Method for producing oblique surfaces in a substrate and wafer having an oblique surface

A method for producing oblique surfaces in a substrate, comprising a formation of recesses on both surfaces of the substrate, until the recesses are so deep that the substrate is perforated by the two recesses. One recess is produced going out from a first main surface in the region of a first surface, and the other recess is produced going out from the second main surface in the region of a second surface, so that the first surface and the second surface do not coincide along a surface normal of the main surfaces of the substrate. Subsequently, flexible diaphragms are attached over the recesses on each of the main surfaces. If a vacuum pressure is then produced inside the recesses, the flexible diaphragms each curve in the direction of the recesses until their surfaces facing the substrate come into contact with one another, generally in the center of the recesses.

CROSS REFERENCE

The present application claims the benefit under 35 U.S.C. §119 of German Patent Application No. 102010062009.2 filed on Nov. 26, 2010, which is expressly incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a method for producing oblique surfaces in a substrate, in particular in planar technology for semiconductor processing, as well as to a wafer having an oblique surface integrated in a substrate, in particular for covering a micromechanical element.

BACKGROUND INFORMATION

In the processing of semiconductors, planar structuring elements can be produced on or in substrates by depositing layers and subsequently structuring them, or through processes of etching the bulk material. This technology is known as planar technology.

An aspect of planar technology relates to the production of non-planar structures such as oblique or curved surfaces.

U.S. Pat. No. 5,174,587 describes an oblique etching process in which a photoresist layer is made to flow under a predetermined high temperature in order to form oblique photoresist edges that are then, in a subsequent etching step, etched together with the substrate situated thereunder to form oblique planes.

U.S. Patent Application Publication No. 2005/0257709 A1 describes a method in which optical or diffractive elements on a substrate are covered by a layer that is to be structured, and through a masking layer a region of the layer to be structured over the optical elements is exposed. The optical elements reflect the radiation and cast the light at an angle into a further region of the layer that is to be structured, so that obliquely exposed regions are formed.

U.S. Patent Application Publication No. 2002/0135717 A1 describes methods by which oblique surfaces can be fashioned by depositing organic layers over structured substrate elements.

In addition, lithographic methods are available, for example gray-tone or half-tone lithography, that can be used to produce regions in a substrate that are structured to different extents. The masking layers here are relatively thin, and the masking material can be selected such that during the etching only a slight abrasion occurs, for example via a high etching selectivity of the masking material relative to the substrate to be etched.

In particular for the covering of microelectromechanical systems having micromirrors by a window, oblique surfaces are often required in order to keep scattered light and undesired reflections of the light source away from the micromirror or from the projected image.

U.S. Patent Application Publication Nos. 2006/1076539 A1 and 2007/0024549 A1 describe methods in which pre-formed coverings having oblique surfaces are placed over micromirror arrays.

German Patent No. DE 10 2008 012 384 A1 describes methods for producing a glass wafer having beveled surfaces that can act as coverings for micromirrors.

However, in order to implement a large-scale production method for oblique and/or curved surfaces in a planar technology environment, it is desirable to provide more efficient and more economical methods.

SUMMARY

In accordance with the present invention, an example method is provided for producing oblique surfaces in a substrate by which oblique or curved surfaces can be produced precisely and with variability and flexibility in dimensions such as inclination, height, or curvature. The present invention also relates to a wafer having an oblique surface integrated in a substrate that can be produced according to an example method according to the present invention in order to provide a covering wafer for a micromechanical element.

According to a specific embodiment, a method for producing oblique surfaces in a substrate includes a formation of recesses on both main surfaces of the substrate, to respective depths whose sum is greater than the thickness of the substrate, i.e., until the recesses are so deep that the substrate is perforated by the two recesses. Here, one recess is produced starting from a first main surface in the region of a first surface and the other recess is produced starting from the second main surface in the region of a second surface, so that the first surface and the second surface do not coincide along a surface normal of the main surfaces of the substrate, i.e., the two surfaces are laterally offset relative to one another. This advantageously results in terraced substrate projections on opposite sides of the recesses. Subsequently, on the main surfaces flexible diaphragms are attached over each of the recesses. If a vacuum pressure is then built up inside the recesses relative to the external pressure, the flexible diaphragms each curve in the direction of the recesses until their surfaces facing the substrate come into contact with one another essentially in the center of the recesses. An advantage of the method is that due to the vacuum pressure the flexible diaphragms fit tightly against the outer edges of the substrate in the edge area of the recesses, in particular against the substrate projections, which form mounting points for a surface that results from the diaphragm surfaces contacting one another inside the recesses. In this way, it can advantageously be achieved that the surface that forms is oblique relative to the main surfaces of the substrate.

A further advantage of the example method according to the present invention is that the inclination, extension, and height of the oblique surfaces inside the recesses can easily be set via the dimensions of the recesses made in the substrate. In addition, the recesses can advantageously be produced in planar technology, so that the method according to the present invention for producing oblique surfaces is compatible with planar technology processes. As a result, the method is efficient, economical, and suitable for large-scale production.

The example method according to the present invention also has the advantage that a large number of geometrically oblique structures can be formed without making significant changes to the production process, and that for the oblique surfaces many materials differing from the substrate material may be used, and that the oblique surfaces can be made very precise and flat.

According to a specific embodiment, the vacuum pressure inside the recesses is produced by applying the flexible diaphragms to the substrate in a vacuum and bringing the substrate, with the hermetically sealed recesses, into an atmosphere of normal pressure, so that as a result of this process a vacuum pressure already exists inside the recesses. In this way, it can advantageously be achieved that the oblique surfaces organize and stabilize themselves.

According to a further specific embodiment of the present invention, a wafer is provided having a substrate that has a first main surface, a second main surface situated opposite the first main surface, and a thickness, the substrate having an opening that extends through the thickness of the substrate, the opening having a first lateral limiting edge and a second lateral limiting edge opposite the first lateral limiting edge. The wafer has a first substrate projection that protrudes into the opening from the first lateral limiting edge at the height of the first main surface, and has a second substrate projection that protrudes from the second lateral limiting edge into the opening at the height of the second main surface. The first and second substrate projection form mounting points for a diaphragm element that extends from the first substrate projection to the second substrate projection and, in the region of the opening, defines a surface that stands at an angle to the first main surface, i.e. obliquely.

Here it is particularly advantageous if the substrate is a silicon substrate and the diaphragm element includes borosilicate glass that is transparent in the optical range. In this way, it can be achieved that the wafer has an oblique window transparent to light that is suitable for covering a micromechanical element, in particular a micromirror, because it effectively eliminates disturbing scattered and reflected radiation.

The above embodiments and developments may be arbitrarily combined with one another to the extent that this is appropriate. Further possible embodiments, developments, and implementations of the present invention also include combinations not explicitly named of features of the present invention described above or in the following with regard to the exemplary embodiments.

The present invention is explained in more detail below on the basis of the exemplary embodiments shown in the schematic Figures.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

In the Figures, identical and functionally identical elements, features, and components are provided with the same reference characters, unless otherwise stated. For reasons of clarity and comprehensibility, components and elements in the figures are not necessarily shown in their true scale relative to one another.

FIGS. 1a-1fschematically show a method sequence according to a specific embodiment of the present invention.

FIG. 1ashows a substrate11having a first main surface12and a second main surface13situated opposite first main surface12. Substrate11has a thickness14. In substrate11, on the side of first main surface12there is made a first recess102, and on the side of second main surface13there is made a second recess103. As is shown here as an example, recesses102,103can have a rectangular cross-sectional surface, but other cross-sectional shapes are also equally possible.

Substrate11can for example be made of silicon or can contain silicon. However, any other substrate material may also be used for substrate11. Recesses102,103can for example be made in planar technology by trench etching or etching using potassium hydroxide (KOH). This results in cuboidal recesses102,103.

The region of second recess103on second main surface13is laterally offset relative to the lateral extension of its cross-sectional surface relative to the region of first recess102. In other words, the side edges in cuboidal recess103in substrate11are laterally offset relative to the side edges of cuboidal recess102in substrate11. However, it can be provided that the region of second recess103has the same shape and dimensions as the region of first recess102, for example a rectangular shape having the same side lengths.

FIG. 1bshows substrate11after the formation of recesses102and103. First recess102has been made in substrate11to a depth104, and second recess103has been made in substrate11to a depth105. Depths104and105can for example be controlled via the duration of an etching process. Here it is provided that the sum of the depth extensions of depths104and105is greater than or equal to thickness14of the substrate. This ensures that the floors of recesses102and103meet one another in the center of substrate11, thus producing a perforation through the substrate in the region of recesses102and103.

FIG. 1cshows substrate11after a further etching of recesses102and103. Recesses102and103can be further etched after the formation of the perforation in substrate11, so that substrate projections107and108can be formed. A first substrate projection107results from the forming of a first recess102, from first main surface12of substrate11, to a depth104that is less than thickness14of substrate11. First substrate projection107then has a thickness that corresponds to thickness14minus depth104. A second substrate projection108results from the formation of second recess103, from second main surface12of substrate11, to a depth105that is less than thickness14of substrate11. Second substrate projection108then has a thickness that corresponds to thickness14minus depth105, and is situated diagonally opposite first substrate projection107, extending past the extension of the perforation and the extension of the overlapping region of the cross-sectional surfaces of the two recesses102and103.

FIG. 1dshows substrate11after a further method step according to a specific embodiment of the present invention. A first flexible diaphragm element118is applied on first main surface12over first recess102, and a second flexible diaphragm element117is applied on second main surface12over second recess102. Flexible diaphragm elements117,118can be for example plastic films. However, it is also possible to use borosilicate glasses such as Pyrex or Borofloat33 for flexible diaphragm elements117,118. Of course, any other material may also be suitable to form flexible diaphragm elements117,118. The thickness of flexible diaphragm elements117,118is preferably approximately 10 to 200 μm, in particular approximately 30 μm, such as for example in the case of borosilicate glass film MEMPAX, or 100 μm for other commercially available borosilicate glasses.

Flexible diaphragm elements117,118each have a surface that is larger than the first and second regions of recesses102and103. In particular, flexible diaphragm elements117,118are attached to main surfaces12,13of substrate11in such a way that a pass-through area (indicated in broken lines) through substrate11is completely covered, and such that regions117a,118aon main surfaces12,13of substrate11outside the first and second regions of recesses102,103are also covered by flexible diaphragm elements117,118. These regions117a,118aact as support areas for the flexible diaphragm elements.

The application of flexible diaphragm elements117,118on main surfaces12,13creates an opening109that is laterally limited by the side edges of recesses102,103and is limited along thickness14of substrate11by flexible diaphragm elements117,118. This opening109is sealed against the external space, and in particular it can be provided that opening109is hermetically sealed against the external space. This can be ensured by corresponding application of flexible diaphragm elements117,118on substrate11, for example by gluing, soldering, anodic bonding, or the like. It can advantageously be provided that flexible diaphragm elements117,118are fixedly attached to substrate11in regions117a,118a. In particular, along main surfaces12,13regions117a,118acan be situated at a distance from the side edges of recesses102,103, so that flexible diaphragm elements117,118have a surface that lies over recesses102,103that is larger than the surface of recesses102,103, and that is not rigidly connected to substrate11.

This facilitates the lateral expansion of the diaphragm in the following method steps, explained inFIGS. 1eand1fbelow.

The application of flexible diaphragm elements117,118can for example take place in a vacuum atmosphere, so that due to the hermetic seal an evacuated space arises inside opening109. However, it can also be possible to attach flexible diaphragm elements117,118to substrate11under normal pressure.

AsFIG. 1eshows, a pressure piprevails in opening109. If a pressure difference is now applied between the external space and opening109, flexible diaphragm elements117,118begin to deform due to the resulting pressure force in regions110or111. If the pressure in the external space is for example pa, and the pressure in the internal space is pi, and moreover pa>pi, pressure forces arise that are indicated inFIG. 1eby arrows. Regions110and111of flexible diaphragm elements117,118are accordingly pressed into opening109from the side of the respective main surface12or13, so that curvatures result in flexible diaphragm elements117,118. The form and degree of the curvature is determined here as a function of the diaphragm element material, thickness, and dimensions of diaphragm elements117,118, the dimensions of opening109, the pressure difference pa-pi, the ambient temperature, and the duration of the curvature process. Given suitable external parameters, the curvature of flexible diaphragm elements117,118takes place to such an extent that flexible diaphragm elements117,118come into contact in regions110or111. Here, flexible diaphragm elements117,118undergo a lateral expansion in regions110,111. Due to the application of diaphragm elements117,118in regions117a,118a, which are situated at a distance from the side edges of opening109along main surfaces12,13of substrate11, in the region of the side edges of the opening there advantageously result parts of flexible diaphragm elements117,118that are not rigidly connected to substrate11by a glued connection, welded connection, soldered connection, or any similar connection. These parts can facilitate the lateral expansion of flexible diaphragm elements117,118in regions110,111.

AsFIG. 1ffurther shows, after the termination of the curvature process inFIG. 1ethere results a region113inside the broken lines inside opening109, within which flexible diaphragm elements117,118are completely in contact with one another with their surfaces facing substrate11. The formation of contact region113is a function of, inter alia, the geometry of the opening. In the present example, substrate projections107,108act as anchor points for surface112, which is mounted via flexible diaphragm elements117,118that contact one another.

In the present example, direction of extension115of surface112forms, with direction of extension114of second (or first) main surface13(or12), an angle116that corresponds to the angle of inclination of oblique surface112. It will be understood that the embodiment shown inFIG. 1fis only an example of the realization of an oblique surface112within opening109. The inclination of an oblique surface can be variably set via the lateral offset of the first and second regions of recesses102,103on main surfaces12,13of substrate11, in combination with the dimensions, i.e., side length, etching depth, and the like, of recesses102,103.

Due to the mechanical tensions in flexible diaphragm elements117,118, oblique surface112inFIG. 1fis extremely smooth and flat. In the method having a glass element as diaphragm element117or118for forming surface112, substrate11, with flexible diaphragm elements117,118, can be brought to a suitably high temperature to enable a flowing of the glass. Here it can be provided that flexible diaphragm elements117,118retain their shape during cooling. However, it can also be provided that flexible diaphragm elements117,118are glued, welded, or soldered to substrate11in region113of their surface contact with one another, or in the region of the side edges of opening109, in particular if normal pressure prevails inside opening109and a high pressure has been applied from outside in order to form surface112. Upon relaxation of this external pressure, the pressure difference between the outer pressure and the inner pressure in opening109decreases, and flexible diaphragm elements117,118could relax back into their original position if they had not been correspondingly joined.

FIG. 2ashows a substrate11in a top view, in which a recess202has been made from the upper main surface. The shape of recess202is for example rectangular. Going out from the main surface facing away from the viewing plane, a recess203has been made in substrate11whose shape is for example that of a parallelogram, as indicated by the broken line inFIG. 2a. Recess202can be offset relative to recess203in such a way that a large offset occurs in the x direction along a sectional line A-A′, while along a sectional line B-B′ there is no offset in the x direction, and along a sectional line C-C′ there is a large offset in the negative x direction. In this way, along the side edge of recess202in the y direction there arises a linearly increasing offset in the x direction. Thus, application of the method explained inFIGS. 1a-1ffor forming an oblique surface inside recesses202,203results in a wavelike surface along the y direction inFIG. 2a.

This is explained by the course of substrate projections207and208along the y direction, as is explained in more detail with reference toFIGS. 2b-2d.FIG. 2bshows a cross-sectional view through substrate11along sectional line A-A′,FIG. 2cshows a cross-sectional view through substrate11along sectional line B-B′, andFIG. 2dshows a cross-sectional view through substrate11along sectional line C-C′.

InFIG. 2b, a first substrate projection207on the underside of substrate11in the x direction is situated opposite a second substrate projection208on the upper side of substrate11. The length of these substrate projections207,208in the x direction continuously decreases in the direction of sectional line B-B′ until, as shown inFIG. 2c, no substrate projections207to208are present on sectional line B-B′. Conversely, however, in the direction of sectional line C-C′ the length of substrate projections207,208increases, but on the respectively other main surface of substrate11. As is shown inFIG. 2d, for sectional line C-C′ a first substrate projection207on the upper side of substrate11in the x direction is situated opposite a second substrate projection208on the underside of substrate11.

A person skilled in the art will understand that in addition to the depicted embodiments inFIGS. 1a-1fand2a, a large number of geometrical dimensions of recesses in a substrate are possible in order to carry out the method according to the present invention and to achieve a large number of oblique and/or curved surfaces inside an opening in a substrate. Concerning this,FIG. 3shows further possibilities for surface structures of oblique surfaces that can be made in an opening in a substrate11.

Reference character31designates an oblique surface as shown inFIG. 1f. Reference character32designates an oblique surface having a constant inclination inside a circular opening. Reference character33designates an oblique surface having a constant inclination inside a triangular opening. Reference character34designates a radially curved surface inside a semicircular opening. Reference character35designates a curved surface having two oblique surface regions having opposite inclination inside rectangular openings.

Reference character36designates a curved surface having two oblique surface regions having opposite inclination inside circular openings. Reference character37designates a radially curved surface inside a circular opening. Reference character38designates a curved surface having three oblique surface regions inside a triangular opening.

FIG. 3ashows an exemplary embodiment of a device having a substrate11and oblique surfaces112and312according to one of the specific embodiments having reference characters35,36, or37inFIG. 3. Substrate11here can correspond to substrate11inFIG. 1aor2a. First and second recesses102,103can be produced according to the method according toFIGS. 1a-1f. In addition, however, third and fourth recesses302and303are formed in substrate11that are offset in mirror-reflected fashion and laterally relative to first and second recesses102,103. The production of third and fourth recesses302and303can preferably be carried out in a manner similar to the production of first and second recesses102,103, in an identical work step.

In addition, flexible diaphragm elements307,308are provided having properties similar to those of flexible diaphragm elements117,118inFIGS. 1d-1f. Here, however, flexible diaphragm elements307,308are dimensioned such that they completely cover both first and second recesses102,103and also third and fourth recesses302and303. By producing a vacuum pressure in first and second recesses102,103and third and fourth recesses302and303hermetically sealed by flexible diaphragm elements307,308, oblique surfaces112and312can be formed in a manner similar to that described in relation toFIGS. 1e-1f.

FIG. 4shows a diagram of a wafer produced using a method according to a further specific embodiment of the present invention. A substrate11having an oblique surface112, which can be one of the above-designated oblique surfaces inFIGS. 1a-1f,2a-2d,3, or3a, faces, with a main surface13, a second substrate411. Substrate411can have, in a cavity under oblique surface112, a microelectromechanical element412, in particular a micromirror or microactuator. Between substrate11and substrate411, spacer elements420, in particular a spacer wafer420, can be attached in order to produce a distance between the two substrates11and411. The thickness of spacer elements420can be selected such that a movement of micromechanical element412is not impaired by oblique surface112.

Oblique surface112can be made in particular of borosilicate glass, and substrate11can include in particular silicon. Oblique surface112can in this case act as an optical window over a micromirror412in order to keep scattered and reflected radiation away from the micromirror or from the projected image. In addition, oblique surface112can act as a protective covering for micromirror412.