WO1989009477A1 - Micromechanical device - Google Patents

Abstract

A micromechanical device comprises an element whose position can be varied consisting of a bimaterial system and which can be placed and maintained in a predetermined position. The instantaneous position of the element is determined by means of incorporated sensors and the measurement signal is combined with the heater current in a common control circuit for the purpose of adjusting the position. The embodiments described include a light modulator, an electrically operated switch and an electrically controlled microvalve.

Description

 Micromechanical device

description

Technical field

The invention relates to a micromechanical device with a position-variable element, which has a fixed and a loose end and consists of layers of different materials with different thermal expansion arranged one above the other. The position of the element is changed by changing the temperature, as a result of which the layers expand to different extents. The position change can be used for different purposes, e.g. B. for switching contacts.

State of the art

In the publication "Micromechanical Membrane Switches on Silicon" (IBM Journal Research Development, Vol. 23, 1979, pp. 376 - 385), K. E. Peters specifies a micromechanical switching element that uses the bimaterial effect. The element changes its switching state at a certain ambient temperature.

In order to be able to influence the level of the changeover temperature, an additional electrode is attached to the switching element, which forms a capacitor with the movable switching tongue and whose position is changed with the aid of electrostatic forces. A disadvantage Such electrostatically operated elements lie in the fact that the electrostatic force decreases rapidly with increasing distance, which is why the position of the position-changing element may only change minimally during the switching process.

Describe in the publication "Micromechanical Silicon Actuators based on thermal expansion effects" (Transducers 1987). Riethmüller, W. Benecke, U. Schnakenberg and A. Heuberger a micromechanical device with a position-changing element, which is produced as a movable tongue from a silicon-metal layer structure and which can be heated with the aid of an electrical resistor.

The position of the bimaterial element can be changed by a defined amount by a predeterminable heating power. However, the current position of the position-changing element cannot be determined exactly since it depends not only on the heating power but also on the ambient temperature. An exact positioning of the position-changing element is therefore not guaranteed.

Presentation of the invention

The invention is based on the object of specifying a micromechanical device with a position-changeable element which can be positioned and regulated. This object is achieved according to the invention in that sensor elements (6, 7) for position detection are mounted on a micromechanical device with a position-variable element and the output signals of the sensor elements (6, 7) are used by means of a control circuit for position control of the element become.

The sensor elements record the current position of the position-variable element and, starting from the known position, allow any desired change in position. The new position can be maintained unaffected by fluctuations in the ambient temperature.

Advantageous embodiments of the invention are characterized in the subclaims. According to claim 2, the device is built on a silicon wafer in (100) orientation. This means that a commercially available chip is used as the starting material. In order to bring about the greatest possible changes in position with low heating outputs, the position-changing element according to this claim consists of a combination of materials with different thermal expansion coefficients. The layer sequence is chosen so that the position-changing element is bent toward the substrate side when the temperature is increased. According to claim 3, the heater is arranged as an electrical resistance between or on the layers so that uniform heating is ensured. Because of the low heat capacity of the position-variable element, a strong temperature increase per electrical output is achieved. Piezo resistors are used as sensors that take advantage of the piezoresistive effect. This static effect is with semiconductors Particularly well developed for silicon and is suitable for measuring tensile or compressive loads. Another advantage is that piezoresistors can be easily manufactured using the technology of integrated circuits.

According to claim 4, the sensors are formed as strips of piezoelectric or ferroelectric material. Depending on the application, the piezoelectric effect or the ferroelectric effect are then used to measure the deflection. If, for example, the change in position of the position-changing element is to be detected, the dynamic piezoelectric effect is suitable.

In one embodiment according to claim 5, the sensor elements are formed as films made of electrically conductive material. Two films each are applied in such a way that they form a capacitor, with a capacitor plate on the position-changing element and the other plate on the stationary substrate. The change in the position of the position-changing element can then be determined by changing the capacitance of the capacitor. This method is characterized by a particularly high sensitivity. The change in position can also be detected with the aid of magnetic effects.

In order to avoid that the position determination of the position-variable element is influenced by the operating temperature, the sensors and the heater are thermally decoupled.

In a particularly advantageous embodiment of the device according to claim 7, sensor and heater signals are linked to one another in a control loop. As a result, the position-changing element is held in a predeterminable position, for example by regulating the heating power. In order to achieve a high degree of miniaturization, the control loop and the micromechanical device are integrated on the same semiconductor chip. As a result, several identical, controllable micromechanical devices can be produced simultaneously on a semiconductor wafer.

The citation by Riethmüller, Benecke, Schnakenberg and Heuberger mentions the further development of a micromechanical device into a light modulator, a switch and a microvalve. However, there is no way out of how the further training should take place.

A further development of the device into a light modulator is characterized in claim 8, in which the position-variable element is coated with a reflective metal layer. The advantage of this device is that it can both be adjusted in a predeterminable direction and - when an oscillating voltage is applied - is suitable for modulating a light beam.

A further development of the device to an electrically driven microvalve is characterized in claim 9. It combines the advantages of known microvalves, such as small dimensions and light weight, with a particularly simple mode of operation and manufacture. According to claim 10, the device is designed as an electrical switch or electrically driven relay. All of the marked developments of the invention are advantageously produced using the methods known in micromechanics and in microelectronics and are compatible with standard IC processes. The individual components are structured using planar lithography processes. The voltage levels customary in microelectronics are sufficient for the operation of a device according to the invention.

The micromechanical device and its further training are characterized by a high degree of miniaturization, high accuracy, great reliability and low costs.

Brief description of the drawings

Four exemplary embodiments are illustrated below with the aid of drawings. Show it:

1 shows a micromechanical device with a position-changing element in top view (a) and sections along the section lines AA '(b) BB ^« (C) CC (d),

2 shows a development of the device into an electrically adjustable mirror in cross section (a) and in top view (b),

3 shows a development of the device to an electrically driven microvalve in cross section (a) and in top view (b), 4 shows a development of the device into an electrically controlled relay in cross section (a) and in supervision (b),

5 shows the method steps for producing a micromechanical device according to the invention.

Ways of Carrying Out the Invention

The position-changing element (1) of the device in Fig. 1 consists of a layer of silicon or a silicon compound (e.g. 4 μm thick) with a low coefficient of thermal expansion. To form a bimaterial, it is partially covered with a metal layer (2) with a significantly higher coefficient of expansion (e.g. a 2 μm thick gold layer). An electrically operated heating resistor (3) (e.g. made of polycrystalline silicon) is arranged between these layers or on the metal layer. Because of the greater coefficient of thermal expansion of the metal, the movable position-changing element is pressed in the direction of an etching pit (4) which is etched into the substrate (5) with the aid of anisotropic etching methods. The dashed line indicates a possible position of the deflected element.

Since the absolute temperature of the two layers (1, 2) determines the current position of the element, this is influenced both by changes in the ambient temperature and by the conditions of heat dissipation. To measure the current position of the element, sensors (6, 7) attached (eg piezo resistors made of silicon), the resistance of which depends on the deflection of the element. The sensors (6, 7) and the heating resistor (3) are linked in a common electrical control circuit so that the position-changing element can be held in any desired position. For the thermal decoupling of the sensors from the heating resistor, the element is composed of three webs which open into a common surface. The sensors (6, 7) are attached to the side webs, and the heating resistor (3) is attached to the center web.

In the development of the device in Fig. 2, the position-changing element (1) is designed as a web with a widened loose end, which is covered with a highly reflective metal layer. The metal layer (2), the heating resistor (3) and the sensors (6, 7) are attached to the narrow area of the web. This development represents an electrically controllable light modulator. In the starting position, an incident light beam (8) is reflected in itself; in the position indicated by the broken line, the light beam (9) leaves the modulator at an adjustable angle of reflection. To increase the reflecting area, many modulators can be operated on a chip in common mode. In order to reflect different parts of a light beam in different directions, the modulators are controlled individually.

The further development of the device shown in FIG. 3 represents a micromechanical valve. The position-changing element (1) is a web a widened loose end that serves as a valve plate. The metal layer (2), the heating resistor (3) and the sensors (6y 7) are attached to the narrow area of the web. The position-changeable element (1) is held at a predeterminable distance from the substrate (5) by a spacer layer (10) (for example an epitaxially deposited silicon layer). The etching pit (4) is designed in the form of a valve opening. By switching on the heating resistor (3), the element (1) designed as a valve plate is pressed against the valve opening. Since the element is more flexible in the area of the narrow web than in the area of the wide loose end, it takes on the shape indicated by the dashed line.

The further development of the device shown in FIG. 4 serves to switch an electrical contact. A switch contact (11) made of metal is attached to the loose end of the position-changing element (1), while the element in the area of the fixed end is formed with a metal layer (2) to form a bimaterial and has a heating resistor (3). Opposite the switching contact (11), two electrodes (12, 13) are arranged on the substrate, which are electrically short-circuited by the contact (11) after activation of the element (1).

The exemplary embodiments shown in FIGS. 2, 3 and 4 are advantageously further developed in that the sensor elements are accommodated on separate webs and are thus decoupled from the heating resistors. The process steps for producing a device according to the invention are shown schematically in FIG. 5.

a) A highly bordoded silicon layer (14) is epitaxially deposited on a silicon wafer in (100) orientation, which serves as substrate (5). It supplies the material for the position-changing element (1). A passivation layer -C15) (e.g. - silicon nitrite) and - as material for both the heating resistor (3) and the sensors (6, 7) - a polycrystalline silicon layer (16) are deposited in succession, which is then doped. b) The heating resistor (3) and the sensors (6, 7) are produced with the aid of lithographic processes and by etching the polycrystalline silicon layer (16). After a passivation layer (17) has been applied, further lithography steps follow. c) A metal layer is deposited and formed into the second layer (2) of the bimaterial by lithographic steps and an etching process. d) The position-changing element (1) is formed by isotropic etching of the epitaxial layer (14) and the etching pit (4) is formed by anisotropic etching of the substrate (5).

Instead of the highly boron-doped silicon layer, a low-doped layer can also be used as the material for the position-changing element. The etching process is then ended by an electrochemical etching stop on the layer surface.

Claims

PATENT CLAIMS Micromechanical device which contains a substrate and at least one layer applied thereon, with a position-changeable element which has a fixed and a loose end and is etched free from the substrate and the layer, the layers arranged one above the other being made of different materials with different thermal expansion ¬ hen, and with a heater to increase the temperature of the position-changing element, characterized in that sensor elements (6, 7) for detecting the position of the position-changing element (1) are attached to the device and the output signals of the sensor elements ( are used 6,7) by means of a control loop for position control of the ^'Elemen¬ tes. Micromechanical device according to claim 1, characterized in that the substrate (5) consists of a silicon wafer with a (100) -oriented surface and that the position-variable element is composed of two layers, the layer closer to the substrate from an epitaxial deposited layer of silicon, a silicon compound or polycrystalline silicon and the overlying layer consists of a metal layer. 3. Micromechanical device according to claims 1 and 2, characterized in that the heater (3) is designed as a resistor and is arranged between the layers, and consists of a silicon or metal layer and that the sensor elements (6,7) are designed as piezoresistors. 4. Micromechanical device according to one of claims 1 to 3, characterized in that the sensor elements (6, 7) are designed as strips of piezoelectric material or ferroelectric material and that for determining the deflection of the position-variable element of the piezoelectric effect or the ferroelectric effect can be used. 5. Micromechanical device according to one of claims 1 to 3, characterized in that the sensor elements (6, 7) are constructed as films made of conductive material, which lie opposite one another in pairs and each of which one on the position-changing element and one is attached to the stationary substrate, and that magnetic or capacitive effects are used to determine the deflection of the position-changing element. 6. Micromechanical device according to one of claims 1 to 5, characterized in that the sensor elements (6, 7) are thermally decoupled from the heating element (3). 7. Micromechanical device according to one of claims 1 to 6, characterized in that the control circuit is integrated on the same semiconductor chip as the micromechanical device. 8. Micromechanical device according to one of claims 1 to 7, characterized in that the position-changing element of the device is at least partially designed as a mirror. 9. Micromechanical device according to one of claims 1 to 7, characterized in that the substrate has a valve opening and the fixed end of the position-changing element is held by a spacer layer (10) at a predeterminable distance from the substrate. 10.Micromechanical device according to one of claims 1 to 9, characterized in that the substrate and the position-changing element are each provided with at least one electrical contact plate (11 or 12, 13), and that corresponding contact plates are electrical Form switches.