WO2006021613A1

WO2006021613A1 - Liquid filled micro-mechanical actuator

Info

Publication number: WO2006021613A1
Application number: PCT/FI2005/000364
Authority: WO
Inventors: Sami MYLLYMÄKI; Eero Ristolainen
Original assignee: Zipic Oy; Ristolainen, Paula; Ristolainen, Noora; Ristolainen, Tero
Priority date: 2004-08-24
Filing date: 2005-08-24
Publication date: 2006-03-02
Also published as: FI20041107A; FI20041107A0

Abstract

A micro-mechanical actuator filled with liquid. The actuator comprises an actuator built on a semiconductor layer (104), the oxide coating under which is replaced by liquid or gas. To the area caused by the actuator motion a voltage signal is fed resulting in change of actuator position and causing liquid-transmitting motion on the actuator second film (104). The actuator can be used for transmission of power and heat. The actuator can be built on the same semiconductor layer as the SOI-microcircuits.

Description

LIQUID FILLED MICRO-MECHANICAL ACTUATOR

Field of invention The invention relates to micro-mechanical components and especially achievement of an actuator filled with liquid.

Background of invention

In recent years systems comprising micro-mechanical components have become general and commercial. Usually different micro techniques can be called micro system technique (MST), which can for instance be defined as technique by means of which different miniature size mechanical, electronic, optical and other components are assembled and made working unites. Typically, the micro system comprises at least one part, the dimensions of which are of micrometer class. For instance, the micro systems can be divided into microelectro-mechanical systems (MEMS), micro-optoelectro- mechanical systems (MOEMS) and microfluidics.

Typically, micro-mechanical systems are based on thin film based surface micro-mechanics. Usually, by means of physical and chemical coating methods, such as amorphous or polycrystalline thin films are formed on silicon wafer, on which then micro-mechanical structures are formed by means of etching techniques, typically used for production of integrated circuits. Typically the moving films of micro-mechanical components are made either of single silicon or of polycrystalline silicon. The film can be made movable so that typically there is under the film a thin insulating layer, which can be corroded off through openings in the silicon film.

Generally, today the most used techniques of surface micro-mechanics are so called SOI-micro-mechanics (Silicon-on-Insulator), where on silicon wafer, working as surface basis, a thin single crystal silicon layer is made so that between them remains a quite thin insulating oxide coating. Then, for instance, the thickness of the silicon basis can be about 50 - 500 μm, the thickness of the oxide coating 25 -200 nm, the thickness of the of the silicon layer about 25-200 nm. A silicon wafer structure like this is typically most suited as raw material for different micro-mechanical components, since thus an above presented etched oxide coating is completed, on which there is a single crystal layer, the mechanical properties of which are of higher quality than those of polycrystalline silicon.

A remarkable factor in the present function of microcircuits concerning both techniques and economy are the cooling devices of microcircuits. According to investigations the loss of power of microcircuits per unit area will continue to increase in the future. Development of wireless terminal equipment to ever greater efficiency even leads to more effective implementation of microcircuits. Regarding efficiency the equipment includes several parts usable for power transmission, as for instance thermal joints, heat distributors and cooling ribs. In the equipment there is need for small and reliable advantageous water coolers, heat machines, heat transmitting materials and thermoelectric coolers. Furthermore, among microcircuit packages there is need to develop new types of packages, by means of which it is possible to minimize heat resistance between the air of microcircuit and package. In micro-fluidics many kinds of components are developed for transportation of liquid or gas in micro-sized channels in the scale of the microcircuit. An example of fluidics is the write head of ink jet printers of today, where this technique is used maybe more extensively. In the write head micro channels are used, from which the ink vaporizes and thus a small amount of ink drifts onto the paper. Micro-fluidics has especially developed the fields of chemistry and biochemistry, where by means of MEMS technology new products are developed, which renew the methods in every day life. By these methods one has succeeded to compensate the laboratory with microcircuit, which is able to analyse a reliable result from a very small amount of samples. The benefit of this is, for instance, by analysis of food quality, bacteria, poisons and allergens, without having to send samples to a traditional laboratory.

By means of micro-fluidics for cooling of microcircuits many different components and methods are developed. Among these the most known are the micro channels, micro motors and different valves, by means of which the flow of coolant is regulated. As coolant besides normal water other liquids are developed. The liquids have often many properties, which influence the use of them. Such ones are for instance the boiling point, thermal conductivity and viscosity and surface tension. Of importance is also the size of the dielectric constant of liquid and whether there are charge carriers in the liquid. Normal arrangement by microcircuit cooling is a cooling rib including hundreds of micro channels and placed in direct contact with the micro circuit. Liquid flowing through the micro channels is pumped by a micro pump and cooled in a micro- size heat machine that releases the heat to the package outside. Such an arrangement is a typical so called SOP structure (System-On-Packet). However, there are in present micro-mechanical cooling systems many problems. The first of them is the insufficient possibility to integrate micro channels connected to the cooling system, which transmit cooling liquid quite close to the electronics to be cooled. This works mainly combining complicated cooling structures with a normal micro circuit. Then the achievement easily includes structures built in several layers, which means manifold raise of price of the of the whole system compared with the price of the actual circuit of electronics. For instance, the SOI-microcircuit includes an insulating oxide coating, the thermal conductivity of which is poor. Then the heat transmission towards the substrate reached by the circuit remains poor and especially by manufacture of active components notable inconveniences arise. In cooling systems according to present technique separately a great number of micro channels are needed, which take much space, and ,which in order to work, need structures that distribute cooling water, as taperings and bypass manifolds. These raise the price of the system and add joints, which reduce the reliability of the whole. Likewise, for pumping of cooling water at least one micro pump is needed, to which the pumping is concentrated. Such a pump includes a stator and a rotor, which wear mechanically, and is complicated to manufacture. It must often be made on separate piece of the microcircuit, so the number of microcircuits grows.

A problem in microcircuits according to present technique is the distribution of heat onto a small area. This area is determined the by components that use high power on a small surface area, a power transistor for instance. A remarkable part of power is released as heat onto a small area. Cooling elements, which are difficult to get close to enough the heat source, due to their big size, can therefore be called powerless, even if their apparent cooling capacity is high.

In systems including many microcircuits according to the present technique, cooling systems must be built between several physically separate microcircuits. The reliability and operating life of the cooling system reduces, due to the complexity of the cooling system. Likewise the technologies of packages get more complicated and to packages new technical demands are made. In actuators according to the present technique the basic division is between magnetostrictive and electrostatic actuators. It is possible to produce also push forces with magnetic actuators, while with electrostatic actuators pulling forces only. It is more difficult to produce magnetic actuators and so the price is a remarkable restriction in taking them to use.

Short presentation of invention

The object of the invention is to develop a liquid-filled micro-mechanical actuator so that the above mentioned disadvantages can be reduced. The goals of the invention are achieved by means of a liquid-filled micro-mechanical actuator characterized in that what is said in the independent claims. The advantageous embodiments of invention are the aim of the dependent claims.

The invention is based on the fact that the liquid-filled micro-mechanical actuator is controlled electrostatically, i.e. by voltage, so that the bend of actuator transmits the power into the liquid, which transmits the power further to another part of the actuator. Into the actuator power is caused in order to produce a motion through the electrodes, the one of which is on top of the structure and the other under the structure for instance on the substrate. The power causes pressure to closed cavity, which is filled with cooling liquid. The pressure discharges in another part of the actuator, where a mechanically bending area is arranged for it. Since the mechanical power bends the electrodes closer to each other, the power caused by cooling liquid bends the part of the actuator outwards, whereat balance of powers is achieved. In causing variation in the balance of power, it is possible to pump liquid between the actuator different parts, whereat the actuator can be used as cooling element.

According to an advantageous embodiment of the invention the actuator position is electrostatically changed in causing between the electrodes of the actuator a difference of voltage. According to an advantageous embodiment the actuator has a part needed for causing mechanical motion and another part into which the liquid is transmitted through the liquid

According to an advantageous embodiment the actuator is closed, i.e. enclosed. This means that the oxide coating of SOI-microcircuit is etched off the actuator area and the emptied area is filled with cooling liquid, which can for instance be water. According to an advantageous embodiment, the, not-compressed cooling liquid ills the whole emptied space, whereat by means of it the power can be transmitted from one actuator part to the other. According to another embodiment the space can also be partly filled with gas or the actuator can be partly open, whereat power cannot be transmitted with liquid, but the actuator can by means of the liquid flow be used as cooling element.

According to an advantageous embodiment of the invention the liquid-filled micro-mechanical actuator is formed at least of two bending elements, to the first element of which power is caused and to the other element, as to its self value, an equally great but opposite power is formed. If there is a number of other elements, the total of powers caused into these power is formed. If there is a number of other elements, the total of powers caused into these elements is as great as the power caused to the first element. According to an advantageous embodiment actuators are so combined that a fluid flow system is formed, by means of which it is possible circulate the cooling liquid inside the SOI-microcircuit to. According to another advantageous embodiment the actuators that form the system are controlled by several different phase signals so that a cooling system is formed that functions by the resonant frequency of the system, whereat in the SOI-microcircuit movable heat flow is maximized.

According to an advantageous embodiment of the invention on that part of the actuator to which power is produced, the stiffness differs from that on the other part. According to this advantageous embodiment the actuator can be used to produce a great and local pushing force. Then the actuator works as transmitter of power in the manner of hydraulic systems. The bending of this advantageous embodiment is influenced by the stiffness of the part to be influenced and the stiffness of other actuator part. The mechanical properties of motion of the actuator can be advantageously influenced by regulation of size and stiffness of the different parts of the actuator.

According to an advantageous embodiment of the invention in the SOI- structure, including substrate, an insulating oxide coating and topmost a semiconductor layer, a liquid-filled micro-mechanical actuator is formed. According to an advantageous embodiment the semiconductor layer of the clutch is of silicon, silicon germanium or some other semiconductor usable in thin film applications. Further, According to an advantageous embodiment of the invention the said semiconductor layer is as to its thickness essentially 25-200 nm and advantageously 70-100 nra. According to an advantageous embodiment of the invention the actuator liquid is a liquid with a high dielectric constant. Then with the actuator a large capacitance can be achieved for instance the dielectric constant of water is 80, when the dielectric constant of silicon dioxide is 4,2.

An advantage of the liquid-filled actuator according to the invention is that by means of the actuator power can be transmitted from one actuator part to another or between actuators. By power transmission the liquid does not loose much by small frequencies, so by power transmission there is no great loss of energy. Further, by means of the actuator an attractive i.e. water-repellent force can be caused, when the electrostatic power, regardless of the voltage polarity, is a force that pulls electrodes together. Water- repellent powers can in a well-known, manner be produced magnetically, so that for production of water-repellent power by means of the actuator according to the invention a more simple and more profitable structure can be achieved than by means of magnets integrated with difficulty.

An advantage of the micro-mechanical actuator according to the invention is that the actuator can be used as cooling element in SOI-microcircuit applications. Of the actuators a movable system can be easily combined into an effectively water moving system, whereat the heat can be effectively taken to an advantageously greater area facilitating continued cooling. It is easy to join the actuator to other actuators placing them advantageously partly overlapping, whereat no separate micro channels are needed. The actuator according to the invention is cheep to manufacture, since it can be made in the same structure as the SOI-microcircuits. The advantage of the actuator is also that the outside cooling elements need not be so effective as earlier, whereby the total price of the system drops.

Short presentation of figures

The invention is disclosed in connection with the advantageous embodiments with reference to the figures, where: Figures Ia, Ib and Ic show the structure of the liquid-filled micro-mechanical actuator according to the invention.

Figures 2a, 2b and 2c show different alternative connection structures of the liquid-filled micro-mechanical actuator according to the invention. Figure 3 shows the connection form of the liquid-filled micro-mechanical actuator according to the invention, which can be used especially for cooling.

Detailed representation of invention

With reference to figures Ia, Ib and Ic in the following the structure of the micro-mechanical liquid-filled actuator is illustrated. Figure Ia shows schematically the crosscut of the of the actuator, dead, and figure Ib shows schematically the crosscut of the of the actuator, active. Figure Ic shows schematically the clutch from above. In figures Ia, Ib and Ic common reference numbering is used. The dimensions of illustrated structures are shown in figures Ia, Ib and Ic so that the invention can be advantageously visualized so that the dimensions do not correspond to the actual dimensions of the clutch.

As manufacturing material of the liquid-filled actuator silicon wafer is used, which comprises the substrate 100, an oxide coating 102, an etched part 103, from which the oxide is removed, and a thin topmost silicon layer, i.e. so called SOI-layer 104. As surface conduction of the structure metal conductors 105 are used. Advantageously the thickness of substrate 100 is of class 50-500 μm, the thickness of oxide coating 102 about 25 - 200 nm and the thickness of SOI-layer 104, about 25 - 200 nm, which essentially corresponds to the thickness of structures generally used in microcircuits. Advantageously the surface conductors 105 is 1 - 10 μm,. The actuator according to the invention is a advantageously filled with liquid, which can be water or something else, for instance FC-72 cooling liquid. The surface area of the microcircuit can change depending on the operational range so that its diameter can be of the microcircuit largeness, that is several millimetres, or it can be local, of a size of few millimetres.

As manufacturing material of SOI-layer 104 I-type semiconductor is used, advantageously unalloyed single crystal silicon. As material of SOI-layer 104 for instance also silicon germanium (SI_xxGβy) or some other thin film semiconductor can be used, whereat the actuator is easier to integrate as part of the manufacture of known microcircuits. Since as material of SOI-layer 104 even other semiconductors than silicon can be used the term SOI can be extended to mean "Semiconductor-On-Insulator". The thickness of oxide coating 102 correlates exponentially in regard to the voltage level needed by use of the component, so in order to minimize the voltage level the oxide coating must be advantageously held as thin as possible. Thus a low voltage actuator is advantageously achieved, as operating voltage of which the typical micro-circuit operating voltage can be used. Substrate 100 is at electrodes 105 advantageously so alloyed that its conductivity is typically of class 0,01 (Ωcm)^'1, whereat time constant RC, which damps the response of input voltage of the actuator clutch, remains advantageously small. Conductors 105 made on SOI-layer can be left out, whereby the alloyed areas of the SOI-layer function as conductors, or between conductors and SOI- layer non-conductive areas can be made. Viewed from above SOI-layer 104 of the actuator can, in mechanical sense, be divided into two areas: a mechanically moving area 112 and an immobile area 114 surrounding it. Further the moving area can be divided into an electrode area tol 16 needed to produce necessary motion and into and other area 118, on which the power transmitted by the 1 liquid is directed.

Between the moving and immobile area there is an interface 120 fitting said areas mechanically to one another. For control of the actuator, that is for voltage supply, an area 116 is formed, which is either an area metallized on the silicon film surface or an area strongly alloyed in the silicon film.

The properties of the actuator according to the invention are as to their essential parts on the other hand influenced by the SOI-layer 104 thickness, on the other hand the by surface areas of areas 116 and 118 of the clutch , and on the other hand by the oxide coating 102 thickness. Furthermore, the function of the actuator is influenced by the properties of filling liquid, which are for instance boiling point, heat capacity, heat conduction, surface tension, dielectric constant and conductivity. Since due to an advantageous embodiment of the invention the actuator can be made in the same semiconductor layer together with other microcircuit and since the thickness both of the SOI-layer and of the oxide coating influences the function of other microcircuit the aim is to influence the clutch properties, mainly the actuator surface area, and regulating the properties of liquid.

The actuator function is mainly based advantageously on that 1 voltage (DC or AC) is conducted to control electrode 16. Typically the largeness of usable voltage can be +/- 0,5 - 100 V. Mostly the actuator is dead in position Ia and active with the voltage in position Ib. Power F , actuator, caused by film bend depending on DC-voltage is calculated according to formula 1 F ^x actuator - ~ — - V ' ² £_Cneae — AJ- ( \Λ^L -ΛJ

2 x²

where

V = DC-voltage

3 air = dielectric constant of air, which comprises (vacuum permittivity) A. = film surface X = distance from substrate

Harmonic power on its part describes the power, by means of which the aim is to pull back the film towards balance position. Therefore the harmonic power is marked with minus sign. The harmonic power can be calculated according formula 2.

Fspπng = -kd (2.) where k = spring constant d = bent distance of film from balance position

Usually in case of t a two-film actuator power F_totai — 2* Fspπ_ng, because the active power of actuator must abolish both springback factors . For instance, in case of figure 2a there are five films, so then F_totai = 5*. F_spri_ng, formulas are valid if the actuator liquid is not compressed or if there is no gas as actuator filling. Otherwise compression of material takes a part of work caused by power directed on the actuator.

The absolute values of power and harmonic power, caused by DC-voltage directed on the actuator are in a certain point of equal size. When the film is still bent the absolute value of harmonic power caused by the actuator is greater than the absolute value of the springback factor. This point is called so called pull-in-point, whereat the substrate pulls the film strongly towards itself. After passing the liquid-filled actuator the moving part of film breaks down. It is the weakest part of actuator in a situation, where the actuator liquid does not get compressed. That is why the pull-in-point should not be exceeded When the active voltage of the actuator is removed the film returns to its initial position due to the springback factor.

Figures 2a, 2b and 2c show advantageous embodiments of the invention. The dimensions of illustrated structures are so presented that the invention can be advantageously so visualized that the dimensions do not correspond to the actual dimensions of the clutch of the invention.

Figures 2a, 2b and 2c show the actuator schematically viewed from above and with them it is expected that attention is paid to the spots, where the structures shown in the figures deviate from the figure 1 structure.

An actuator according to the invention and shown in figure 2a can also be made of five films 200 and activating electrode 201 can for instance be positioned in the middle of the film. By means of figure 2a the aim is to pay attention to how complicated systems of the actuator basic form can be built, which have several films and electrodes. Figure 2b shows the actuator schematically viewed from above and by means o them the aim is to pay attention to the manufacturing method of the invention. The actuator can be made circular by etching the oxide coating through a small hole in the silicon layer. By regulation of etching time the size of the etched figure can be changed, be manufactured Then greater figure 220 can for instance be manufactured repeating small figures 222. The size of the actuator films can change and large films can be joined with smaller films 224. Furthermore, the embodiments of the actuator according to the invention are not dependent of geometry, but the embodiments can be of any geometric form, as square or polygon. The manufacturing method of the actuator can also be some other method, where an actuator according to the claims of the invention is achieved.

Figure 2c shows an actuator according to the invention, where five films 203 and two electrodes 204 have been used. Here expressly one wants to point out that the micro-mechanical actuator can be used for cooling of component 205 so that the component to be cooled is at least partly overlapping the actuator. Then effective cooling can be achieved, when the cooling liquid is in direct connection with the component to be cooled, for instance with the power transistor channel. The structure shown by the figure can be carried out in many other ways, which can change within the frames of the claims.

A cooling system according to an embodiment of the invention can be made for instance according to figure 3 so that the system is formed of six film parts and of two activation electrodes. The function of system is controlled by voltage-fed separately phased to control electrodes, as effectively as possible. Phase shift is so timed that liquid-circulation between films 1 - 2 -3 - 4 takes place as effectively as possible. Embodiments of the actuator or those of the system produced by it are not dependent of the number of films or electrodes or the position in regard to each other.

The surface areas of films or electrodes can change within the frames of the claims. Furthermore, the actuator embodiments are not dependent of the manufacturing method but can be produced by different methods of different materials.

Clutch structures according to the invention are especially usable in different kinds of communication equipment, as for instance GSM- and UMTS-mobile stations and 4^th generation broadband networks. Other usable systems are among others Bluetooth, WLAN IEEE 802.11 HIPERLAN (High Performance Radio Local Area Network). In these systems the main equip-ment uses frequencies that change about between 900MHz - 5,8 GHz.

For the professionals on the field it is obvious that when the technique advances the basic concept of the invention can be realized in many ways. The invention and its embodiments do thus not be restricted to the above examples but can change within the frames of the claims.

Claims

1. A micro-mechanical actuator comprised of a chamber space formed by means of a level (104) and edge parts (102) on top of a semiconductor layer (100), characterized in that level (104) is moving mechanically and comprising a first portion and with it a second portion moving oppositely phased, whereat the first moving portion is fitted to work as response to voltage fed to the portion and that there is liquid or gas working as transmitter of the motion between said portions.

2. An actuator according to claim 1 characterized in that level (104) has a substantially moving portion (112) and an immobile portion (114), the moving surface area of which comprises a first area (116), which comprises an area causing the mechanical motion and a second area (118), which comprises an area caused by transmission of liquid or gas.

3. An actuator according to claim 1 characterized in that it is filled with gas or liquid transmitting power of actuator.

4. An actuator according to claim 1 characterized in that it is filled with gas or liquid which transmit heat energy.

5. An actuator according to any above claim characterized in that it is made in the SOI-structure, which comprises the substrate, an insulating oxide coating and topmost a semiconductor layer, onto which said actuator including the layer of conductors on it is formed.

6. An actuator according to claim 1 characterized in that the said semiconductor layer is of unalloyed single crystal silicon, silicon germanium or some other material used in film-microcircuits.

7. A cooling or power transmitting system using liquid or gas as medium characterized in that the cooling or power transmitting system is formed by means of one or several actuators according to the invention.