DE19941701A1 - Manufacturing method for cantilevered structure used for sensor application involves plasma etching of substrate in window region over portion of substrate thickness in front of support or functional layer - Google Patents

Abstract

A support layer or functional layer (3-6) is deposited on the side of a substrate (1). A substrate material for a cantilevered structure in a window region (12) is etched in the side opposite to a layer side up to the support layer or functional layer. The plasma etching of the substrate in the window region is performed over a portion of the substrate in front of the support layer or functional layer. An Independent claim is also included for a sensor structure on a semiconductor wafer.

Description

The invention relates to a method for manufacturing self-supporting structures and a sensor structure according to the The preambles of claim 1 and 6 respectively.

State of the art

Sensor applications with self-supporting structures are in various embodiments have become known. The self-supporting structures are regularly used to a "transducer", e.g. B. in the form of an electro-thermal or electro-mechanical transducer, from one or more Functional layers of a substrate mechanically and / or to decouple thermally. In the case of thermal sensors thereby the heat transfer from certain points of the Functional layer or layers to surrounding areas be minimized. At z. B. Accelerometers should the cantilever structures a free swing of the enable sensitive sensor areas.

The shape of such self-supporting structures, such as membranes, Bridges or tongues largely determine the thermal and  mechanical properties of the sensor system. In the Bulk micromechanics, in which the bulk material is a Semiconductor wafers for providing an "undermining" (Cavity) or a window opening to create self-supporting structures are used for the Creation of a cavity or window opening in the most cases anisotropic wet chemical etching techniques for Commitment.

In surface micromechanics there are "cavity" areas realized through sacrificial layer techniques. Here, too Sacrificial layer regularly through anisotropic wet chemical Etching techniques or steam etching etched away.

The etching solutions for bulk micromechanics are generally basic in nature, e.g. B. KOH (potassium hydroxide) or TMAH (tetramethylammonium hydroxide). However, structuring a semiconductor wafer bulk material with such etching solutions has disadvantages. The diffusion of cations from the etching solution (e.g. K ⁺ from KOH) into the substrate can disrupt electronics integrated on the chip.

To solve this problem, solutions from EDP (Ethylenediamine pyrocatechol) used. Etching solutions made of EDP are toxic, however.

Furthermore, the anisotropy achieved in the etching is the desired etching rate and the etching homogeneity depending on the etching solutions and any additives are often unsatisfactory. Due to undesirable reactions, it occurs e.g. B. failure of Silicates that happen to be deposited on the structure.

Furthermore, anisotropic wet chemical etching processes in ( 100 ) -oriented silicon wafers lead to the formation of inclined etching flanks along the 111 plane, which form an angle of approximately 55 ° to the 100 surface. As a result, the sensor element requires approximately 1.4 times the etching depth more space on the silicon wafer, as would be the case compared to a process with vertical flanks.

With sacrificial layer techniques of surface micromechanics can such problems are fixed. However, they are achievable layer thicknesses of the sacrificial layers often too low to adequate thermal and / or mechanical To achieve decoupling.

Object and advantages of the invention

The object of the invention is based on a substrate a sensor structure with self-supporting structural elements a window area penetrating the substrate places that have a comparatively smaller space requirement required.

This object is achieved by the features of claim 1 and of claim 6 solved.

Advantageous continuations of the Procedure specified.

The invention is based on a method for producing self-supporting structures for sensor applications, comprising the following process steps:
Applying at least one carrier or functional layer to one side of a flat substrate, the functional layer serving to provide sensor properties and
Etching away substrate material to form the self-supporting structures in a window area from the side of the substrate opposite the layer side to the carrier or functional layer.

The essence of the invention is that the etching of the Substrate in the window area at least one Section of the thickness of the substrate before the carrier or Functional layer performed with a plasma etching process becomes. With this procedure, the knowledge exploited that adjustable with a plasma etching process Flank slopes up to exactly vertical wall areas can be achieved. In this way, the space requirement for a sensor structure can be significantly reduced because of the Share of comparatively strongly inclined etching edges is reduced.

In a further particularly advantageous embodiment of the Invention becomes the subsection of plasma etching immediately placed in front of the carrier or functional layer. Through this Measure can have the following problem, especially with the Use of silicon wafers as substrate material, be opposed. Leave in wet chemical etching only angular due to the crystal structure of the silicon Etch structures. This can be in the transition area to a Membrane or a bridge structure or a tongue structure lead to voltage breaks. By means of a physical chemical plasma etching occurs a "rounding" of sharp corners or edges, which is a cheaper Resulting in the course of tension in the structures. If one any additional masking can be used realize any window shape. Especially in this Connection, it is also advantageous if the etching of the substrate in the window area completely with a Plasma etching takes place.

In a further advantageous embodiment of the invention is the at least one carrier or  Functional layer applied and then for generation of tongue or bridge structures in the window area structured. If a directly on the substrate Functional layer follows, this also has the tasks of a To meet carrier layer, d. H. it should be the required have mechanical properties to be self-supporting To be able to train structure. However, often is first at least one carrier layer are applied and thereon then at least one functional layer is deposited. In this It is favorable if, before structuring the structuring the at least one carrier layer at least one functional layer takes place.

To avoid a tongue or bridge structure and in particular a functional layer arranged on it Etching the window area of an unwanted etching is further proposed that before Etching process on the bridge or tongue structure Protective layer is applied. In the simplest case it can this protective layer can be a photoresist layer that is then easily removed by ashing leaves.

The method described can be used in particular Sensor structure on a semiconductor wafer with self-supporting Structural elements over the semiconductor wafer create penetrating window area in which the Flank course of the window area walls, based on a Substrate normal, is in the range of less than +/- 10 °.

drawings

An embodiment of the invention is in the drawings shown and in the description below Details of further advantages and details explained. It demonstrate  

Fig. 1a-f shows a detail of a sensor structure in a sectional view in different process stages of the manufacturing process.

Description of the embodiment

To produce a sensor structure with a self-supporting bridge 7 (see in particular FIG. 1f), a layer system 2 with carrier layers and a functional layer is first applied to a silicon wafer 1 as an example (see FIG. 1a). In the exemplary embodiment, an SiO ₂ layer 3 , a subsequent silicon nitride layer 4 , which in turn is covered by an SiO ₂ layer 5 , serve as carrier layers which are applied directly to the silicon wafer 1 . With the at least one further functional layer 6 arranged thereon, a sensor-sensitive area is produced according to FIG. 1b. This can be done, for example, using a photolithographically generated mask (not shown) and a subsequent etching process.

According to the sectional view of FIG. 1c is made of the support layers 3, 4, 5, a bridge structure structured, wherein on the bridge 7, the structured functional layer 8 is positioned.

The structuring process for producing slot-shaped regions 9 , 10 next to the bridge 7 is preferably carried out by means of a chemical-physical plasma etching process. The slit-shaped areas for the plasma etching process can be masked by means of a photolithographically structured photoresist layer (not shown). The etching process is carried out until the silicon wafer 1 is exposed in the slot-shaped regions 9 , 10 . The photoresist mask (not shown) is then removed and the bridge structure is covered with a protective layer 11 (see FIG. 1d).

The protective layer 11 has the task of protecting the functional layer in particular from unwanted etching during the subsequent etching process.

In this etching process (see FIG. 1e), the silicon wafer 1 is provided with a window cutout 12 from the rear. This makes the bridge 7 self-supporting. The window cutout 12 is preferably etched into the silicon wafer material 1 by a plasma etching process, which has a thickness d. In this way, as can be seen in FIG. 1e, ideally 90 ° steep edges 13 , 14 of the window cutout 12 can be realized. In contrast to a wet chemical etching process in which the flanks have an inclination of e.g. B. have 55 °, is significantly reduced by the structure with steep flanks, the area consumption on the wafer. Here too, the masking for this etching step can be carried out by means of a photoresist structure produced by photolithography.

At an angle of approximately 55 ° of the flanks 13 , 14 , approximately 1.4 times the etching depth of "flank requirement" is required for a sensor element in comparison to a 90 ° flank to accommodate the sensors on the wafer. With an etching depth of z. B. 380 microns can thus be achieved considerable space savings.

In a final step, e.g. B. by an ashing process, the protective layer 11 (for example, a photoresist layer) removed (see Fig. 1f).

Claims (6)

1. A method for producing self-supporting structures for sensor applications, in which at least one carrier layer or a functional layer is applied to one side of a substrate, the functional layer serving to provide sensor properties, and in which the substrate material from the side opposite the layer side to form the self-supporting Structures in a window area is etched away as far as the carrier or functional layer, characterized in that the etching of the substrate ( 1 ) in the window area ( 12 ) is carried out using a plasma etching method at least over a portion of the thickness (d) of the substrate in front of the carrier or functional layer becomes. 2. The method according to claim 1, characterized in that the at least one partial section, in which the plasma etching is carried out, is located immediately before the carrier or functional layer ( 3 , 4 , 5 , 6 ). 3. The method according to any one of the preceding claims, characterized in that the etching of the substrate ( 1 ) in the window area ( 12 ) is carried out entirely with a plasma etching process. 4. The method according to any one of the preceding claims, characterized in that first the carrier or functional layer for producing tongue or bridge structures ( 7 ) is structured over the window area ( 12 ) and then the window area ( 12 ) is etched. 5. The method according to claim 4, characterized in that before the etching of the window area ( 12 ) a protective layer ( 11 ) is applied to the structured, at least one functional or carrier layer. 6. Sensor structure on a semiconductor wafer with self-supporting structural elements over a window area penetrating the semiconductor wafer, characterized in that the edge advance of the window area walls ( 13 , 14 ), based on a substrate normal, is in a range of +/- 10 °.