HERMETICALLY SEALED ENCLOSURE AND METHOD FOR DESIGNING THE WELD CONNECTION FOR SUCH AN ENCLOSURE

Abstract

A base substrate of an enclosure has a functional region and a cover substrate covers the functional region. The base substrate and cover substrate are directly connected together in a hermetically tight manner via at least one laser bonding line so the functional region is hermetically enclosed in the enclosure. For the connection between the substrates a minimum shear force is specified that the laser weld connection is to withstand, a minimum length is determined, by an empirically determined force per laser bonding line length P, for the total length of all bonding lines, and a contact surface width B is selected such that a ratio A.sub.i/A.sub.w, formed from a contact surface A.sub.i, at which the base substrate and the cover substrate can touch one another, and a laser bonding surface A.sub.w covered by the laser bonding lines with a width w, is in the range from 1 to 10.

Claims

1-17. (canceled)

18. A method for designing a laser weld between a base substrate and a cover substrate of an enclosure, wherein the base substrate has a functional region and the cover substrate, which is in contact with the base substrate, covers the functional region, the method comprising: directly connecting the base substrate and the cover substrate to one another hermetically tightly via at least one laser bonding line so that the functional region is hermetically enclosed inside the resulting enclosure, wherein a minimum shear force F.sub.min, which the laser weld is intended to withstand, is specified for the connection between the cover substrate and the base substrate, and in that a sum of lengths L.sub.ges of all laser bonding lines is selected to be greater than a required minimum length L.sub.min of the length of all laser bonding lines, L.sub.min being determined by dividing the specified minimum shear force F.sub.min by an empirically determined force per unit laser bonding line length P so L.sub.min=F.sub.min/P, and in that a contact area width B, measured in a plane of a front face of the base substrate, which faces toward the cover substrate, as a shortest route between the functional region and an exterior of the enclosure, is selected so that a ratio J=A.sub.i/A.sub.w formed from a contact area A.sub.i, on which the base substrate and the cover substrate can touch, and a laser bonding area A.sub.w spanned by the at least one laser bonding line with a width w on the front face of the base substrate, which faces toward the cover substrate, lies in a range of from 1 to 10.

19. The method of claim 18, wherein the at least one laser bonding line comprises a plurality of laser bonding lines and a number N of closed paths of laser bonding lines with width w and a distance H between midpoints of two neighboring laser bonding lines of at least the width w are arranged around the functional region, the number N being determined as a smallest number N for which the total length L.sub.ges of all laser bonding lines, formed from the number N multiplied by a length of a contour line that delimits the functional region, is greater than the minimum length L.sub.min.

20. The method of claim 19, wherein the distance H between the midpoints of two neighboring laser bonding lines with the width w is selected in a range of from 1 w to 5 w.

21. The method of claim 20, wherein the distance His in the range of from 1.01 w to 2.5 w.

22. The method of claim 18, wherein the contact area width B is selected in a range of from 100 to 1000 m.

23. The method of claim 18, wherein the width w of the at least one laser bonding line is selected in a range of from 20 m to 75 m.

24. The method of claim 23, wherein the width w of the at least one laser bonding line is selected in the range of from 30 m to 60 m.

25. The method of claim 18, wherein the force per unit laser bonding line length P is determined empirically by producing a plurality of test specimens, in which a first substrate consisting of a cover substrate material is connected to a second substrate consisting of a base substrate material by laser bonding lines, the total length L.sub.ges of the laser bonding lines in the test specimens being selected equally, a shear force resistance of the test specimens being determined by applying an increasing shear force to the connection of the first substrate and the second substrate, determining a force at which the connection is destroyed, and evaluating a failure probability distribution.

26. The method of claim 18, wherein the minimum shear force F.sub.min is specified in such a way that, when producing a plurality of test specimens, in which a first substrate consisting of a cover substrate material is connected to a second substrate consisting of a base substrate material by laser bonding lines so that they are designed for a minimum shear force F.sub.min, and when this minimum shear force F.sub.min is applied more than 50% of the test specimens do not break along a contact area by failure of the weld connection but break at an edge of one or more of the substrates.

27. A hermetically sealed enclosure, comprising: a base substrate having a functional region; and a cover substrate which is in contact with the base substrate and covers the functional region, wherein the base substrate and the cover substrate are directly connected hermetically tightly to one another via at least one laser bonding line, and wherein the functional region is hermetically enclosed inside the resulting enclosure, wherein a ratio J=A.sub.i/A.sub.w formed from a contact area A.sub.i, on which the base substrate and the cover substrate can touch, and a laser bonding area A.sub.w spanned by the at least one laser bonding line with a width w on a surface of an interface between the base substrate and the cover substrate lies in a range of from 1 to 10, and a contact area width B, measured in a plane of a front face of the base substrate, which faces toward the cover substrate, as a shortest route between the functional region and an exterior of the enclosure, lying in a range of 100 m to 1000 m.

28. The enclosure of claim 27, wherein the area A.sub.w spanned by the at least one laser bonding line is selected so that the connection between the cover substrate and the base substrate has a failure shear force in a range of from 10 N to 1000 N.

29. The enclosure of claim 28, wherein the failure shear force is in the range of from 50 N to 500 N.

30. The enclosure of claim 27, wherein a total length L.sub.ges of all laser bonding lines is selected by a design method as claimed in claim 18.

31. The enclosure of claim 27, wherein the at least one laser bonding line comprises a plurality of laser bonding lines, the laser bonding lines having a width w, and a distance H between midpoints of two neighboring laser bonding lines being selected in a range of from 1 w to 5 w.

32. The enclosure of claim 27, wherein the cover substrate is formed as a transparent thin-film substrate having a thickness of less than 200 m.

33. The enclosure of claim 27, wherein the cover substrate and the base substrate adjoin one another directly on the contact area A.sub.i, so that the connection in the laser bonding area A.sub.w spanned by the at least one laser bonding line is free from extraneous materials comprising connecting materials or an absorbing layer.

34. The enclosure of claim 27, wherein the base substrate has a flat bottom substrate, which forms a bottom face of the functional region configured as a cavity, and an intermediate substrate, which forms side walls of the cavity, with a front face facing toward the cover substrate, and in that the bottom substrate and the intermediate substrate are connected hermetically tightly to one another via at least one laser bonding line, or in that the functional region in the form of a depression with a bottom face and side walls, which forms a cavity together with the cover substrate as a top face, is formed in the base substrate.

35. The enclosure of claim 27, wherein the cover substrate and/or the base substrate consists of glass, glass ceramic, silicon, sapphire or a combination of the aforementioned materials.

36. The enclosure of claim 27, characterized in that the width w of the at least one laser bonding line lies in a range of from 20 m to 75 m and/or the at least one laser bonding line comprises a plurality of laser bonding lines and the width w of all laser bonding lines varies by at most 30% over a total length L.sub.ges of the laser bonding lines.

37. A sensor unit and/or a medical implant, comprising: an enclosure comprising: a base substrate having a functional region; and a cover substrate which is in contact with the base substrate and covers the functional region, wherein the base substrate and the cover substrate are directly connected hermetically tightly to one another via at least one laser bonding line, and wherein the functional region is hermetically enclosed inside the resulting enclosure, wherein a ratio J=A.sub.i/A.sub.w formed from a contact area A.sub.i, on which the base substrate and the cover substrate can touch, and a laser bonding area A.sub.w spanned by the at least one laser bonding line with a width w on a surface of an interface between the base substrate and the cover substrate lies in a range of from 1 to 10, and a contact area width B, measured in a plane of a front face of the base substrate, which faces toward the cover substrate, as a shortest route between the functional region and an exterior of the enclosure, lying in a range of 100 m to 1000 m.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0075] Preferred elaborations and embodiments of the invention are represented in the drawings and will be explained in more detail in the following description, reference signs which are the same referring to structural parts or elements which are the same or similar or functionally equivalent.

[0076] In a schematic form

[0077] FIG. 1 shows a perspective view of substrates connected by a laser bonding line,

[0078] FIG. 2 shows a plan view of a hermetic enclosure,

[0079] FIG. 3 shows a sectional view of the hermetic enclosure from the side,

[0080] FIG. 4 shows a section through laser bonding lines along the welding direction,

[0081] FIG. 5 shows a section through laser bonding lines perpendicularly to the welding direction,

[0082] FIG. 6 shows a diagram of the failure probability of laser-welded test specimens in the shear test for three different total lengths of the laser bonding lines as a function of the shear force exerted,

[0083] FIG. 7 shows a diagram of the characteristic failure force of the laser-welded test specimens as a function of the total length of the laser bonding lines,

[0084] FIG. 8 shows a diagram of the empirical constant determined for the bonding strength per unit length,

[0085] FIG. 9 shows a micrograph of a cross-section polish of two substrates connected to one another by laser bonding lines,

[0086] FIG. 10 shows three examples of fracture patterns for the failure of the weld connection when the failure shear force is exceeded, and

[0087] FIG. 11 shows three examples of fracture patterns in which one or both substrates are fractured by the action of force without prior failure of the weld connection.

DETAILED DESCRIPTION OF THE INVENTION

[0088] FIG. 1 represents a perspective view of two substrates 3, 4 connected by a laser bonding line 2. A first substrate 3 is in this case placed on a second substrate 4 so that the two substrates 3, 4 touch directly. The area on which the two substrates 3, 4 touch is referred to as the contact area A.sub.i.

[0089] If the surfaces of the two substrates 3, 4 are smooth, the surfaces placed on one another have a spacing from one another which can no longer be optically determined. This is usually the case for a spacing of less than about 250 nm. With such small spacings, adhesion forces already occur between the two substrates 3, 4 during the placement. These adhesion forces occur in a region which is referred to as the touching contact area A.sub.c. The touching contact area A.sub.c is less than the total contact area A.sub.i.

[0090] For the hermetically tight connection of the two substrates 3, 4 in the region of the touching contact area A.sub.c, laser welding is carried out by introducing a laser bonding line 2. Along the laser bonding line 2, material is melted with an ultrashort-pulse laser and recooled, so that the two substrates 3, 4 are connected to one another if they adjoin one another very tightly as in the region of the touching contact area A.sub.c. In a laser bonding area A.sub.w generated by the laser welding, the two substrates 3, 4 are materially bonded to one another so that there is no longer any spacing between the substrates 3, 4. At about 20 m to 75 m, the width of the laser bonding lines 2 is thin, so that in the surface-wide connection of the substrates 3, 4 represented as an example in FIG. 1 only a very small part of the touching contact area A.sub.c, or of the contact area A.sub.i, is additionally welded by laser treatment.

[0091] Surprisingly, it has been found that the contribution to the shear force resistance of the connection of the two substrates 3, 4 by the laser bonding area A.sub.w, despite the very small area compared with the total contact area A.sub.i and the touching contact area A.sub.c in the example represented in FIG. 1, is very much greater than the contribution of the adhesion forces in the region of the touching contact area A.sub.c. The laser bonding area A.sub.w may accordingly be used not only to hermetically tightly seal a gap existing between the two substrates, but also to increase the resistance of the connection against exerted shear forces.

[0092] FIG. 2 shows a plan view of an exemplary embodiment of a hermetic enclosure 1. The enclosure has a length a, a width b and a height c (cf. FIG. 3). A functional region 20 in the form of a hollow space or cavity 21, into which a functional element 22 is hermetically tightly encapsulated, for example a sensor or a transponder, is formed in the enclosure 1.

[0093] Typical measurements of an enclosure are a=5 mm, b=5 mm, c=2.5 mm, although larger-area and flatter (for example a=10 mm, 10=5 mm, c=0.9 mm) or more compact (a=3 mm, b=4 mm, c=2 mm) ones are also possible.

[0094] In order to form the enclosure 1, a cover substrate 14 is placed onto a base substrate 10 (cf. FIG. 3) and touches the base substrate 10 on the contact area A.sub.i. The contact area A.sub.i corresponds to a front face 16 of the base substrate 10, cf. FIG. 3.

[0095] Via a plurality of laser bonding lines 2, the cover substrate 14 is connected hermetically tightly to the base substrate 10. The laser bonding lines 2 extend parallel to the side walls of the cavity 21, the cavity 21 being rectangular in the example represented, and correspondingly having four side walls. In the example, two bonding lines 2 in each case extend parallel to one of the side walls of the cavity, the thickness of the side walls corresponding to a contact area width B. Only those bonding lines 2 that are not separated from one another by the functional region 20, or the cavity 21, are regarded as neighboring one another. The laser bonding lines 2 in this example form two closed rectangular paths around the functional region 20.

[0096] The enclosure represented in FIG. 2 was obtained from a wafer stack in which a wafer for the base substrate 10 and a wafer for the cover substrate 14 were placed on one another and connected to one another. A wafer stack which comprises a multiplicity of enclosures 1 connected to one another was thereby obtained. The laser bonding lines 2 were respectively elaborated over the entire width, or length, of the wafer stack. The individual enclosure 1, as is represented in FIG. 2, was obtained by singulating the multiplicity of enclosures.

[0097] FIG. 3 shows a sectional view of the hermetic enclosure 1 of FIG. 2 from the side along the section line marked by A-A in FIG. 2.

[0098] It may be seen in the sectional representation of FIG. 3 that the base substrate 10 in this exemplary embodiment consists of a bottom substrate 11 and an intermediate substrate 12. A connection of the bottom substrate 11 and the intermediate substrate 12 was elaborated hermetically tightly in a similar way to the connection between the cover substrate 14 and the base substrate 10, or the intermediate substrate 11, via a plurality of laser bonding lines 2.

[0099] The side walls of the resulting cavity 21 are formed here by the intermediate substrate 12, and the bottom of the cavity 21 is formed by the bottom substrate 11. In the example represented, the functional element 22 is arranged inside the cavity 21 on the bottom substrate 11.

[0100] FIG. 4 shows a section through laser bonding lines 2 along the welding direction. The welding direction is the direction along which the laser beam was guided over the substrates 11, 12, 14 to be connected, the individual pulses locally overlapping multiply so that a weld seam is created by accumulation of heat above the focal points 32. The cross section of the seam is pyriform and is referred to as a weld pear 30. The weld pear 30 represents the region of the substrates 11, 12, 14 that has been processed by the respective laser pulse in such a way that the material was heated above the glass transition temperature T.sub.g, or the melting temperature, and the respectively neighboring substrates 11, 12, 14 are able to be materially bonded. The scan speed is selected in conjunction with the pulse repetition rate of the ultrashort-pulse laser so that a continuous laser bonding region is created in the region of the laser bonding line 2.

[0101] The laser beam is focused so that a focal point 32 is placed at a distance T from the connection plane between the two respective substrates 11, 12, 14. Starting from the focal point 32, the weld pear 30 is then formed with a height HL by the energy transferred onto the respective substrate 11, 12, 14 by the laser pulse.

[0102] FIG. 5 shows a section through laser bonding lines 2 perpendicularly to the welding direction. It may be seen in this sectional representation that the respective laser bonding lines 2 have a width w in relation to the connection plane between the substrates 11, 12, 14 respectively to be connected, i.e. in this case once between the bottom substrate 11 and the intermediate substrate 12 and a further time between the intermediate substrate 12 and the cover substrate 14. Since the width of the weld pears 30 varies along the height HL of the weld pears 30, the width w of the laser bonding lines 2 may correspondingly be adjusted by selection of the depth T of the focal point 32 in relation to the respective connection plane. A distance H respectively between two neighboring laser bonding lines 2, in each case measured from midpoint to midpoint, is preferably selected so that the laser bonding lines 2 do not overlap. Accordingly, the distance H is greater than or equal to the width w. It is furthermore desirable to embody the enclosure 1 as compactly as possible, and accordingly to select the contact area width B, which corresponds here to the width of the side wall of the cavity 21, cf. FIG. 3, to be as small as possible. Accordingly, a distance between two laser bonding lines 2 is preferably selected to be at most five times the width w.

[0103] FIG. 6 shows a log-log representation of the cumulative failure probability of laser-welded test specimens in the shear test for three different total lengths of the laser bonding lines as a function of the shear force exerted in N. A first curve 101 shows the cumulative failure probability for 30 test specimens with a laser bonding line length of in total 20 mm, a second curve 102 shows the cumulative failure probability for 30 test specimens with a laser bonding line length of in total 40 mm, and a third curve 103 shows the cumulative failure probability for 30 test specimens with a laser bonding line length of in total 60 mm.

[0104] FIG. 7 shows a diagram of the characteristic failure force of the laser-welded test specimens as a function of the total length of the laser bonding lines. A fitted affine function may be used to determine the empirical constant P. The slope of the function corresponds to the constant P. The y axis intercept corresponds to the adhesion force provided by a touching contact area A.sub.c, cf. FIG. 1.

[0105] FIG. 8 shows a diagram of the empirical constant P determined for the bonding strength per unit length for the three curves 101, 102, 103, cf. FIG. 6. It may be seen that, within the scope of the error tolerance, the values obtained for the constant p are independent of the laser bonding line length of the respective test specimens.

[0106] FIG. 9 shows a micrograph of a cross-section polish of two substrates 10, 14 connected to one another by laser bonding lines 2 with reference to the example of substrates 10, 14 consisting of a borosilicate glass. The laser bonding lines 2 may be seen clearly because of the refractive index changes that occur during the heating and recooling.

[0107] FIG. 10 shows three examples a, b and c of fracture patterns for the failure of the weld connection between two substrates when the failure shear force is exceeded. It may be seen clearly here that the two substrates have been separated from one another substantially without further damage along the weld seams, or laser bonding lines.

[0108] FIG. 11 shows three examples a, b, c of fracture patterns in which one or both substrates are fractured by the action of force without prior failure of the weld connection. It may in each case be seen clearly that the fracture lines do not extend along the original surfaces of the substrates here, but rather the respective substrates themselves have been destroyed. Parts of the respective substrates are chipped.

[0109] Although the present invention has been described with the aid of preferred exemplary embodiments, it is not restricted thereto but may be modified in a variety of ways.

LIST OF REFERENCE SIGNS

[0110] A.sub.i contact area [0111] A.sub.c touching contact area [0112] A.sub.w laser bonding area [0113] 1 enclosure [0114] 2 laser bonding line [0115] 3 first substrate [0116] 4 second substrate [0117] 10 base substrate [0118] 11 bottom substrate [0119] 12 intermediate substrate [0120] 14 cover substrate [0121] 16 front face [0122] 20 functional region [0123] 21 cavity [0124] 22 functional element [0125] 30 weld pear [0126] 32 focal point [0127] A section line [0128] a length of enclosure [0129] b width of enclosure [0130] C height of enclosure [0131] B contact area width [0132] HL height of laser bonding line [0133] T depth of laser bonding line [0134] W width of laser bonding line [0135] H distance between two laser bonding lines [0136] 101 first curve [0137] 102 second curve [0138] 103 third curve [0139] 5 KW cumulative failure probability [0140] S shear force [0141] p empirical constant [0142] F.sub.v failure shear force [0143] L laser bonding line length