DE4211898C2 - - Google Patents

Description

The invention relates to a micromechanical mirror for La water applications according to the preamble of claim 1.

The laser mirrors according to the prior art usually have an egg A glass substrate evaporated dielectric layers with different refractive indices in alternating order. The shift dic ken are to be designed depending on the wavelength so that the multibeam reflection occurring at the interfaces of the layers leads to the desired reflectance by interference. This results in typical layer thicknesses, which are around a quarter of the light wavelength (λ / 4). However, such a mirror is not easily mini turisizable and also has a relatively large, inert mass, so that it cannot be moved very quickly. In addition, such mirrors are right relatively expensive components, because they individually on high optical quality must be polished, steamed and assembled or held.

However, modern lasers are particularly characterized by increasingly smaller marks measurements with extremely high power density. Best known are the semiconductor lasers and those pumped by semiconductor lasers Solid state laser. For the operation of such lasers are mirrors with very good optical properties and precisely defined reflectance such. In most cases, the mirrors are in "monolithic" Construction using a dielectric layer sequence directly on the la seractive component applied. It is for a number of applications however advantageous if at least one mirror of the laser-active medium is arranged separately. On the one hand, you get in many cases a better radiation quality, on the other hand you can separate this Make the mirror movable or adjustable. For example  frequency modulation of the laser radiation is carried out or in In the event of a slight tilt of the mirror the laser on or off are switched, which with suitable control to generate so-called called giant pulses (Q circuit) leads to very high peak power.

Through the DE magazine "Electronics", 22. (1990), pp. 114-126 is a micromechanical mirror for laser applications on a silicon sub known strat, which is largely suspended freely.

Through the US magazine "IEEE Journal of Quantum Electronics", vol. 24 No. 6, June 1988, pp. 1172-1177 it is known to form mirror surfaces of thin dielectric layers on a substrate and from the DE book "Mikromechanik", by A. Heuberger - Springer Verlag 1989, Pp. 399-405 are teaching examples for microfabrication using methods of Given semiconductor technology.

Due to the increasing demand for further miniaturization of laser systems However, the requirements for the compactness of laser pie are also mentioned are always higher. DE-OS 39 25 201 of the applicant is a diode pumped, miniaturized solid-state laser on an optical bench known from silicon, whereby a compact microsystem can be realized.

The invention has for its object a miniaturized mirror to create the type mentioned above, in addition to the semiconductor substrate (Silicon) no longer has any other substrate materials, micromecha is deflectable, very fast movements at high frequencies can lead and can be used as adaptive mirror optics.

This object is achieved by the measures outlined in claim 1 solves. Refinements and developments are in the subclaims specified and in the following description are exemplary embodiments explained and sketched in the figures of the drawing. Show it

Fig. 1 is a schematic diagram of a mirror embodiment on silicon substrate base dielectric coating which is arranged freely suspended in the optically active region,

Fig. 2 is a schematic diagram of a cross-section through a dielectric layer system shows Mul prior to the thinning of the silicon substrate,

Fig. 3 is a schematic diagram of a coating of a laser mirror with locally different reflectance properties for forming a so-called. Gaussian mirror,

Fig. 4 is a schematic diagram of a cross-section through a Ausführungsbei play for a movable laser mirror with electrostatic deflection,

Fig. 5 is a schematic picture in the plan view of an exemplary embodiment of a movable mirror for parallel deflection,

Fig. 6 is a schematic picture in the plan view of an embodiment of ei nes movable mirror for tilting movements,

Fig. 7 is a schematic image for an embodiment of an adaptive mirror with adjustable curvature of the reflective membrane by targeted application of negative or positive pressure in the pressure chamber.

The invention provides a mirror, for example to work out of silicon in micromechanical construction, the actual reflecting surface made of a dielectric multilayer thin film exists. The silicon substrate is now in the optically active area completely removed by etching technology, so that a free-floating, op Table active film is created. This also makes the mirror for Lichtwel length in the visible range - in which silicon is opaque - usable.

In FIG. 1, a simple embodiment is sketched of this invention. In its simplest form, the mirror consists of a highly polished silicon substrate of the quality customary in microelectronics with the crystal orientation (100) or (110). Other single-crystalline substrates such as. B. GaAs, InP or quartz can be used in principle. This substrate is provided with suitable multilayer dielectric layers. Coatings made of silicon dioxide (SiO ₂ ) and silicon nitride (Si ₃ N ₄ ) are particularly suitable, since they are very well compatible with standard silicon technology. However, layer sequences of SiO ₂ and TiO ₂ are also possible. Layers of this type can be produced for example by deposition from the gas phase (CVD, LPCVD) or with plasma support (PECVD). Evaporation and sputtering is also possible. The thicknesses and the number of individual layers are calculated according to the desired optical properties (reflection, transmission). They typically move at λ / 4 (approx. 100 to 200 nm for visible light). Fig. 3 is illustrated as a coating step in a greatly enlarged scale.

After the production of the optically active coating, the silicon substrate in the active mirror region is completely removed by etching technology, so that the dielectric layer package acts as a free-floating mirror ( FIG. 1). This can be done with conventional wet chemical etching solutions, such as. B. KOH can be achieved. By appropriately setting the stratified mechanical stresses, it can be ensured that this stratum is highly planed.

For some applications, it is advantageous that the reflection property of the mirror can be designed to be variable over its surface, in order to eliminate undesirable diffraction effects or at least to minimize them considerably. In particular, there are applications in which the reflection properties are to be varied according to a Gaussian function (Gaussian mirror). By including the microelectronic manufacturing techniques, such as in lithography, selective wet chemical or dry etching processes, it is possible to produce such profiles economically by appropriately designing the multilayer systems with locally different layer thicknesses. An exemplary embodiment is illustrated in FIG. 3.

The Fig. 4 is a schematic image of a further embodiment in which the mirror can also be actively moved by a more complex structure. For this purpose, the above-described mirror is etched in such a way that the optically active region is suspended on thin bending beams made of silicon or a suitable thin film in a freely movable manner. For reasons of integration, it is advisable to use electrostatics to apply the force. For this purpose, the silicon substrate is electrically contacted, so that the mirror represents an electrode of a capacitor arrangement. This unit is connected to a second silicon substrate, into which a through opening for the transmission of the light beam is worked. Furthermore, a shallow depression is provided, which defines the distance between the electrodes and the movable part and thus also the freedom of movement of the mirror. Capacitor electrodes are applied within this recess. These two substrates are made by suitable methods, such as. B. anodic bonding or direct Si-Si bonding mitein connected. The mirror can be moved electrostatically by applying an external voltage to the electrodes.

In addition to electrostatics, other force principles, such as for example, piezoelectric or magnetics can be used. This can be done using a corresponding small control element, or a permanent magnet the mirror arrangement can be added.

Depending on whether a parallel displacement or a tilting movement of the mirror is desired, the suspension of the movable part and the arrangement of the electrodes are designed differently. Fig. 5 shows an arrangement in plan view, which for a parallel deflection - z. B. for frequency modulation of the laser - is designed. In this case, for example, a highly symmetrical, diagonal arrangement of the bending beams is selected. The counter electrode on the lower cover wafer is not divided.

In contrast to this, FIG. 6 shows an arrangement which is suitable for tilting movements of the mirror, for example in order to implement a Q switch. The suspension is selected here, for example, in the form of two goal beams. In this case, the electrode surfaces are divided into two separate halves which can be controlled independently of one another.

Another possibility that results from the thin-film arrangement of the mirror is the targeted warping of the mirror surface by applying a negative or positive pressure. So that z. B. adjust the focal length of the mirror so that targeted focusing is made possible. An exemplary embodiment of this is outlined in FIG. 7. The base mirror element is connected to a transparent substrate - for example glass - so that a closed cavity is formed, which is connected to an external pressure reservoir or a pump by a selectively controllable channel.

This creates a mirror, which due to its design conditioned very small mass, very fast movements with high fre can realize sequences. Due to the lack of a substrate in the optical Anti-reflective coating on the back does not have to be carried out. The laser mirrors proposed here can be processed using the Mi krosystemtechnik be manufactured so that their dimensions in the order of magnitude of modern laser-active elements in small and medium Their power range is (laser diodes typically 300 µm · 500 µm · 30 µm, solid-state lasers typically 500 µm · 1 mm · 1 mm).

Claims (7)

1. Micromechanical mirror for laser applications with layers of different refractive indices vapor-deposited in alternating order, on a silicon substrate, the mirror being suspended largely freely floating, characterized in that the mirror consists of a silicon or single-crystalline (semiconductor) substrate, which has multilayered substrates dielectric or metallic layers are provided and the substrate in the active mirror area is completely removed by etching. 2. Micromechanical mirror according to claim 1, characterized net that the active mirror area in its reflection properties is designed to be variable, profiles with locally different Layer thicknesses are produced. 3. Micromechanical mirror according to claim 1 or 2, characterized records that the optically active mirror area as a Gaussian mirror is trained. 4. Micromechanical mirror according to one of claims 1 to 3, there characterized in that the optically active mirror area ätztech niche on thin bending webs from the semiconductor substrate or with a suitable thin film is hung freely movable. 5. Micromechanical mirror according to one of claims 1 to 4, there characterized in that the semiconductor substrate makes electrical contact is so that the mirror bil an electrode of a capacitor array det, which is connected to a second semiconductor substrate and a shallow recess, in which capacitor electrodes are applied, pre is to see the electrode distance to the moving part and thus determines the freedom of movement of the optically active mirror area.   6. Micromechanical mirror according to claim 5, characterized net that the two semiconductor substrates by anodic bonding or direct Si-Si bonding. 7. Micromechanical mirror according to one of claims 1 to 6, there characterized in that the optically active mirror area by sub pressure or overpressure can be specifically varied in its curvature and thus the focal length of the mirror is changeable.