -15 0. 15 30 45 
Angular position (degree) 
(b) 
Figure 8. Single phase torape/position characteristics. 
1 
F- 0 . 5  
E 
- 
01 
P 0. 
3 
L 
$ - . 5  
e 
L 
-1 
5 2 . 5  90 -60 -22.5 15 
Angular position (degree) 
Figure 9. Three-phase torque/posi tion characteristics. 
6 


IC-processed micro-motors: design, technology, and testing

Micro-motors having rotors with diameters between 60 and 120 pm have been fabricated and driven electrostatically to con-tinuous rotation. These motors were built using processes derived from IC micro-circuit fabrication techniques. Initial tests on the motors show that friction plays a dominant role in their dynamic behavior. Observed rotational speeds have thus far been limited to several hundred rpm which is a small fraction of what should be achievable if only natural frequency were to limit the response. Experimental starting voltages are at least an order-of-magnitude larger than had been expected (60 volts at minimum and above 100 V for some structures). Observations of asynchronous as well as synchronous rotation between the driving fields and the rotors can be explained in terms of the torquelrotor-angle characteristics for the motors. I

Yu-chong Tai

Long-sheng Fan

Richard S. Muller

CiteSeerX

IC-processed micromotors: Design, technology, and testing

Design Considerations for a Practical Elecuostatic Micromotor,&quot;

Design Considerations for Microfabricated Electric Actuators,&quot;

Micromechanical Elements and Methods for Their Fabrication,&quot; U.S. Patent 4,740,410, issued to the Regents of the

Muller,&quot;IC-processed Electrostatic Micro-motors,&quot;

Muller,&quot;Integrated movable micromechanical structures for sensors and actuators,&quot;

https://authors.library.caltech.edu/31523/1/TAImems89.pdf

IC-processed micro-motors: design, technology, and testing

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