Protective wafer including inclined optical windows and device

Abstract

A method for manufacturing a protective wafer including a frame wafer and an optical window, and to a method for manufacturing a micromechanical device including such a protective wafer having an inclined optical window. Also described are a protective wafer including a frame wafer and an optical window, and a micromechanical device including a MEMS wafer and such a protective wafer, which delimit a cavity, the protective wafer including an inclined optical window.

Claims

1. A protective wafer, comprising: a frame wafer; and an optical window situated in a through-opening of the frame wafer; wherein: the frame wafer includes at least one first trench on a front side of the frame wafer and at least one second trench on a rear side of the frame wafer; the at least one first trench and the at least second trench are situated as an interdigital structure and form a meander-shaped spring structure that surrounds the through-opening; a first spring area of the spring structure, that is at a first circumferential position relative to the through-opening, has a first spring length in a respective direction from the first spring area towards the through-opening; a second spring area of the spring structure, that is at a second circumferential position relative to the through-opening, has a second spring length in a respective direction from the second spring area towards the through-opening; and the first spring length is greater than the second spring length.

2. A micromechanical device, comprising: a MEMS wafer; and a protective wafer, wherein: the protective wafer includes a frame wafer and an optical window situated in a through-opening of the frame wafer, the frame wafer includes at least one first trench on a front side of the frame wafer and at least one second trench on a rear side of the frame wafer; the at least one first trench and the at least second trench are situated as an interdigital structure and form a meander-shaped spring structure that surrounds the through-opening; a first spring area of the spring structure has a first spring length; and a second spring area of the spring structure has a second spring length; the first spring length is greater than the second spring length; the MEMS wafer and the protective wafer delimit a cavity; the cavity has an internal atmospheric pressure that is different from an external atmospheric pressure; and the optical window is arranged to be deflected out of a rest position at which the optical window is parallel to the MEMS wafer into a position at which the optical window is at an incline with respect to the MEMS wafer in that the optical window is farther deflected in a vicinity of the first spring area than in a vicinity of the second spring area.

3. The micromechanical device of claim 2, wherein the protective wafer further includes a glass solder connecting the optical window to the frame wafer.

4. The protective wafer of claim 1, wherein, due to the first spring length being greater than the second spring length, the spring area is deflectable to a greater degree than the second spring area.

5. The protective wafer of claim 1, wherein, due to the first spring length being greater than the second spring length, an edge region of the optical window at which the optical window is attached to the first spring area is deflectable to a greater degree than another edge region of the optical window at which the optical window is attached to the second spring area.

6. The protective wafer of claim 1, further comprising a glass solder connecting the optical window to the frame wafer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1a-f show a method according to the present invention for manufacturing a micromechanical device including an inclined optical window based on cross-sectional views of a chip.

(2) FIG. 2 shows a top view onto a detail of a micromechanical device according to the present invention in the form of a chip including an optical window and circumferential resilient elements having corresponding trenches.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

(3) The manufacturing method is described hereafter for a protective wafer including cavities, through-holes and a transparent inclined window in optical quality. The sequence of the manufacturing method is described here only by way of example and may also take place deviating therefrom. The method is described by way of example based on FIGS. 1a through 1f in a sectional view and based on FIG. 2 in a top view.

(4) The micromechanical device according to the present invention includes a protective wafer 10 and a MEMS wafer 400. Protective wafer 10 is made up of a frame wafer 100 and optical windows 300. Frame wafer 100 is preferably made of silicon, and the optical windows 300 are made of glass having a thermal coefficient of expansion adapted to silicon.

(5) In a first step, depressions are introduced on front side 110 and on a rear side 120 of frame wafer 100, in particular with the aid of KOH etching (FIG. 1a). This is first recess 115 on front side 110, and second recess 125 on rear side 120. Thereafter, a through-opening 150 is introduced (FIG. 1b), over which optical window 300 is to be positioned. Through-opening 150 may be achieved by trench etching or also with the aid of KOH pre-etching of this area, prior to the two-sided KOH etching of the depression. First recess 115 on front side 110 is used to situate optical window 300 recessed with respect to the wafer surface of frame wafer 100. In this way, it is ensured that it is not possible to damage optical window 300 after the insertion. Scratches and the like due to mechanical action are thus avoided. In the same trench etching process step, first trenches 211 are circumferentially introduced around the area of optical window 300. On rear side 120, an etching mask 30 for rear-side second trenches 221 is applied (FIG. 1c). Second trenches 221 extend between first trenches 211 of front side 110 and form an interdigital structure with these. The trench depth on front side 110 and rear side 120 should be selected in such a way that laterally and vertically only a thin remaining thickness of the silicon remains. The front side and rear side trenches, in the cross section, result in a spring structure 200, a meander-shaped element which due to the thin silicon walls acts like a bellows (accordion). Bellows 200 may be both expanded in the wafer plane and be bent out of or into the wafer plane. The stiffness regarding the expansion and bending may be set via the number of the trenches and via the wall thickness of the silicon. The design of bellows 200 is to be implemented accordingly.

(6) The insertion of optical windows 300, which previously were circumferentially provided with glass solder 310, takes place with the aid of flip chip technology even before the introduction of second trenches 221 (FIG. 1c).

(7) After the insertion of optical windows 300, frame wafer 100 completely equipped with windows is heated on a heating plate, and when the softening temperature of glass solder 310 has been reached, window 300 is pressed onto the window frame of frame wafer 100 with the aid of a pressure difference between the front side and the rear side. Glass solder 310, which is present between glass window 300 and frame wafer 100, softens and thereby spreads. After cooling, a hermetic joint is thus established between glass window 300 and frame wafer 100. Inserting optical windows 300 even before the rear-side second trenches 221 are completed is advantageous to avoid or minimize a deflection of the bellows out of the wafer plane due to the applied pressure difference.

(8) This is followed by the etching of second trenches 221 with the aid of the already completed mask 30 (FIG. 1d). Advantageously, the mask is made of oxide and thus does not have to be removed after the trench etching. As a result, protective wafer 10 is created.

(9) For the creation of a hermetic bond of protective wafer 10 with a MEMS wafer 400, in particular an actuator or sensor wafer, a glass solder 310, for example, is applied to rear side 120 of frame wafer 100 (FIG. 1d).

(10) The creation of the wafer assembly is best carried out using MEMS-customary wafer assembly processes and equipment. An underpressure (up to a vacuum) or an overpressure with respect to the outside world may be set in cavities 500 between frame wafer 100 and MEMS wafer 400. This pressure difference achieves a resulting force on optical window 300 and thus a deflection of the resilient elements of spring structure 200. An underpressure (vacuum) in cavity 500 results in a deflection into the wafer plane (FIG. 1e), and an overpressure results in a deflection beyond the wafer plane (FIG. 1f).

(11) To achieve a tilt of optical window 300, spring structure 200 is to be designed in such a way that a first spring area 220 is provided, for example on three sides of optical window 300, which is ensured a large first spring length 225 a low stiffness with respect to a movement out of the wafer plane. On the fourth window side, spring structure 200 is to be designed in such a way that such a movement preferably does not take place. Correspondingly, a second spring area 240 having a small second spring length 245 is situated here. On this side, however, bending should be possible in second spring area 240, which allows an inclined position of optical window 300. The tilt angle may essentially be set via the pressure difference, i.e., the difference between the internal atmospheric pressure in cavity 500 and the external atmospheric pressure, the stiffnesses and spring lengths of the bellows elements. FIG. 2 schematically shows such a design in a top view.

(12) The tilting or inclined position of optical windows 300 is only provided when the pressure difference between cavity 500 and the outside world is preserved. The degree of the deflection of optical windows 300 may thus be used as a test criterion for the tightness of cavity 500.

(13) In this way, the tightness test may be carried out both during the manufacturing processes and later during operation of the device. In the operating mode, the interfering, stationary reflection appears in the scan range of the micromirror if a component is not tight, and thus if an optical window is not inclined.

LIST OF REFERENCE NUMERALS

(14) 10 protective wafer

(15) 100 frame wafer

(16) 110 front side

(17) 115 first recess

(18) 120 rear side

(19) 125 second recess

(20) 150 through-opening

(21) 300 optical window

(22) 310 glass solder

(23) 30 rear-side etching mask

(24) 200 spring structure

(25) 211 first trenches

(26) 221 second trenches

(27) 220 first spring area

(28) 225 first spring length

(29) 240 second spring area

(30) 245 second spring length

(31) 400 MEMS wafer

(32) 500 cavity