WO2005050801A1

WO2005050801A1 - Conductive posts for use in air gap-based, optoelectronic components

Info

Publication number: WO2005050801A1
Application number: PCT/DE2004/002432
Authority: WO
Inventors: Hartmut Hillmer; Sören Irmer; Jürgen Daleiden; Amer Tarraf
Original assignee: Universität Kassel
Priority date: 2003-11-14
Filing date: 2004-11-01
Publication date: 2005-06-02
Also published as: DE10353546B4; DE10353546A1

Abstract

The invention relates to optoelectronic components and to methods for producing the same. The invention specifically relates to vertically oriented components having a layer structure (69) comprising a plurality of optically effective layers (53, 56, 57, 58, 59) and at least one air-filled cavity (55). Said layer structure (69) is supported by means of at least one outer marginal support (67) that is associated with a marginal area of the layer structure. In order to increase the mechanical stability of the layer structure, at least two layers of the layer structure are firmly interconnected by means of a connecting element (68). Said connecting element is not located in the marginal areas of the layer structure but is positioned as close as possible to the optically used area (51) of the layers (53).

Description

Conductive holding posts in air gap-based, optoelectronic components

STATE OF THE ART

The present invention relates to optoelectronic components and methods for their production. In particular, it relates to vertically oriented components with a layer structure with a plurality of optically active layers and at least one gas or liquid-filled, in particular, however, air or vacuum-containing layer space, the layer structure being held by means of at least one outer edge support assigned to a respective edge region of the layer structure ,

In the past ten years, vertically oriented, optoelectronic components have gained more and more economic interest compared to components oriented horizontally on a wafer. This group includes, for example, the so-called "Vertical Cavity Surface Emitting Lasers", abbreviated as VCSEL, or "Vertical Cavity Fabry-Perot Filters", abbreviated as VCFPF.

The main advantages of such vertically oriented components compared to horizontally oriented components are that vertically oriented components can already be tested on the wafer prior to assembly (packaging), ie they do not need to be separated from the wafer in order to be able to be tested. This means that the test procedures can be made much more efficient. Another advantage is that a more effective coupling to glass fibers is made possible, as a result of which a simplified structure can be implemented in a chip housing which contains the vertically oriented optoelectronic components. With optical filters and lasers, there are two mirror arrangements, separated by the cavity. Their geometry and in particular their length is of great importance for the optical properties. In the case of vertically oriented optical filters and lasers, the cavity is generally very short, which is why a high, longitudinal mode spacing is achieved. With VCSEL, the active volume is very low, so low threshold currents can be achieved. The manufacture of the mirrors, which are designed as DBR (Distributed Bragg Reflectors), is currently still relatively difficult, since a very high reflectivity of the mirrors of> 99.7% is required to meet the requirements for commercial use such components are specified.

A special branch within the development of the aforementioned vertically oriented components is the technological approach to form the layer structure of a DBR mirror by an alternating sequence of material membrane layers and intermediate air layers (air gaps). In other words, the layer structure of a DBR reflector contains, for example, a number n of membrane layers and a number of n-1 intermediate air gaps.

The technical structure, the mode of operation in various technical applications, as well as the most important optical properties of vertically oriented, air gap-based, DBR components is disclosed in “H. Hillmer, J. Daleiden, C. Prott, F. Römer, S. Irmer, V. Rangelov, A. Tarraf, S. Schüler, M. Strassner: “Potential For Micromachined Actuation

Of Ultra-wide Continuously Tunable Optoelectronic Devices. ", Applied Physics B 75, 3-13 (2002), published August 8, 2002.

The use of air gaps as a separation layer between the individual membrane layers is advantageous because an interface (air gap / membrane material) formed by these two layers has a very high jump in refractive index. As a result, the above-mentioned high reflectivity of 99.7% of the DBRs with only a few periods, e.g. with indium phosphide (InP) / air with 3 membrane layers and 2 intermediate air gaps. There is also a positive side effect: optoelectronic components, the above Using air gap mirrors and an air gap cavity (see SzBog reference Hillmer et.al.) can, for example, be influenced in a targeted manner with regard to an important optical property, such as their transmission spectrum, for example if the optically effective length L of the cavity between the two DBR reflectors is increased or is reduced. Depending on the setting of the cavity length L, the maximum of the transmitted wavelength range as a sharp peak is at different wavelengths. This tuning can be done by means of capacitive (in connection with conductive membranes) or thermal actuation, and appropriate control.

Such air gap-based components from the prior art, however, have the disadvantage of relatively low mechanical stability, although with increasing miniaturization of such components, the influence of inertial forces on such layers tends to decrease, as the above-mentioned publication shows. Nevertheless, the influence of mechanical tension in the aforementioned individual membrane layers yet so large that certain deformations can occur that are difficult to control, particularly in industrial manufacturing processes, that can cause unwanted changes in the optical and mechanical properties of the components and therefore lead to an increased reject rate of the components. This relatively large sensitivity to mechanical inaccuracies is due to the extremely high aspect ratio of a DBR layer structure mentioned above, the following values being given as examples for the order of magnitude:

The diameter of the actual layer structure is approximately 40 micrometers, the distance between the individual layers being only a few 100 nanometers, and the thickness of a layer likewise being only a few 100 nanometers to a few micrometers. Such fine layer spacings have so far been difficult to produce, and in particular the mechanical and thus the geometric stability of the layer structure is not yet stable enough. The European patent application EP 1 320 157 discloses as a solution to this problem to provide a so-called "support layer" on the side walls of the layer structure. However, this measure still provides too little stability, especially in the case of unfavorable aspect ratios.

Therefore, the object of the present invention is to provide an improved structure of the components mentioned at the outset and a corresponding manufacturing method for the components.

ADVANTAGES OF THE INVENTION

The object with the features of claim 1 solves the first-mentioned object. Advantageous developments and improvements of the respective subject matter of the invention can be found in the subclaims.

According to the main aspect of the present invention, a so-called “connecting element” is provided according to the invention for mechanically connecting at least two material membrane layers of the layer structure, the connecting element being apart from it when using only one supporting edge area, and otherwise, for example when the layer structure is composed of two edge areas is supported, preferably located as close as possible to the optically active area between these edge areas.

In other words, and in relation to the topology introduced above, vertically oriented, that is to say essentially “holding posts” which run essentially perpendicular to the individual layer planes of the membrane layers, are created as connecting elements of the membrane layers of a DBR reflector or other optoelectronic components with a fine layer structure The immediate result of this is that the entire layer structure or a certain subset thereof, depending on which membrane layers are connected to one another by a holding post, gains increased mechanical stability, as a result of which the above-mentioned disadvantageous deformations of the individual layers can be considerably reduced From an application point of view, this results in the advantage that the optical properties can be considerably improved and the reject rate in the series production of such components can be significantly reduced.

The publication mentioned above also discloses a way such as such a layer structure - but without that Connecting elements according to the invention can be produced.

Three main steps are used:

1. First, a multi-layer structure is made by epitaxy or other deposition methods.

2. the lateral structure is then produced by a dry etching process, that is to say a vertical structuring of the so-called mesa is carried out, and

3. Artificial, ie only temporarily existing layers of the layer structure, so-called sacrificial layers, are removed by a highly selective wet-chemical etching process in order to form the aforementioned air gaps. Masks are used that contain various membrane geometries and structures indicating mechanical stress, as well as quality control elements and alignment markings. There, a filter structure is disclosed, each of which is suspended at an angle of 90 ° on four arms, which in turn are attached to square, supporting frame blocks on the outside.

If this method is to be used as the basis for producing the component according to the invention, and the connecting elements according to the invention are to be integrated into the manufacturing method as holding posts, then the connecting elements or at least their outer layer consist of a material with a high selectivity to a sacrificial layer, which is then etched away as an interlayer in the aforementioned manufacturing process in an etching step. A connecting element is preferably arranged as close as possible to the optically active region of the layer structure, but without disturbing the beam path therein. Depending on the so-called clamping length, which is defined between the above-mentioned edge supports, more than one connecting element can be used per connecting line between two edge supports. When using four edge supports and a circular filter structure, this results, for example, in the use of four holding posts, which are provided essentially and preferably at the same radial distance from the center of the filter arrangement.

All membrane layers of the layer structure are preferably connected to one another by a connecting element. This means that all membrane layers can be mechanically stabilized.

According to a further, particularly advantageous aspect of the present invention, a connecting element contains a coherent, metallized area which has at least two

Layers of the layer structure connects together. As a result, the interconnected layers can be electrically contacted in a targeted manner, and / or a targeted heat dissipation or heat distribution is possible in the membrane layers surrounded by air, so that heat generated during operation can be dissipated specifically to a heat sink. It is essentially irrelevant whether the holding post consists entirely of a conductive material or only partially as long as the conductive connection between the preselected layers is established.

If the component provided according to the invention now has two opposing layer structures with the above-mentioned intermediate cavity, the basic result is Structure of an optical filter, as it is often used for technical purposes.

According to a further preferred aspect of the present invention, at least one connecting element is provided in an arrangement as described above, the end portion of which, facing the cavity, projects into the cavity with a predetermined length, so that, in other words, a “nub” is formed which extends over the border between the cavity and the first membrane layer is raised. Such knobs can also be present on several connecting elements. This solves an otherwise existing problem, namely in the event of an actuation of two membranes, as mentioned above, for example by electrostatic attractive forces between the Membranes can be prevented that the attraction force between the membranes becomes too great and the component is destroyed from a certain actuation voltage that is selected too large. At this tension, the so-called "pull in" tension, the membranes come out of step according to technology the technology, so without it The knobs provided according to the invention come into contact and stick firmly to one another. The component is therefore destroyed without the knobs after the "pull in" voltage has been exceeded.

The effect of the knobs is therefore that a spacer is automatically formed to maintain a minimum distance for the individual layer packages. This minimum distance then corresponds to the height of the knobs or, for example, twice the height of the knobs, if the end sections of two knobs face each other exactly and then touch. In any case, contact of the membranes, which are movable per se, is prevented during a "tuning". If air is used as the gas filling for the interstices, a simple manufacturing process results for the component, since during the manufacturing process it itself is not particularly sealed against the environment.

Furthermore, the materials of the DBR reflectors can also have low conductivity, for example if they consist essentially of silicon nitride or silicon dioxide. Due to the lack of conductivity of the membranes, electrostatic actuation is not possible here. According to the present invention, however, conductive connecting elements can be used to enable electrostatic actuation of membranes or entire membrane packages with opposing connecting elements or opposing existing metal layers separated by the air gap.

A high current density in the active optical layers is particularly important for electrically pumped lasers. If further in an advantageous manner, therefore, the invention

Connecting elements (holding posts) consist of a conductive core material which is located in a non-conductive shell of suitable thickness, so only certain layers can be contacted in a targeted manner by this type of arrangement. This makes it possible to inject charge carriers into an optically active zone and to keep the current flow in the active zone laterally limited by electrical interaction (current confinement).

Furthermore, the connecting elements can be used for heat conduction or heat distribution to other layers or heat sinks. A lot of heat is often generated, especially in the case of active optical components, but its dissipation especially in air gap-based membrane arrangements otherwise a major problem according to the prior art.

Furthermore, the connecting elements can be provided with a contact via which a voltage can be applied in a targeted manner during laser operation. The polarization of the light can thus be influenced in a targeted manner. Among other things, modulation of the light possible.

DRAWINGS

Embodiments of the invention are shown in the drawings and explained in more detail in the following description.

Show it:

1 shows the production of a layer structure according to the invention, as mentioned above on the basis of four (a-d) or five (a-c, e, f) production stages, in order to illustrate the basic structure and the essential method steps in its production;

2 schematically shows the structure of an electrically pumped, air gap-based VCSEL that can be mechanically actuated as a preferred exemplary embodiment of the present invention in a top view;

3 shows a sectional representation along a line III-III of the arrangement as shown in FIG. 2 in a top view;

Fig. 4 shows another preferred embodiment of an air gap-based, opto-electronic filter component with mechanical actuation as a top view;

5 shows the cross section of the filter component in FIG. 4 along a line V-V;

DESCRIPTION OF THE EMBODIMENTS

In the figures, identical reference symbols designate identical or functionally identical components.

The production of a layer structure according to the invention, as can be used for optoelectronic components as described above, is described in more detail below with reference to FIG. 1.

In a first step, see FIG. 1A, a plane-parallel layer structure is produced (epitaxy), the following materials being selected:

For layer 10: indium phosphide (InP), for layers 11, 12 gallium indium arsenide (GalnAs). These materials are deposited using MOCVD (metal organic chemical vapor deposition). GalnAs is required as a sacrificial layer, which is selectively etched away in a later process step and then replaced by a gas or a liquid 17.

After the deposition, vertical cavities 13 are generated in a second process section, for example by means of a photoresist mask by dry chemical etching.

In the following process step, see FIG. IC, the sacrificial layers are etched by wet chemistry. Then the cavities are covered with a material 14, in this case with a material of relatively high thermal conductivity and good e Electrical conductivity, completely or partially filled, see Fig. 1D according to the left or right picture. In a further method step, with reference to FIG. 1E, the sacrificial layers 11, 12 are now etched using wet chemistry and replaced by gas or liquids 17. In the simplest case, it is air. The vertically drawn structures 14, 15, and 16 serve as holding posts in the sense of the present invention.

Contacting individual layers is also possible. For this purpose, after the method steps already described by means of FIGS. 1A to IC, a non-conductive material 15 is first filled into the cavity 13. Then e.g. by means of dry chemical etching with a photoresist mask in the middle, a new insulated cavity is created (FIG. IF), which is then filled with a conductive material 16 (FIG. IG). The sacrificial layers are then again removed by wet chemistry and replaced by gas or liquids (17).

With reference to FIGS. 2 and 3, further explanations are given below of an exemplary embodiment of the present invention which represents an electrically pumped, air-gap-based VCSEL with mechanical actuation.

In a first production step, the individual layers lying one above the other, shown in FIG. 3, are produced epitaxially:

An alternating layer sequence of p-doped GalnAs 54 and InP 53, consisting of a total of six layers, is applied to a substrate 62, for example made of p-doped InP. Following an intrinsic GalnAs Layer 61, which defines the later length of the cavity, an n-doped InP layer 58, an n-doped GalnAs layer 60, a highly doped n ⁺ -InP layer 59, quantum well (QW) layers embedded in InP 57 deposited from gallium indium arsenide phosphide (GalnAsP) and a highly doped p ⁺ -InP layer 56. Finally, a layer sequence consisting of a total of 8 layers of p-doped GalnAs 54 and InP 53 is applied. As the next process step, a first lithography for electrically conductive connecting elements for the individual sub-layers is carried out in the form of holding posts. In a further dry chemical process section, cavities 50 are etched vertically downward thereon, which are later to be used to accommodate metallizations 52 for metallized hollow posts as connecting elements according to the exemplary embodiment.

The lithography for the metallization of the hollow posts is then carried out in a further process step, and in a subsequent metallization step one

Metal layer 52, which is also used below as a mask for further etching, is applied to the inner walls of the above-mentioned cavities 50 in order to form the outside of the holding posts 68 according to the invention.

After the central optical area 51 has been created using an additional mask, e.g. made of silicon nitride, the corresponding measurement can be etched and the contacts 63 and 64 can be applied. Finally, all GalnAs sacrificial layers, with the exception of the inner areas of the large edge supports 67, are etched away by wet chemistry and replaced by gas or liquid.

The QW layers 57 consisting of GalnAsP are etched much more slowly. After the undercut, the membrane structure is held by edge supports 67, since these have not been completely undercut due to their size. Due to the contacts 65 formed between the p ⁺ layer and the hollow posts 68, which are very close to the optically active area 66 due to the metallized hollow posts and the laterally etched QW layers, the current flow is restricted in the area 66, which is necessary for laser operation of the VCSEL is very advantageous. Furthermore, the heat generated in this area can be dissipated via the metal layer 52 of the holding posts.

A further preferred exemplary embodiment is described below with reference to FIGS. 4 and 5, which contains an air gap-based filter with mechanical actuation:

A total of six doped GalnAs, see reference numeral 88, and InP layers 87 are alternately deposited on an InP substrate 89. An intrinsic layer of GalnAs 85, which defines the later length of the cavity, and a layer package of 7 alternating dielectric layers made of silicon nitride 83 and silicon 84 are then applied. Then, by means of lithography and subsequent dry chemical, anisotropic etching, cavities 80 are created in the upper layer package, which are as close as possible to the optical beam path used later, but do not yet influence it. In the next step, the cavities are partially or completely filled with conductive material 82, whereby support posts 90 are formed.

After mesa etching and structuring of contacts 86 and 93, controlled underetching, selective etching of all GalnAs layers and selective etching of the silicon layers with the exception of the large surface area areas 91. The remaining free-standing silicon nitride membranes are now supported in addition to the edge area by the holding posts in the vicinity of the optically active area. A voltage applied to the contacts 93 and 86 then causes attractive forces between the layer 87 and the conductive holding posts 90, which makes mechanical actuation possible.

The present invention can, with certain adaptations familiar to the average person skilled in the art, deviate from the above exemplary embodiment for many different purposes and in different embodiments:

For example, for electrically pumped (actuatable), micro-mechanically tunable VCSEL, for filters to build detectors, multiplexers, demultiplexers, modulators, switches or amplifiers, whereby the advantages mentioned above come into play.

Furthermore, the present invention can be modified in many ways in order to obtain certain technical properties. It is understood in particular that the connecting elements according to the invention do not necessarily have to be produced by the production method, as was described above by way of example, since there is a wide variety of production variants in the prior art. The choice of the above-mentioned materials should not restrict the inventive idea as long as a relatively firm, mechanical connection can be made between the connecting element according to the invention (holding post) and a respective membrane layer. Finally, the features of the subclaims can be combined with one another essentially freely and not by the order presented in the claims, provided that they are independent of one another.

Claims

Conductive holding posts in air gap-based, optoelectronic components

1. Optoelectronic, vertically oriented component containing a layer structure (69; 92) with a plurality of optically active layers (10; 53, 56, 57, 58, 59, 62; 83, 87, 89) and at least one gas or liquid-filled layer interspace (17; 55; 94), the layer structure being held by means of at least one outer edge support (67; 91) associated with an edge region of the layer structure, characterized in that at least two of the layers of the layer structure are held by a connecting element ( 14, 15, 16; 68; 90) are firmly connected to one another, the connecting element being located off the edge region of the layer structure.

2. Optoelectronic component according to claim 1, wherein the connecting element (14, 15, 16; 68; 90) is arranged as close as possible to the optically active region (66) of the layers.

3. Optoelectronic component according to claim 1, wherein the connecting element (14, 15, 16; 68; 90) is oriented substantially perpendicular to a layer plane.

4. Optoelectronic component according to claim 1, wherein all layers of the layer structure (69; 92) are connected to one another by connecting elements (14, 15, 16; 68; 90).

5. Optoelectronic component according to claim 1, wherein the material of the connecting element (14, 15, 16; 68; 90) a high selectivity to a sacrificial layer (11, 12; 54, 60, 61;

84, 85, 88) in an etching process of the manufacturing process, in which the intermediate layers (17; 55; 94) are etched away as sacrificial layers.

6. The optoelectronic component according to claim 1, wherein the connecting element (14, 15, 16; 68; 90) contains a conductive, coherent area (52; 82) which connects at least two layers of the layer structure (69; 92) to one another.

7. The optoelectronic component according to claim 1, wherein the connecting element (14, 15, 16; 68; 90) contains a conductive, coherent core region and a non-conductive shell is provided.

8. The optoelectronic component according to claim 1, wherein the component has two mutually opposite layer structures with an intermediate cavity.

9. Optoelectronic component according to the preceding claim, wherein at least one connecting element (14, 15, 16; 68; 90) is provided in a layer structure (69; 92) whose end portion facing the cavity protrudes into the cavity with a predetermined length.

10. Optoelectronic component according to the preceding claim, wherein two layer structures each having at least one connecting element are provided opposite one another, the end sections of the opposite connecting elements touching each other when the layer structures are bent vertically.

11. Optoelectronic component according to one of the preceding

Claims designed as a Bragg mirror component (DBR).

12. Manufacturing method for a vertically oriented, optoelectronic component comprising a layer structure (69; 92) with a plurality of optically active layers (10; 53, 56, 57, 58, 59; 83, 87, 89) and at least one gas or liquid-filled layer interspace (17; 55; 94), the layer structure being held by means of at least one outer edge support (67; 91) assigned to a respective edge region of the layer structure, comprising the steps:

a) producing the layer structure (69; 92) using suitable sacrificial layer materials for the interstices (17; 55; 94), b) creating a vertical cavity (13; 50; 80) which is used to form a connecting element (14, 15 , 16; 68; 90), c) removing the sacrificial layers (11, 12; 54, 60, 61; 84, 85, 88) d) filling the cavity (13; 50; 80) with a material of preselected electrical and thermal Conductivity and pre-selected mechanical strength.

13. The manufacturing method according to the preceding claim, wherein the cavity (13; 50; 80) is filled with materials of different electrical conductivity or different thermal conductivity in each of the preselected areas, with specific spatial areas of the cavity being specifically filled with a respective material.

14. Manufacturing method according to the preceding claim, wherein the cavity (13; 50; 80) is filled so that an internal core and an outer shell made of different materials.