Electromechanical energy conversion in the nanometer gaps

Electromechanical energy conversion in the nanometer gaps E.G. Kostsov Institute ofAutomation and Electrometry, Russian Academy ofSciences, Novosibirsk, Russia, [email protected]

The summary The design the new principle of electromechanical energy conversion that allows one to carry the electromechanical energy conversion in the nanometer gap, and significantly (up to two orders of magnitude) increase MEMS specific energy output, operation speed and power. The energy conversion takes place in the nanometer gap (5 — 200 nm), when the electric energy accumulated during reversible electrostatic pressing of the free metallic film (moving electrode) to the surface of the thin crystalline dielectric (ferroelectric film, FF) with high dielectric permeability c (more than 3000-5000) is transformed into mechanical energy . The tension ofthe metallic film caused by electrostatic forces in converted into the mechanical motion of the moving element of the device. With this approach, the specific energy output of 0.3 — 1 10 J/mm2 and driving force of 0.01-0.3 N can be achieved starting from the first microseconds of the voltage pulse. An experimental investigation of new electromechanical energy converter is performed. Keywords: MEMS, micromotors ,electromechanical energy conversion, nanometer gap, high dielectric permeability.

Introduction MEMS are one of the most active fields of modem microelectronics. The main problem of microelectromechanics is the creation ofthe innovative micromechanical constructions with wide potential applications. This activity builds on the achievements of modem microelectronic technologies, inheriting the main advantages of integrated circuits: low cost, high reliability and reproducibility ofparameters, and wide practical applications. The electromechanical energy converters are the main elements of MEMS. These converters can utilize different principles of energy conversion: electromagnetic and capacitive (jiezoelectric, electrostatic). The latter are preferred for MEMS design as more technologically appropriate. The classical capacitor electrostatic motors are not widely used because of very low energy output caused by the low electric field strength E in the operating gap, and because they need high operating voltage. The use of the microelectronics technology for the creation of the electrostatic micromotors allows one to make operation gaps as small as several micrometers, and thus to get much higher values of E and electric field density compared to the macroscopic prototypes. With the help of silicon deep etching technology the gaps of about 2 of the elemental actuator can be as sm can be created, so the specific capacity and specific energy output high as 4 pF/mm2 and 108 J/mm2 respectively, and the driving force can achieve the value of 106 i0 N. The operation principle ofthese microactuators is as follows: the moving electrode is pulled in the interelectrode gap with the pulling force equal to (V23C/ôx)/2. The drawbacks ofthese microactuators are the small

values of the main parameters C, F and the small range of moving element (moving platform, MP) motion — on the order of 5 — 50 rim. To increase the power of the device it is necessary to use many microactuators in parallel and, consequently, use a significant part of the integrated circuit surface. A number of important practical applications require higher values of the main parameters, so MEMS designers are constantly striving to improve these parameters. that can be achieved by further decreasing the gap value is limited both by possible gap The increase in breakdown (or corresponding voltage decrease) and by difficulties arising because of the need for more and more precise electrode motion. This work describes the new principle of electromechanical energy conversion that allows one to cany the electromechanical energy conversion in the nanometer gap, and significantly (up to two orders of magnitude) increase MEMS specific energy output, operation speed and power [1]. The energy conversion takes place in the nanometer gap (5 — 200 nm), when the electric energy accumulated during reversible electrostatic pressing of the free metallic film (moving electrode) to the surface of the thin crystalline dielectric (ferroelectric film, FF) with high dielectric permeability c (more than 3000-5000) is transformed into mechanical motion. The tension ofthe metallic film caused by electrostatic forces in converted into the mechanical motion ofthe moving element ofthe device. With this approach, the specific energy output ofO.3 — 1 106 J/mm2 and driving force of0.01-0.1 N can be achieved starting from the first microseconds of the voltage pulse. The new microelectronic element has the following structure: conductor — dielectric with high dielectric permeability — nanometer air (or vacuum) gap — moving conductor (thin metallic film). The latter can be repeatedly and reversibly pressed to the surface of the dielectric by the electrostatic forces in a few microseconds time, and after the end of the voltage pulse it can equally quickly recoil from this surface under the effect of the mechanical forces. Micro- and Nanoelectronics 2007, edited by Kamil A. Valiev, Alexander A. Orlikovsky, Proc. of SPIE Vol. 7025, 70251G, (2008) 0277-786X/08/$18 · doi: 10.1117/12.802501 Proc. of SPIE Vol. 7025 70251G-1

Characteristics of thin-film structure of M - a ferroelectric - a mobile electrode at influence of an electric field of the nanometer gap (air or vacuum) as a function of the interelectrode Fig. 1 shows the specific capacity gap size. This figure also shows the specific capacity ofthe gap when a dielectric layer of thickness d, with a specific c/d value is placed in the interelectrode space, with the total distance to the moving electrode equal to d. It can be seen that for high values of c/d (more than 1 O) the capacity, and, consequently, the attraction force between the

Fig. l.The dependence of specific capacity on air gap d for M-FF-P system

O8//j 11

0,c

>0,4

7

0,2

0,0

4 100

200

o300 dA z,

400

500

Fig.2.The ratio ofvoltage drop at the gap V to the applied voltage V for various d and e/dffm' : 1 1Ob0; 2 -io; 3 2*108; 4

fl7

two electrodes are close to those typical for the nanometer air gap. Besides, in this case almost all the applied voltage drops in the nanometer gap, and only small part of it drops on the dielectric layer, thus it can be said that the electrostatic force typical for the nanometer air gap is applied to the moving electrode (petal), see fig.2. Analysis ofthe pressing force between the moving electrode F — O.5V2(dC3/d3) and the surface ofthe ferroelectric film for the given values ofV and c/d (fig.3) shows that high values of the force may be obtained only for c/d higher than lO (c is larger than 500). For the linear dielectrics c/d is smaller than 1 0, and so the voltage drop across the nanometer gap is insignificant. It is important that large voltage ofup to 100 V can be applied to the described structure (to the nanometer gap, in effect) without breakdown, because FF have high breakdown strength of more than 100 V/im. In addition, charge

Proc. of SPIE Vol. 7025 70251G-2

is weakly accumulated in the FF under the effect of the electric field, and relaxation after the end of the voltage pulse happens quickly, which is explained by the high quality crystalline structure of FF. Fig.4 shows the schematic of the experimantally studied thin film structure that consists of metal, ferroelectric

E

a

C)

cc1

a 0

3,0x108

dm z,

Fig.3.Variation of the force F, which is pressing the petal to the FF surface at the fixed values of V (100V), as a function of d -

c/dffm': i-io; 2_3.3*108; 3 108;4 J7

ferroeiec-ic fikr

Fig.4 Structure M —FF- mobile electrode

film, moving electrode (thin metallic film) and base (silicon, sapphire). Lantan modified barium strontium niobate (BSN) film Ba5Sr05Nb2O6 +1% La was used as the ferroelectric. The dielectric permeability of this film was 3000-5000. The synthesis of the FF takes place at the "non-orienting" surface of the electrode in such a way that the transition layer between the ferroelectric and the electrode does not affect the specific capacity. Fig. 5 shows a typical curve showing dielectric properties of one of NBS films. The main parameters of these films are described in [2].


In the first few microseconds after the voltage pulse is applied to this structure the moving electrode is pressed to the FF surface with the force of more than i03 kg/cm3, see fig.3. The force depends on the size of the gap, and the size of the gap is defined by the FF surface roughness. The microroughness size on the FF surface is on the order of 3500

3OOO 2500 2000 1500

1000 500

0

-20

-40

20

40

V Fig.5 The dependence of c on V for M-FF-M system, film Ba05Sr05Nb2O5 +1% La, c/d = 2.3 i09

5-50 nm, depending on the technology of FF synthesis. It is established experimentally that the shear force of the

two surfaces after their electrostatic pressing, that is, when the is tumed on is about 5 iO N/J. The comparison of the specific capacity of the M-FF-M structures and experimental M-FF-P structures allows one to estimate d value as a function of V using formula C, CMFFMCZ/(CMFFM+CZ), where C is the capacity of the air gap. The typical experimental values for one ofthe FF samples is shown on Fig.6. As can be seen, for large value ofV the gap width d can be as low as 30-50 A° depending on the FF surface quality and its manufacturing technology. By altering the substrate temperature during the FF synthesis, FF growth speed and thickness one can control the roughness of the FF surface and the d value so it varies from 30 to 1000 A. For the described principle of the electromechanical energy conversion the moving electrode should be quickly separated from the ferroelectric surface after the end of the voltage pulse — the time of the turn off. It is established experimentally that the separation time is in the nanosecond range, see fig.7. The separation of the

3000 2500

d=2.4j.tm,c =1450

2000

cld=6*108

1500

1000

500 I

I

0

20

.

4O

Fig.6. Various d with increasing V in the process of rolling

moving electrode from the ferroelectric surface is so quick because the electrostatic pressing of the thin metallic film to the microroughness on FF surface is accompanied by the reversible local deformation of the film. After the end of the voltage pulse whose descending part has less than 0.1 is duration the accumulated energy is freed thus leading to a very quick separation of the metallic film from the ferroelectric surface. The duration of this process is less than 0.1 j.ts, and it is further influenced by the hardness of the ferroelectric surface -it's Moh's


hardness is equal to 5.5 clock frequency.

— and a

small volume charge in the FF. Thus the energy conversion can be performed at high

120 100

80v 2000

60

V

40

1000

20

0

0 ns Fig.7 Turn-off time of contact M-FF

New principle of electromechanical transformation of energy To transfer the action ofthe large mechanical forces developed in the nanometer gaps along large distances the 3D design ofthe moving electrode (jetal) was used. The 3D energy converter, free metallic film, has to have certain mechanical properties to with stand large driving force: the longitudinal elasticity modulus (Young's modulus) has to be larger than 10' ' N/rn2, its thickness is 1-2 tim. It has to have breaking strength and tensile strength of more than 10 -50 N/rn. The length ofthe petal is defined by the motion range of the moving plate, and the size ofthe step made during one cycle. Fig. 8 shows the schematic of the operation principle of the linear micromotor based on the 3D design of the moving electrode. Micromotor consists of the two plates. The first is the stationary plate (SP), which is a silicon base with an electrode and ferroelectric film 3 applied to its surface. The second is the moving plate (MP), 1, with the metallic petals synthesized on its' surface that move with respect to SP along the guides 2. They are separated by the gap de

A

F1

/

UF2Jg K

/1/2 Li

Fig.8 Illustration of a principle of transformation of electric energy in mechanical at consecutive electrostatic pressing a thin metal film to a surface of a ferroelectric. A - point of attachment of a petal, Fl - force of draft, 1 — slider, 2 —guide, 3 -surface of a ferroelectric film, 4 -Fr, tensioning of a metal film, 6- Si - substrate, 7 - nanogap with the width of 10— 200 tm.


Some experimental devices When the voltage pulse is applied between the petals and the electrode, the electrostatic force reversibly rolls larger and larger parts ofthe petal onto the ferroelectric surface. The free moving metallic film is bent and stretched, and the motions is transferred to the plate 1 . Thus electromechanical energy conversion is performed. The rolling length lr(t) grows during the voltage pulse, and, consequently, the motion step of the MP h(t) increases. h(t) value and the speed of motion of the petal part that is being rolled onto the ferroelectric are determined by the MP mass M, the duration ofthe pulse t,, its amplitude V and friction coefficient k. It is established experimentally that the MP step is in the nano- and micrometer range [3-4], with high reproducibility and low hysteresis. The typical curves that show the energy performance capabilities of one of the samples of the linear micromotor with the number ofpetals equal to 40 (their thickness is 1 .5 pm and width is 500 rim) are shown in figs. 9A and 9b [3-4]. The driving force ofthe micromotor is more than 0.3 N, and the acceleration ofthe 5 grams load is more than several g. A number ofnew microelectromechanical devices can be designed using the high driving force that can be produced by the described energy converter, e.g., fast-acting (microsecond range) commutators with the two states. The switching between the states happens when the moving element acceleration is in several thousand g range [5].

1,2

2.,/'..

O8

''

//

::

0

\\\ ,

500

:'

1000

I

,

1500

2000

t, J_ts Fig.9. Time variation ofa sample velocity MP for M l 1 g and 6 g (2),

tjmp 400 jis, coefficient of friction k =0.3

The described electromechanical energy conversion principles can be used to design power devices, micromotors and microactuators, as well as completely new microelectronic devices. One such device is fast acting gas charging microvalve. This microvalve operation principle is based on the blocking of the opening in the plate on whose surface metal — ferroelectric structure is created. The opening is blocked by the thin metallic film that is pressed onto the ferroelectric surface by the electrostatic force when the voltage pulse is applied [6]. Fig.1O shows the schematic ofthe microvalve design. A voltage pulse causes an elastic metallic films to change its shape and roll with the large force onto the surface of the plate (over 500 atmospheres), thus blocking a pinhole. The pressing force depends on the quality of the ferroelectric film, and, in particular, on the dielectric permeability, as well as metallic film area and voltage pulse amplitude. In this case metallic film area was 15 mm2 and the capacity ofstructure was 4 i09 F, at amplitude ofimpulse ofan electrical pressure 30 V, and for a lift-off of a metal film from a surface of a ferroelectric (the ) it is necessary to put effort not less than 1- 1,5 N. On Fig. 11 is shown change of capacity of structure of M-FF-nanogap - M depending on V, process of closing of a pinhole of the microvalve, the size c/d is equal 6 108.


The valve can be switched from closed state to open state after the end of the voltage pulse, the duration of the process t0ff is approximately equal to the duration of the descending part of the impulse and can be less than 1 ts.

applied \\voltage

thin metal film ferroelectric film

-

J / sapphire substrate

— electrodesf

aperture /

Fig. 10 Electrostatic Micro-Valve

5. 4, 3. LL

1 0

0

/ 10

/ 20

30

40

50

V Fig. 11 Change of capacity of structure of M-FF-nanogap M as functions V during closing a a pinhole.

The design of this microvalve does not assume high geometric precision. The precision is also not required to create multielement array of microvalves.


Conclusion When the thin crystalline dielectric films with the high dielectric permeability value are used in the capacity electromechanical energy converters moves the main stage of the energy conversion takes place the nanometer operating gap, and so the parameters of the converters can be significantly improved: it is possible to -increase specific energy output and corresponding power; -increase the field strength in the operating gap

-increase clock frequency -increase the motion range ofthe moving plate. The described energy converters have possible applications across an entire MEMS spectrum.

REFERENCES 1 . E.G. Kostsov, Ferroelectric-based electrostatic micromotors with nanometer gaps, IEEE Transaction on U!trasonics, Ferroelectric and Frequency Control, Special Issue on Nanoscale Ferroe!ectric, v.53, N 12, pp. 2294 — 2299, 2006. 2. E.G. Kostsov, Ferroe!ectric barium-strontium niobate films and multi-layer structures FERROELECTRICS 314: 169-187 2005. 3. I.L.Baginsky and E.G.Kostsov, Linear electrostatic micromotors for nano- and micro-positioning, Procceding SPIE, v. 5401, pp.613-619, 2004, 4. I.L. Baginsky, E.G. Kostsov, High-energy capacitive electrostatic micromotors, J. Micromech. Microeng. 13,

190—200, 2003. 5. E. G. Kostsov, A. A. Kolesnikov, High-speed electrostatic microswitchboards on the basis of ferroe!ectric films, Ferroelectrics, v. 351, pp. 138-144, 2007. 6. Kamyshlov V.F., Kostsov E.G. , Microe!ectromechanica! Micro-Valve, Journal of Nano and MICROSYSTEM TECHNIQUE, N 12, pp.S'7-6O, 2006.
