Micromechanism linear actuator with capillary force sealing

A class of micromachine linear actuators whose function is based on gas driven pistons in which capillary forces are used to seal the gas behind the piston. The capillary forces also increase the amount of force transmitted from the gas pressure to the piston. In a major subclass of such devices, the gas bubble is produced by thermal vaporization of a working fluid. Because of their dependence on capillary forces for sealing, such devices are only practical on the sub-mm size scale, but in that regime they produce very large force times distance (total work) values.

BACKGROUND 
This invention addresses the general problem of motive power for 
micromachines, i.e., those mechanical devices whose individual components 
are typically between 1 .mu.m and 1000.mu.m in size. The essence of the 
current invention is the introduction of gas pressure-driven pistons which 
use capillary forces to seal the working fluid in the working device and 
to transmit the forces which result from the pressure acting on the 
working fluid to the piston. The result is a new class of microminiature 
linear activators which make available piston forces some 2-3 orders of 
magnitude larger than do conventional electrostatic microactuators at 
similar operating voltages. 
In the field of micromechanical technologies, there is a great need for 
devices which can provide useful work to active micromechanical 
assemblies. (For the purposes of this application, an active 
micromechanical assembly would be a micromachine having driven moving 
parts, for example a gear train, whereas a passive micromechanical 
assembly would depend on the deformation of fixed elements, such as 
cantilevers, in response to external motion or other external conditions.) 
The same need is commonplace on the macroscopic scale; an assembly of 
gears and pivots and linear motion guides is not a lathe until some source 
of motive power is added. On the macroscopic scale such motive power is 
often provided by internal and external combustion engines, although the 
energy provided by these sources may first be transformed into forms 
useable by electric motors or hydraulic actuators, which devices may 
directly drive the desired assembly. 
Unfortunately, the standard macroscopic sources of motive power do not 
scale well into the microscopic regime with which we are currently 
concerned. They are either too complicated to manufacture at such 
dimensions or the physical laws that govern their operation do not scale 
favorably, resulting in inadequate performance. Several types of motive 
power have been investigated in the prior art for application in the 
microscopic regime, notably electrostatic motors, piezoelectric drives, 
and thermal bimorphs, and these have proven useful in some cases. In 
general, however, the force produced is limited, and/or is available over 
a rather small range of linear displacement, and hence is not adequate for 
a large number of potential applications. Further, some of the available 
options (piezoelectric drives, thermal bimorphs, and others) do not adapt 
easily to integrated circuit processing techniques, an important factor 
for applications requiring many actuators or when many complete mechanisms 
must be built. 
For the foregoing reasons, there is a need for a new type of microminiature 
linear activator that provides accessible work per operating cycle (force 
x length of stroke) vastly greater than is achievable using currently 
available microscopic actuators. A further need is for such a device that 
is easily manufactured in large quantities on a silicon wafer (the current 
arena for development of micromachines) using fabrication techniques 
compatible with the enormous suite of techniques developed for fabrication 
of integrated circuits. 
SUMMARY 
The present invention is directed to a new class of microminiature linear 
activators that satisfies the aforementioned needs. This class of devices 
comprises the harnessing of vapor pressure, controllably produced through 
the action of a heating means on a working fluid or by a gas manifold 
system, to drive the linear motion of a piston in a cylinder bore. The 
working fluid is sealed within the actuator (thus preventing blowby of the 
vapor) by capillary forces alone. These same capillary forces act to 
couple the force of the vapor pressure acting over the cross-sectional 
area of the cylinder bore onto the piston. As the piston in this size 
regime may be considerably smaller than the cylinder bore, this coupling 
acts to substantially increase the ultimate force of such devices. For 
.mu.m-scale devices, the force generated is some 2-3 orders of magnitude 
greater than that from prior art electrostatic comb-drive devices, and the 
range of piston displacement is many times the piston "diameter". (As the 
piston is usually not round, the term "diameter" will be used as 
representing a characteristic cross-sectional dimension when the piston 
has a cross-sectional aspect ratio near one. If a round piston or cylinder 
bore is being discussed, its size will be expressed as a radius.) The 
needs identified in the Background are all addressed by this invention, 
which provides a compact, controllable, and powerful source of linear 
motive power for micromachines. A subclass of such devices can be 
micromachined on silicon wafers using standard silicon micromachining 
techniques. Numerous embodiments and other features, aspects, and 
advantages of the present invention will become better understood with 
reference to the following descriptions and appended claims.

DESCRIPTION 
The essence of the present invention is illustrated schematically in FIG. 
1. The miniature actuator comprises a body 10 which is essentially 
impermeable to gas and liquid. By a series of conventional micromachining 
processing steps, a cavity is formed in body 10 to form pressure chamber 
11. A cylinder bore 13 is similarly formed in body 10 to connect said 
pressure chamber to the exterior of body 10. A piston 12 is located within 
said cylinder bore 13. The mutual dimensions of piston 12 and cylinder 
bore 13 allow non-binding linear motion of piston 12 along the axis of the 
cylinder bore 13. Piston 12 is pressing against some means (not shown) 
which provides a restoring force against motion to the left in the FIG., 
and which prevents piston 12 from completely exiting the cylinder bore 13. 
The combined void defined by pressure chamber 11, cylinder bore 13, and 
piston 12 is partially or wholly filled with a working fluid 14, said 
fluid being liquid (at the normal range of temperatures found exterior to 
the device) and capable of wetting the surface of piston 12. Heating 
element 15 is located near the pressure chamber and, when activated, 
serves to vaporize a portion of the working fluid 14, creating vapor 
bubble 16. The volume of vapor bubble 16 increases monotonically with 
increasing heat input from element 15, which is controlled by a 
controlling means (not shown). (It is also possible in principle to form 
the vapor bubble 16 by a forming means comprising a source of pressurized 
gas, a feed valve, and a bleed valve. However this class of embodiments 
requires special design to avoid capillary sealing at the access holes for 
the feed and bleed valves. For the present the forming means will be 
assumed to be thermal in nature as described above.) 
When the surface of said vapor bubble 16 reaches the shoulder 17 of the 
piston 12, a capillary meniscus 18 forms between the shoulder 17 and the 
cylinder bore 13. As the vapor bubble 16 continues to increase in volume, 
the vapor begins to push into the interstices between the piston 12 and 
the cylinder bore 13. As the working fluid 14 wets the surface of the 
piston 12, the capillary meniscus 18 remains attached to the shoulder 17 
until a critical contact angle at the piston surface is exceeded. Thus, 
the capillary meniscus 18 bulges to the left in the FIG., increasing its 
surface area and thus its energy. The system as a whole will attempt to 
adjust to reduce this energy, but, as long as the capillary meniscus 18 
remains attached to the shoulder of the sealing end the only path toward 
such reduction is for piston 12 to move to the left in the FIG., thus 
reducing the surface area of the capillary meniscus 18. Piston 12 
therefore experiences a force to the left as the vapor bubble 16 grows. 
The force described above is capillary in nature, and exists in addition to 
the pressure of the vapor bubble 16 on the end of the piston 12. The 
principles of capillary effects are well-known in the art. In particular, 
the capillary force scales as the circumference of the sealing end of the 
piston 12, whereas the vapor pressure on the end of the piston 12 scales 
as the area of the piston. As the size of the actuator decreases, 
therefore, the proportion of the force on the piston 12 due to capillary 
effects increases. The exact dimensions associated with crossover to 
capillary dominance (i.e., those conditions under which more than half of 
the piston force is transmitted to the piston by capillary forces) depends 
on the design of the actuator, but will generally occur when the piston 12 
has an area less than about 10-100 square microns. 
Note that the capillary meniscus 18 also provides the only means to seal in 
the pressure of the vapor bubble 16, whereas prior art electrothermal 
actuators of this general type depend on conventional sealing means, such 
as o-rings or moving diaphragms. The present miniature actuator is thus 
simpler in design than prior art, as well as dependent on a different 
physical phenomenon both for transfer of force and for sealing of gas. The 
capillary seal will be effective until the pressure in the vapor bubble is 
great enough that the capillary meniscus 18 detaches from the piston 
shoulder 17. This is the exact point when the capillary transfer of force 
to the piston 12 fails. Accordingly, the capillary sealing functions 
throughout the operating range of the actuator. Note that the above 
discussion concerns an extremely simplified device so that the principles 
of operation may be succinctly expounded, and should not be taken to limit 
the scope of the current invention. 
The external environment of this type of actuator is also important. For 
example, if the actuator is surrounded by a liquid which is mutually 
soluble with the working fluid but has a much higher heat of vaporization, 
the actuator will eventually stop working as expected, as the heating 
power required to produce a given amount of force will greatly increase. 
Any such liquid environment, where substitution of the working fluid with 
the environmental liquid will significantly alter the operational 
properties of the actuator, will be called incompatible with the working 
fluid. On the other hand, if the environmental liquid has similar 
properties to those of the working fluid, an actuator will continue to 
operate properly in that environment, and the environmental liquid and 
working fluid are compatible. Another case is when the environmental 
liquid and the working fluid are mutually immiscible. Here intermixing 
does not occur, and the operation of the actuator is unaffected. Finally, 
when the external environment is gaseous, the capillary meniscus formed at 
the interface between the working fluid and the atmosphere serves to seal 
the working fluid in the actuator, except for a small amount of 
evaporative loss. The actuators made possible by the present invention 
will operate successfully in a wide range of external environments, but 
the working fluid must be carefully chosen to match the working 
conditions. 
It is necessary to briefly explain the source and certain properties of the 
capillary force on a piston in an actuator of the type illustrated in FIG. 
1 and described above, so that design of non-functional devices can be 
avoided. As mentioned above, the force on the piston is generated by the 
deformation of the capillary meniscus as the pressure of the vapor bubble 
increases. This deformation increases the surface area of the capillary 
meniscus; since this surface has a positive energy, the energy of the 
system also increases upon deformation. There is thus a restoring force 
generated to reduce the surface area of the capillary meniscus. This 
restoring force is countered by the attachment of the capillary meniscus 
to the shoulder of the piston. As the meniscus is attached to said 
shoulder until a critical geometric condition is satisfied, the restoring 
force is transmitted to the piston. 
It is clear from the above that the ultimate source of the capillary force 
on the piston is simply the pressure of the vapor bubble on the capillary 
meniscus. The capillary meniscus thus serves to concentrate the on-axis 
force from the vapor pressure on the meniscus to act on the piston. As the 
piston also experiences this pressure on its end, the total force pushing 
the piston out of the actuator is a function of the pressure in the vapor 
bubble multiplied by the cross-sectional area of the cylinder bore, rather 
than that of the piston. In large actuators having close tolerances this 
difference is negligible. However, in micron-sized devices having cylinder 
bore-piston clearance on the order of the piston dimensions, the 
concentration of force by capillary interactions can be quite significant. 
For example, if the piston is 2 .mu.m in diameter and the cylinder bore is 
4 .mu.m, capillary forces increase the force on the piston by a factor of 
2 over the simple action of vapor pressure on the end of the piston. The 
capillary seal thus not only decreases complexity of the actuator, but 
increases operating effectiveness. 
The amount of force that can be transmitted to the piston by the capillary 
meniscus is determined by the surface tension .gamma.(dynes/cm) 
characterizing the vapor-liquid interface of the working fluid. The 
surface tension of water is representative of potential working fluids at 
some 70 dynes/cm. The capillary force transmitted is 
EQU F.sub.cap= 2.pi.r.gamma.cos.theta. (1) 
where r is the radius of the piston (assumed to have a round cross-section 
for this discussion) and .theta. is the critical angle of wetting. .theta. 
is close to zero for desirable piston-working fluid combinations, and thus 
cos.theta. will be taken as equal to 1 to calculate the maximum capillary 
force F.sub.mc. 
The maximum operating pressure of the actuator is given by setting the 
force of the vapor pressure P on the capillary meniscus equal to the 
capillary force. Assume that the cylinder bore is round in cross section, 
and that its radius is .alpha.r, r still being the radius of the piston. 
The force on the capillary meniscus is then .pi.(.alpha.2-1) r.sup.2 P. 
This gives 
EQU P.sub.max =2(.alpha.+1).gamma./(.alpha..sup.2 -1)r (2) 
or roughly 140/r dynes/cm2 when .alpha.=2. (Alternately, P.sub.max 
.apprxeq.140,000/rPa, or 20/r psi when r is in microns.) As small 
operating pressures are difficult to control particularly with external 
control units, this argues toward activators of the present design having 
cross-sectional areas of less than about 10000 .mu.m.sup.2. 
Note that the maximum capillary force F.sub.mc does not depend on the 
clearance between the cylinder bore and the piston. As this force results 
from the action of the pressure of the vaporized working fluid on the 
capillary meniscus, the vapor pressure required to produce a given 
capillary force on the piston depends inversely on the difference in 
cross-sectional area between the cylinder bore and the piston. (Small 
.alpha. near one gives large working pressures in Equation 2.) One 
consequence is that an actuator having a small value of .alpha.(i.e. near 
1) may, owing to the action of the higher operating pressure on the end of 
the piston, actually produce more piston force than a second actuator with 
a large value of .alpha., even though only the second actuator exhibits 
capillary dominance. Hence, capillary dominance is not a design goal. 
Capillary sealing, however, is required for the operation of this new 
class of microminiature activators. A careful look at the capillary 
sealing mechanism is thus appropriate. 
The effectiveness of a capillary seal in other than microgravity conditions 
clearly depends on the dimensions of the piston and cylinder bore. 
Trivially, if the cylinder bore is 2 meters in diameter and the piston is 
1 meter in diameter, the working fluid will simply run out on the floor, 
leaving an nonfunctional device. This is why O-rings and the like were 
developed, to allow conventional devices to use capillary sealing effects. 
The question of stability of a capillary seal is a critical design 
problem, and must be addressed to let one skilled in the art practice the 
current invention without undue experimentation. 
As illustrated in FIG. 2, the force of gravitation on the working fluid 
acts to increase the pressure at the bottom of the cylinder bore 20 
compared to that at the top of the cylinder bore 21 by 2.rho..alpha.gr, 
where .rho. is the density of the working fluid 22 and g is the 
acceleration due to gravity or other influences (such as vibration). This 
pressure is offset by the capillary force developed by distorting the 
shape of the capillary meniscus 23 and its angles of contact with the 
piston and the cylinder bore. This force is difficult to calculate 
exactly, but it will produce a maximum pressure of perhaps 5-10% of the 
maximum working pressure Pmax in opposition to the force of gravity. Using 
this estimate, one can establish design rules for the maximum clearance 
between cylinder bore and piston as a function of piston radius. The 
maximum clearance .delta. between cylinder bore and piston is (.alpha.-1) 
r, where .alpha.r is the cylinder bore radius and .alpha. obeys the 
expression below: 
EQU .alpha.(.alpha..sup.2 -1).ltoreq.0.1.gamma./.rho.gr.sup.2 ; .alpha.&gt;1. (3) 
Equation 3 is one example of a design rule for clearance of a device which 
depends on capillary sealing. Clearly different orientations and external 
conditions will produce different rules, but they will always depend on 
the type of force balancing argument outlined above, which can be easily 
adapted by one skilled in the art. 
Equation 3 can be solved by any of a number of iterative techniques for 
nonlinear equations or by direct solution of the third-order polynomial 
equation. If we assume the need to contain fluid under accelerations of 10 
Gs and use water as the working fluid, we find that .alpha..sub.max 
(.alpha..sub.max.sup.2 -1).apprxeq.10.sup.5 r.sup.-2 where r is measured 
in microns. The earlier discussion of maximum working pressure suggested 
that r should not be more than about 100 .mu.m. For this case, 
.alpha..sub.max .apprxeq.2.3, and the maximum clearance 
.delta..apprxeq.130 .mu.m. In contrast, if r=1 .mu. m under the same 
conditions, .mu..sub.max .apprxeq.50, giving a maximum clearance 
.delta..apprxeq.49 .mu.m. It is clear that actuators of the present type 
having a reasonable working pressure will have little problem in sealing 
the working fluid using only capillary action. 
The nature of the cylinder bore is important in design of miniature 
activators of the above design. The principal factor driving cylinder bore 
design is that smooth piston motion is desired, even when moving against 
an external load. This consideration has several consequences. For 
example, the cylinder bore should be reasonably smooth, so that the 
capillary meniscus can slide freely as the vapor bubble expands. Roughness 
of the cylinder bore will produce some degree of stick-slip piston motion. 
As was discussed briefly above, the bore can take any number of 
cross-sectional shapes. Micromachining techniques lend themselves most 
easily to roughly square or rectangular bores. 
A more important criterion, however, is that the cross-sectional area of 
the cylinder bore should be essentially constant along its length, or at 
least along that portion of its length designed to interact with the 
capillary meniscus. The reason is that the total force transmitted to the 
piston is a function of the vapor pressure multiplied by the 
cross-sectional area of the cylinder bore at the location of the piston 
shoulder. Consider the case, illustrated schematically in FIGS. 3A-3C, 
where there is a restoring force on the piston 44. (For simplicity we will 
assume here that the actuator 40 is lifting a weight 45. The restoring 
force represented by element 45, however, need not be constant for the 
following discussion to be valid in essence.) The cross-sectional area of 
the lower cylinder bore 41 is 1, that of the middle cylinder bore 42 is 
0.5, and that of the upper cylinder bore 43 is 1 again. (The piston 44 is 
centered and small enough to move axially without making contact with the 
cylinder bore.) 
In FIG. 3A, the capillary meniscus 49 is in the lower cylinder bore 41, so 
that a certain constant vapor pressure P is needed to keep the piston 44 
steady against the external load 45. The pressure and volume of the vapor 
48 are controlled by adjusting the power of the heating means 47, and 
thereby the amount of the working fluid 46 which is vaporized. To move the 
piston 44, e.g., outward, the power delivered by the heating means 47 is 
increased so that the equilibrium volume of vapor 48 having pressure P is 
larger than in the initial condition. 
In FIG. 3B, the capillary meniscus 49 has reached the intersection between 
the lower cylinder bore 41 and the middle cylinder bore 42. At this point, 
the cross-sectional area of the cylinder bore is reduced abruptly to half 
of its original value. As a result, the vapor pressure required to support 
the external load 45 beyond the constriction increases to .sqroot.2P, 
because the force transmitted to the piston 44 is a function of the 
pressure of the vapor 48 multiplied by the cross-sectional area of the 
cylinder bore. On reaching this transition point between lower and middle 
cylinder bores, further outward motion of the piston 44 will not occur 
until the pressure of the vapor 48 has increased from its original value. 
(The increased pressure is indicated by the higher density of the vapor 48 
in FIG. 3B.) A considerable increase in the power delivered by the heater 
means 47 is required to produce the required increase in pressure. In 
effect, then, the actuator `sticks` in a certain range of heater power 
before outward motion once again continues. 
Once the vapor pressure is increased to .sqroot.2P, the piston 44 is again 
free to respond to changes in heater input 47. Now consider the situation 
shown in FIG. 3C where the piston 44 has moved far enough out that the 
capillary meniscus 49 enters the upper cylinder bore 43. As the 
cross-sectional area of this region is again equal to 1, the actuator 
suddenly has .sqroot.2 times the vapor pressure required to support the 
external load 45. The piston 44 responds by moving outward until the 
pressure of the vapor 48 is reduced to P. Depending on the geometry of the 
cylinder bore and pressure chamber, this may require a great deal of 
piston motion. Large changes in cylinder bore cross-sectional area thus 
produce dramatic `stick-slip` type piston motion. 
The `stick-slip` phenomenon described above can be used to the designer's 
benefit in certain circumstances. It is possible to use this effect to 
help retain the piston in the cylinder bore. Consider again the operation 
of the actuator shown in FIG. 4. When the capillary meniscus 49 is in the 
lower cylinder bore 41, the motion of the piston 44 in and out of the 
actuator is smoothly controlled by the power delivered by the heating 
means 47 (the method for such control is not shown here). When the 
capillary meniscus 49 reaches the point of transition between the lower 
cylinder bore 41 and the middle cylinder bore 42, however, no further 
motion will occur until the power delivered by the heating means 47 is 
greatly increased. The relevant power levels can be calibrated for any 
given actuator design, and incorporated into the design of the controlling 
means for the heating means 47 so that adequate heating power to drive the 
capillary meniscus 49 into the middle cylinder bore 42 cannot be 
delivered. In this case (for which the upper cylinder bore 43 is not 
necessary), the presence of a known (not necessarily constant) restoring 
force and the change in the cross-sectional area of the cylinder bore 
combine to retain the piston in the actuator. 
The `stick-slip` phenomenon can also be used to deliver an impact to a 
target when the piston 44 `slips` into the upper cylinder bore 43, rather 
like a miniature hammer. When the capillary meniscus 49 moves into the 
upper cylinder bore 43, the piston is suddenly subject to a force greater 
than the restoring force (twice the restoring force, in the example). This 
excess force will accelerate the piston 44, which will deliver a blow to a 
target whose magnitude depends primarily on the properties of the 
restoring means, the amount of piston travel following the initial 
`slippage`, and the volume of high pressure vapor (determined by the 
design of the actuator). Such blows against a piezoelectric element would 
generate an electrical pulse when the power to the heater means 47 exceeds 
a given value, the system thus generating a control signal or acting as a 
stage in an analog to digital converter. 
In a similar manner, changes in the cross-sectional shape of the cylinder 
bore that leave the cross-sectional area constant will also produce 
`stick-slip` piston motion. The cause is the excess energy required to 
distort the initial meniscus shape into the shape required by the new 
cross-sectional shape. This source of `stick-slip` motion is much smaller 
in magnitude than that resulting from significant changes in 
cross-sectional area, but must still be considered in design of actuators 
of the present type for precision applications. 
Having completed the discussion of the capillary sealing/drive mechanism, 
we now introduce useful variations in the basic actuator mechanism. As 
shown in FIG. 4, the same basic principle of capillary action can be 
applied to make an actuator having an arbitrary number of cylinder 
bore-piston pairs 51-52. All such pairs in a given actuator are in common 
contact with a single pressure chamber 50 containing a heating means 53. 
The unit is initially filled by a working fluid 54, within which is 
formed, by action of the heating means 53, a vapor bubble 55. When the 
vapor bubble 55 intersects the respective pistons 52, capillary menisci 56 
form between the walls of the cylinder bores 51 and the end of the 
respective pistons 52 proximate to the vapor bubble 55. These capillary 
menisci 56 transfer force to the respective pistons 52. 
Because the cylinder bores 51 are all connected to a common pressure 
chamber 50, they are all driven by the same vapor pressure P. When the 
vapor bubble 55 is too small to contact the pistons 52, said pistons 
experience an outward force equal to their cross-sectional area multiplied 
by P. The vapor pressure P.sub.init required to increase the volume of the 
vapor bubble can be approximately determined from Equation 1 to be 
2.gamma./.alpha. r. (This is simply the maximum capillary force on a 
meniscus drawn across a cylinder bore having radius .alpha.r.) The initial 
outward force on the pistons 52 is thus equal to 2.pi.r.gamma./.alpha.. 
Once the capillary menisci 56 are formed, however, the outward force on 
the pistons increases to 2.pi.r.gamma., a factor of .alpha. larger than 
the initial force. (Note that this implies that when multiple cylinder 
bore-piston pairs are involved, the vapor bubble will grow to form the 
desired capillary menisci 56 before any of the pistons experiences an 
outward force greater than the 2.pi.r.gamma. value.) The capillary menisci 
56 also serve as sealing means which allow the vapor pressure P to be 
increased, thus increasing the outward force on the pistons 52 as 
described by Equations 1 and 2. 
The principles of operation and design having been described above, 
attention is now turned toward specific embodiments of the various 
subsystems. These are chosen to illustrate general features, and not to 
thereby limit the scope of the present invention. 
Two possible heating means for forming vapor bubbles by local heating are 
illustrated in FIGS. 5A and 5B. In FIG. 5A an electrical heating means is 
illustrated. The actuator body 60 has a resistance element 61 attached, 
embedded, or otherwise incorporated into its physical structure. In the 
embodiment shown, the resistance element 61 is embedded in the actuator 
body 60. The resistance element 61 is connected to a power controlling 
means 62, which provides electrical power to said resistance element. The 
actuator body is filled with a working fluid 63. The heat from the 
resistance element 61 has partially vaporized the working fluid, creating 
the vapor bubble 64. (A vapor bubble forms because of surface tension and 
the dynamics of heat transport in this type of device.) Any cylinder 
bore-piston pairs functionally connected to this assembly are not shown. 
The flow of heat in and around the actuator body 60 is driven by the input 
of heat into the actuator body from the resistive element 61, removal of 
heat from the actuator body by conductive, convective, and radiative 
transfer with the surroundings of the actuator body, and conductive and 
convective transfer within the working fluid 63 and the vapor bubble 64. 
As a result, modeling the performance of any given actuator design and 
application is a difficult problem whose details differ for each 
application. A general rule, however, is that more power from the power 
controlling means 62 produces a bigger vapor bubble 64. This will be true 
providing that the surroundings of the actuator remain thermally static 
and that neither the working fluid nor the vapor make a transition to 
turbulent material flow (which would change the amount of internal heat 
transfer). In difficult cases, it is possible to arrange a feedback signal 
from the piston motion or piston force back to the power controlling means 
62 to maintain the relevant device parameter at the desired value, which 
may be a function (possibly multivariable) of time and/or other functional 
parameters. 
FIG. 5B schematically illustrates the use of an optical heating means. An 
optical absorbing region 65 is in thermal contact with the actuator body 
60. An access hole 66 allows the light output of a fiber-optic waveguide 
67 to contact the optical absorbing region 65. The input end of the 
fiber-optic waveguide 67 connects to an optical controlling means 68, 
which provides a source of light with sufficient power to heat the optical 
absorbing region 65 sufficiently to vaporize the working fluid 63, thus 
forming a vapor bubble 64. The intricacies of control are much as 
described above. Note that this is only one embodiment of an optical 
heating means. 
The fiber-optic waveguide 67 can be replaced or augmented by any of a 
number of optical contrivances, specifically including systems comprising 
lenses, mirrors, and gradient-index optics, which serve the purpose of 
coupling the output of the optical controlling means 68 to the optical 
absorbing region 65. Also, the optical controlling means 68 may be any 
suitable optical source, including lasers, LED's, and incandescent bulbs. 
In the discussion of FIG. 1 above the possibility of forming a vapor bubble 
in a microminiature activator of the type described herein by balancing 
gas flow in and out of the pressure chamber of the actuator in order to 
maintain a constant volume of gas at a constant pressure. Such a bubble 
forming means is illustrated in FIG. 6. This FIG. shows an actuator having 
a cylinder bore 70 and a piston 71, containing a working fluid 72. There 
is also a gas bubble 73. This gas bubble is not composed primarily of the 
vapor of the working fluid 72, but rather of gas injected through input 
aperture 74. The source of this gas is pressurized reservoir 78, and the 
rate of gas flow into the gas bubble 73 is controlled by feed valve 76. 
The gas could also come from another external source or from an on-chip 
gas pump. The gas making up the gas bubble 73 is free to escape the bubble 
through output aperture 75. Note that the surfaces of the capillary bore 
70 proximate to the input and output apertures 74 and 75, and the surfaces 
of these selfsame apertures, must be treated so that the working fluid 
does not wet said surfaces. Otherwise a capillary meniscus will form 
across output aperture 75, forming an effective high-pressure seal against 
gas flow from gas bubble 73. (The seal pressure is high relative to the 
working pressure of the actuator because the cross-sectional area of the 
output aperture 75 is small compared to that of the capillary bore 70.) 
Suitable surface treatments would include a thin film of low-friction 
polymer (e.g., PTFE) or a self-organizing monolayer with inactive end 
groups. 
The amount of gas which can escape per time interval is proportional to the 
product of the conductance .beta..sub.b of the bleed valve 77 and the 
difference in the pressure in the gas bubble 73 and the environment into 
which exhaust tube 79 exits. Similarly, the amount of gas which can enter 
the bubble from the pressurized reservoir 78 per time interval is 
proportional to the conductance .beta..sub.f of the feed valve 76 and the 
difference in the pressure of the reservoir 78 and the gas bubble 73. 
These valves are adjustable so that the conductances can be changed at 
will. 
To maintain a given amount (number of atoms) of gas in the gas bubble 73, 
the flow rate into the bubble should equal the flow rate out. This 
condition leads to the equation 
EQU P=(.beta..sub.f P.sub.i +.beta..sub.b P.sub.o)/(.beta..sub.f +.beta..sub.b) 
(5) 
where P is the pressure in the gas bubble, P.sub.i is the pressure in the 
pressurized reservoir 78, P.sub.o is the pressure to which the exhaust 
tube 79 discharges, and where there is no absorption of gas into the 
working fluid. The pressure and volume of the gas bubble 73 will thus 
remain constant when this condition is fulfilled. The combined use of the 
feed and bleed valves 76 and 77 thus allows control over the size and 
pressure of the gas bubble 73. Such a control scheme is most likely to be 
of use in fluidic applications, but there is no fundamental reason that 
this scheme, and others obvious to those skilled in the art, may not be 
implemented using electronic or mechanical control. 
Maintaining alignment of the actuator piston is of concern in some 
subclasses of the present invention, and should thus be discussed briefly. 
Although the magnitude of the restoring force is difficult to estimate, it 
is unlikely that this effect will be adequate to maintain acceptable 
piston alignment in all devices. Several other possibilities will thus be 
discussed here. This discussion is not intended in any way to limit the 
invention beyond the claims presented. 
FIGS. 7A-7E show two possible ways beyond capillary deformation in which 
piston alignment can be maintained. The first (FIGS. 7A-7C) is a brute 
force use of aspect ratio. If the length L of the piston 82 remaining in 
the actuator 80 at its maximum extension is much greater than the 
clearance between the piston 82 and the cylinder bore 81, said piston will 
not be free to move more than .beta./L radians out of alignment with the 
axis of the cylinder bore. This misalignment angle can be made as small as 
required through proper design of the actuator. This is illustrated by the 
misalignment of the two tilted pistons; the tilt of the piston in the 
short actuator is clearly more than that of the piston in the longer 
actuator. 
The above scheme has certain limitations, however. It works best when the 
cylinder bore has smooth walls, when the piston has no sharp edges, and 
when the cross-sectional shapes of the piston and cylinder bore are 
similar. As these conditions deteriorate, an activator depending on the 
above scheme for piston alignment will experience stick-slip motion, 
possibly to the extent of jamming the actuator. The technique also 
requires actuators having large (for this microscopic regime) dimensions. 
This approach will be of use in some situations, but other alignment means 
must be available for more general application of this class of 
microminiature linear actuators. 
Another approach is shown in FIGS. 7D and 7E. This again depends on 
material interference for its effect, but in this case a plurality of 
lands 84 are raised on the surface of the piston 83. The lands 84 restrict 
the lateral motion of the piston, thus maintaining the desired piston 
alignment. (In some cases the piston may actually ride upon certain of the 
lands; this, however, is not required.) The lands must offer enough 
clearance for free linear motion of the piston in the cylinder bore, but 
the degree of clearance required may be a small fraction of the 
piston-cylinder bore clearance. As a result, this scheme gives much better 
alignment than that of FIGS. 7A-7C, and allows alignment specifications to 
be met in a smaller actuator. Formation of the lands on the piston is a 
difficult matter however with conventional silicon micromachining 
processes. Although the lands 84 can in principle be placed on the walls 
of the cylinder bore 85 as suggested in the FIG., there is some danger in 
that case of said lands producing a `stick-slip` motion owing to the 
resulting variation in the cross-sectional area of the cylinder bore (see 
discussion of FIG. 3 above). If the lands 84 must be on the walls of the 
cylinder bore 85 for some other design requirement, the `stick-slip` 
effect can be minimized by proper placement of the lands, i.e., so that 
the average cross-sectional area of the cylinder bore 85 with lands 84 is 
approximately constant. Variations on the above embodiments are possible, 
and the above discussion is not intended to limit the claimed material. 
A demonstration actuator has been constructed possessing the characteristic 
features of the present invention. Further, this demonstration actuator 
was constructed using conventional silicon microfabrication and 
micromachining technology. Accordingly, a detailed description of this 
demonstration actuator appears below. This description is not intended to 
limit the scope of the claimed invention, but rather to demonstrate that a 
member of the class of claimed inventions functions in accord with 
expectations. 
The actuator is produced using a multilayer deposition and etching process 
on a silicon wafer. Specifically, two polycrystalline silicon depositions 
separated by a patterned and etched sacrificial glass layer are used to 
form the actuator. The heater element with contact pads, the piston, and 
the restoring spring with anchor areas are made using the first 
polysilicon deposition. The cylinder cap with anchor areas is made with 
the second polysilicon deposition. The moving components, which comprise 
polycrystalline silicon, are freed from the solid composite by etching 
away sacrificial layers of SiO.sub.2. The top view of the actuator design 
appears in FIG. 8. All elements shown are made of polysilicon, but three 
different areas of the device are emphasized. The darkest are in direct 
contact with the silicon substrate, the light areas are the suspended 
moving elements (and the heater), and the outlined area 98 surrounding the 
heater-piston area is a cap formed over these elements to enclose them. 
Contact pads 90 provide a means for making electrical contact to heating 
element 91. Polysilicon contacts 92 make contact with the heating element 
91, but the smaller cross-sectional area of element 91 concentrates the 
majority of the heat production in that element. The heating element 91 
heats the working fluid 93 to form a vapor bubble. Water has been used 
with success, but it is possible that other fluids such as 
chlorofluorocarbons or low-viscosity oils may work better. 
The piston 94 is 6 .mu.m wide and 2 .mu.m high, and is designed for a total 
travel of 10 .mu.m. Said piston is restrained by springs 96 so that at 
rest, one end of the piston is immersed in the working fluid 93. The 
springs provide a restoring force to piston motion, and also allow the 
force produced by the actuator to be measured. The springs are anchored in 
place by anchor pads 97, which are fixed to the substrate. The piston 94 
is constructed with a plurality of lands 95 (two in this specific 
instance) on the surface of the piston proximate to the silicon substrate. 
Cap layer 98, in consort with anchor areas 99, forms a chamber over the 
heating element 91, the working fluid 93, and one end of the piston. This 
configuration has produced over 1 .mu.N of force, in excellent agreement 
with the model for capillary sealing, and has operated for hundreds of 
cycles without apparent loss of working fluid. 
The techniques used to form the above structure are well-known in the art, 
and hence will not be described in detail. However, a listing of the 
process used is helpful in understanding the structure seen in FIG. 8. 
Unless mentioned, standard etching procedures are used to pattern layers, 
and layers are not patterned unless mentioned. 
1. Begin with a silicon wafer. 
2. Deposit SiO.sub.2 to act as a dielectric layer. 
3. Deposit silicon nitride (SiN) as an etch stop layer. 
4. Deposit a ground plane layer of poly-Si. (This layer is patterned and 
etched. It is not essential to the construction of the device.) 
5. Deposit a SiO.sub.2 sacrificial layer. This will define an air gap when 
removed. 
6. Protect all regions of wafer with photoresist except where piston lands 
are wanted. Etch partway through the SiO.sub.2 layer in these regions to 
produce dimples. 
7. Etch down to the stop etch layer formed in step 3 in regions 90 and 97 
in FIG. 8. 
8. Deposit 2 .mu.m of poly-Si to form the piston 94, springs 96, and 
heating element 91-92 in FIG. 8. This layer must be of the proper 
resistivity to enable the functioning of the heating element. The poly-Si 
also fills the dimples formed in the SiO.sub.2 layer in step 6 to form the 
piston lands 95. 
9. Etch the poly-Si layer to form the shapes shown in FIG. 8. 
10. Deposit SiO.sub.2 (another sacrificial layer). 
11. Etch regions 99 down to the stop etch layer formed in step 3. 
12. Deposit 1 .mu.m of poly-Si. 
13. Etch the poly-Si layer to form the cap 98 over the piston 94 and the 
heating element 91-92. 
14. Remove all SiO.sub.2 in an HF dip, so that an air gap separates the 
piston, springs, and heating element from the surrounding poly-Si 
structures. 
15. Flush with deionized water. 
This is the endpoint of the process if water is to be used as the working 
fluid. As mentioned earlier, there is reason to believe that other media 
may work better, but these have not been examined at this time. A useful 
variation of step 8 in the above process list is to substitute a 
three-layer sandwich of doped poly-Si, poly-Si, and doped poly-Si for the 
2 .mu.m of poly-Si. If such a sandwich is annealed in place, the resulting 
structure is nearly stress-free, giving more design freedom. Note again 
that the detailed description above is of only one of a wide range of 
possible embodiments of the present invention, and is not intended to 
limit the scope of that invention.