Hydrophobic coating for reducing humidity effect in electrostatic actuators

A touch mode electrostatic actuator and method of making the same, having first and second electrode driven surfaces positioned to move between a spaced apart relationship and a contact relationship when dielectric layer on each of the first and second electrode surfaces is subjected to a source of electrical potential to selectively actuate and discharge the dielectric layers to cause the movement between the relationships. The electrostatic actuators of this invention includes a hydrophobic layer for preventing absorption of water thereon, the hydrophobic layers being adapted to cause condensed water to form drops and prevent formation of a continuous water layer. The hydrophobic layer may be a coating on a hydrophilic actuator or, alternatively, may be made entirely from a hydrophobic material. The preferred hydrophobic material is selected from a group consisting of organic materials such as octadecyltrichlorosilane, perfluoro-decyltrichlorosilane, tetrafluoroethylene, and inorganic materials such as a diamond type carbon layer. The hydrophobic layer is electrically isolating and chemically stable in its intended environment. In the preferred embodiments, the hydrophobic material is applied to the hydrophilic dielectric layer by a method selected from dipping, plasma deposition, Langmuir-Blodgett technique, sputtering and CVD deposition.

FIELD OF THE INVENTION 
The present invention relates to an electrostatic actuator. More 
particularly the invention relates to a hydrophobic coating used to reduce 
humidity and improve performance of the electrostatic actuator. 
BACKGROUND OF THE INVENTION 
Electrostatic actuators have become selected as the solution of choice for 
actuators that employ low power, operate at high speed, require low cost 
to produce, and are of small size. These devices present significant 
advantages: over thermal devices by requiring much less power; over 
electromagnetic devices using less power and having smaller size; or 
piezoelectric actuators that have a higher cost and have a much smaller 
amplitude of motion. 
To date, however, there are no commercially available electrostatic 
actuators. Of particular concern are electrostatic actuation in the 
presence of dielectrically isolated electrodes, where specific problems 
are incurred. 
In electrostatic actuators, the desired displacement is the result of the 
attractive electrostatic force generated by the interaction between a 
distribution of opposite sign charges placed on two bodies, one of which 
is moveable. For the purposes of this invention, these two bodies are 
known as actuator plates. The actuator plates are placed apart by a 
predetermined distance. The charge distribution is then generated by 
applying a potential difference between two conductive electrodes that are 
part of the actuator plates. The actuator will be in the ON state or mode 
when a potential difference is applied between the electrodes and will be 
in the OFF state when the electrodes are at the same potential. 
One family of patents describes fluid control employing microminiature 
valves, sensors and other components using a main passage between one 
inlet and exit port and additionally a servo passage between inlet and 
outlet ports. The servo passage is controlled by a control flow tube such 
that tabs are moved electrostatically. U.S. Pat. No. 5,176,358 to Bonne et 
al teaches such a fluid regulating device, while divisional U.S. Pat. Nos. 
5,323,999 and 5,441,597 relate to alternative embodiments. 
The actual electrostatic device is only briefly described in the above 
patents, wherein at least one tab formed as part of a dielectric layer 
moves toward and away from an aperture upon activation of a means for 
varying the potential of at least one electrode associated therewith to 
generate an electrostatic force. 
The above referenced patents identify another family of patents for further 
information on microvalves using electrostatic forces. The pending U.S. 
patent application referred to in those first discussed patents has 
matured into U.S. Pat. No. 5,082,242 to Bonne et al. This patent describes 
a microvalve that is an integral structure made on one piece of silicon 
such that the device is a flow through valve with inlet and outlet on 
opposite sides of the silicon wafer. The valves are closed by contact with 
a valve seat where surfaces must be matched in order to avoid degradation 
of valve performance. Two patents, U.S. Pat. Nos. 5,180,623 and 5,244,527 
are divisional patents relating to the first patent. These patents 
generally describe operation of the electrostatic valve as being driven by 
various kinds of voltage sources. Specifically, the valve is said to 
operate as a two position valve with fully open and fully closed positions 
by applying a DC voltage between electrodes. Also, operation as a 
proportional control valve is disclosed as being effected by applying a 
voltage proportional to the voltage necessary to close the valve. Finally, 
These patents describe operation of the valve with a pulse width modulated 
voltage signal to modulate gas flow through the valve. 
In some electrostatic actuators, the actuator plates have to come in 
intimate contact during the normal operation cycle. These actuators are 
sometimes referred to as touch-mode electrostatic actuators. In order to 
prevent electrical shorting during the touch phase of the operation cycle, 
the conductive electrodes are isolated from each other by dielectric 
layers. In order to get the maximum work from a specific device, large 
electric fields are usually developed between the two conductive 
electrodes. The non-linear character of the electrostatic attraction 
results in a snapping action, where the actuator plates move toward each 
other with accelerations as high as 10.sup.8 g and speeds that exceed 
10.sup.3 m/sec. After the impact, the free surfaces of the actuator plates 
are pushed against each other by the large electrostatically generated 
pressure. 
This operation mode creates the possibility of very large mechanical impact 
and strong interaction forces being developed between the actuator plates. 
Some of these forces can continue to act after removal of the potential 
difference between the actuator plates. In some cases, these forces are 
stronger than the restoring forces available for bringing the electrodes 
in their original position. In such a case, the two electrodes remain 
temporarily or permanently attached and the actuator stops functioning as 
intended and desired. This condition is sometimes referred to as 
`stiction.` 
Present day touch-mode electrostatic actuators fail to operate properly and 
effectively in humid environments, such as where the dew point exceeds 
5.degree. C. or more, and are functionally inoperative above a dew point 
of 10.degree.-15.degree. C. In high humidity, electrostatic actuators 
exhibit an uncontrolled (vibration type) movement even when driven with a 
DC voltage. It has been discovered herein that layers of water build at 
the exposed surfaces of the electrostatic actuators, producing a 
cancellation of the electrostatic field/force at the interface. 
It would be of great advantage in the art if a method could be found that 
would reduce the sensitivity of electrostatic actuators to environmental 
humidity. 
It would be another great advance in the art if touch mode electrostatic 
actuators could be provided for use with out-of-doors and in many 
unprotected environments. 
Other advantages will appear hereinafter. 
SUMMARY OF THE INVENTION 
It has now been discovered that the above and other objects of the present 
invention may be accomplished in the following manner. Specifically, the 
present invention provides an improved electrostatic actuator device and 
method of making the same, in which the exposed surfaces of the actuator 
are hydrophobic. This can be accomplished in two ways: (1) by coating the 
surfaces of the hydrophilic dielectrics with hydrophobic layers so that 
water will not be absorbed on the surface; and (2) by using as a 
dielectric layer a hydrophobic material. 
Condensed water, if any, will form drops on the layer, rather than a 
continuous conductive layer of water. Accordingly, the electrostatic 
actuation will be effective up to much higher levels of humidity in the 
environment. 
The hydrophobic material is intended to be both electrically isolating and 
chemically stable in its intended environment. The preferred hydrophobic 
material is selected from the group consisting of organic or inorganic 
materials such as octadecyltrichlorosilane, perfluorodecyltrichlorosilane, 
other fluorinated layers such as tetrafluoroethylene, a diamond type 
carbon layer, or a diamond like noncomposite such as Dylyn.RTM. (trademark 
material from Advanced Refractory Technology). In the preferred embodiment 
for making the device of this invention, the hydrophobic layer can be 
applied by a method selected from spin coating, dipping, plasma 
deposition, Langmuir-Blodgett technique, and CVD deposition.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention recognizes and solves a problem that is experienced 
with electrostatic actuators, especially touch mode electrostatic 
actuators where the properties of the two surfaces that come into contact 
have a determining effect on the behavior of the actuator. 
Since the electrostatic field is zero inside conductive materials, layers 
of conductive materials on the contact surfaces of electrostatic actuators 
will zero the field at the interface and cancel the electrostatic force. 
As a result, the actuation process will be terminated, irrespective of the 
applied voltage. 
A dielectric is used to isolate the two electrodes in the electrostatic 
actuators. Most if not all dielectrics are hydrophilic, such as silicon 
dioxide, silicon nitride (usually covered with a thin, native oxide layer) 
and the like. Examples of such electrostatic actuators with hydrophilic 
surfaces are found in the above referenced U.S. Pat. No. 5,176,358 and the 
patents related thereto. 
When such devices are employed in a humid environment, water will easily be 
adsorbed and/or condensed on the dielectric surfaces, thus forming a 
relatively conductive layer. The actuation process will be severely 
affected. As shown in the prior art device of FIG. 1, the two electrodes 
11 and 13, having hydrophilic dielectric surfaces 15, will not function 
when a potential is applied from voltage source 17 because water 19 has 
formed on the hydrophilic surfaces 15. Water 19 builds up on the exposed 
surfaces 15 so that the field at the interface is canceled and plates 11 
and 13 will move apart under the restoring elastic force that supports the 
plates. 
Turning now to FIG. 2, the same electrostatic actuator elements are 
present, including electrodes 11 and 13 with dielectric 15 coated thereon. 
Application of potential via voltage source 17 is not impeded by the 
presence of water 19, however, because hydrophilic dielectric surfaces 15 
are coated with a hydrophobic layer 21. Alternatively, the device in FIG. 
2 could have layers 15 made from a hydrophobic material directly. 
In these types of electrostatic actuators, a strong field develops in the 
gap between the dielectric surfaces 15 as long as an air gap exists at the 
interface. This creates an attractive electrostatic force E between the 
two electrode plates 11 and 13, so that these plates will move toward each 
other. When electrode plates 11 and 13 come into contact at their 
dielectric surfaces 15, the water layers in FIG. 1 will also come in 
contact and, given the conductive character and the high dielectric 
constant of water, the field E will be almost zero at the interface. The 
plates 11 and 13 will separate under the restoring force of the mechanical 
structure. As the air gap is formed again, the plates will again be pulled 
together, and so on. The actuator itself will vibrate instead of staying 
closed. Such behavior has been observed whenever electrostatic actuators 
driven with DC voltages are used in humid environments. Of course, this 
behavior is highly detrimental to valve type applications. 
The effect of the relatively high surface conductivity induced by the 
absorbed water layer can be reduced by driving the actuator with a square 
wave AC voltage, however even with an AC drive, the electrostatic pressure 
generated by an actuator with hydrophilic surfaces drops significantly in 
the presence of high humidity. 
A series of tests were made to compare the prior art uncoated design of 
FIG. 1 with the device of this invention as shown in FIG. 2. Both devices 
were operated as electrostatic actuators at an alternating current square 
wave voltage of 25 Volts peak, and at a frequency of 100 Hz. In a 
controlled atmospheric environment, the dew point in .degree.C. was 
increased at an ambient temperature of 21.degree. C. and the electrostatic 
pressure developed to hold the dielectric electrode plates in contact was 
evaluated. 
FIG. 3 is a plot of the results of these tests. As can readily be seen, the 
prior art device of FIG. 1 produced a curve 25 that dropped off rapidly at 
a dew point of about 6.degree.-8.degree. C. while the present invention 
device of FIG. 2 remained unaffected by humidity at a dew point over 
15.degree. C. 
It is clear from these tests that the present invention provides a superior 
range of performance. In point of fact, the present invention is operable 
under conditions where the prior art devices can not even operate, due to 
the adverse effects of humidity on the hydrophilic dielectric surfaces of 
the actuator. 
By coating the exposed surfaces of the hydrophilic dielectric with a 
hydrophobic material, or by using hydrophobic materials as dielectric, 
water adsorption on the surface is prevented. Water drops, if any, will 
not wet the surfaces and will easily be pushed away by the operation of 
the electrodes. 
The actuation process will work properly up to much higher levels of 
humidity in the environment. As a result, because of the present invention 
and the use of hydrophobic coatings and/or hydrophobic materials, 
electrostatic actuators can operate in a less controlled atmosphere, 
opening up opportunities for a broad range of industrial, commercial and 
hope applications. Microvalves for pneumatic controls and micropumps will 
have a much more diverse field of application. 
While particular embodiments of the present invention have been illustrated 
and described, it is not intended to limit the invention, except as 
defined by the following claims.