Hermetically sealed enclosure and method for designing the weld connection for such an enclosure

Abstract

The invention relates to a method for designing a laser weld connection between a base substrate (10) and a cover substrate (14) of an enclosure (1), wherein the base substrate (10) has a functional region (20), and the cover substrate (14), which is in contact with the base substrate (10), covers the functional region (20), wherein the base substrate (10) and the cover substrate (14) are directly connected to one another in a hermetically tight manner via at least one laser bonding line (2) such that the functional region (20) is hermetically enclosed in the interior of the formed enclosure (1). According to the invention, for the connection between cover substrate (14) and base substrate (10) a minimum shear force Fmin is specified that the laser weld connection is to withstand, a minimum length Lmin is determined, by means of an empirically determined force per laser bonding line length P, for the total length of all bonding lines (2), and a contact surface width B is selected such that a ratio Ai / Aw, formed from a contact surface Ai, at which the base substrate (10) and the cover substrate (14) can touch one another, and a laser bonding surface Aw covered by the laser bonding lines (2) with a width w, is in the range from 1 to 10. Further aspects of the invention relate to such an enclosure (1) and to a sensor unit and / or medical implant comprising such an enclosure (1).

Description

[0001] Hermetically sealed enclosure and method for designing the welded joint for such an enclosure

[0002] The invention relates to a hermetically sealed housing comprising a base substrate having a functional region and a cover substrate in contact with the base substrate and covering the functional region. The base substrate and the cover substrate are directly hermetically sealed to one another via at least one laser bonding line, and the functional region is hermetically enclosed within the formed housing. Furthermore, the invention relates to a method for designing the laser-welded connection between the substrates and the use of such a housing.

[0003] Hermetically sealed enclosures, for example, are designed to protect a component or components inside the enclosure from adverse environmental conditions. Applications for such hermetically sealed enclosures can be found, for example, in electronics applications to protect sensitive electronic components, and also in optics applications to encapsulate optical components. Further applications can be found, in particular, in the areas of medical implants, microfluidic chips, augmented reality, and mobility sensors (e.g., pressure sensors).

[0004] Transparent materials such as glass are particularly desirable for the housing in optical applications. However, glass materials are also advantageous over conventional metal housings, such as those made of titanium, in electronic applications where wireless communication or wireless charging is desired, as these materials do not shield the radiation used. An example of such a hermetically sealed housing is known from EP3812352 A1. The housing comprises at least a base substrate and a cover substrate, which form the housing and enclose a functional area inside. The cover substrate and the base substrate, which are selected, for example, from a glass material, are connected to one another by laser bonding lines.

[0005] A method for producing a transparent part for protecting an optical component using a laser process for generating laser bonding lines is also known from European patent specification EP 3 012 059 B1.

[0006] In these known laser processes, two substrates are placed on top of each other and any remaining distance or gap between the substrates is sealed by laser welding so that the connection between the two substrates is hermetically sealed.

[0007] The resulting housings must meet high mechanical requirements, especially when used as implants. One measure of the mechanical strength of the connection between two housing parts is its resistance to shear forces. The greater the stability of the connection, the higher the shear forces the connection can withstand without coming loose. When connecting two substrates, the shear force resistance depends on the size of the contact area over which the two substrates are in contact.

[0008] To ensure sufficient shear resistance, the known hermetic enclosures have comparatively thick walls, which can be several millimeters thick. In addition to increasing the wall thickness, it is also possible to design the parts to be joined so that they have a positive fit. However, this also requires a lot of installation space and is complex, especially for components made of glass and the like. This makes a particularly compact design of the hermetic enclosures difficult.

[0009] One object of the invention can therefore be seen in providing a hermetic enclosure that has particularly thin walls and simultaneously meets the mechanical requirements. A further object of the invention can be seen in providing a method for designing a laser weld between components of an enclosure, with which a particularly compact and simultaneously sufficiently durable enclosure can be obtained based on a given mechanical requirement.

[0010] Disclosure of the invention

[0011] A method for designing a laser weld between a base substrate and a cover substrate of a housing is proposed. The housing to be formed comprises at least the base substrate with a functional area and the cover substrate. The cover substrate is in contact with the base substrate and covers the functional area. The base substrate and the cover substrate are directly hermetically sealed to one another via at least one laser bonding line, so that the functional area is hermetically enclosed within the formed housing. It is provided that a minimum shear force resistance Fmin is specified for the connection between the cover substrate and the base substrate, which the laser weld is intended to withstand, and that the sum of the lengths L gesof all laser bonding lines is selected to be greater than a required minimum length Lmin of the length of all laser bonding lines, where Lmin=Fmin / P is determined by dividing the specified minimum shear force Fmin by an empirically determined force per laser bonding line length P and where a contact surface width B, measured in the plane of the end face of the base substrate facing the cover substrate as the shortest distance between the functional area and the outer part of the housing, is selected such that a ratio J=A / A W formed from a contact area A, at which the base substrate and the cover substrate can touch, and a laser bonding area Aw swept over by the laser bonding lines with a width w on the end face of the base substrate facing the cover substrate in the range from 1 to 10.

[0012] Preferably, a number N of closed paths of laser bonding lines with width w and a distance H between the centers of two adjacent laser bonding lines of at least the width w are arranged around the functional area, wherein the number N is determined as the smallest number N for which the total length L ges of all laser bonding lines formed from the number N multiplied by the length of a contour line which delimits the functional area is greater than the minimum length Lmin.

[0013] For the purposes of this application, a contact surface is the intersection of the inclined surfaces of the two substrates to be brought into contact. The contact surface refers to a partial area of ​​the contact surface where the distance between the two substrates is so small that it can no longer be measured optically. In particular, in the area of ​​the contact surface, the distance between the surfaces of the adjacent substrates is less than 250 nm. In general, the contact area is greater than or equal to the contact surface.

[0014] In other words, two substrates are first arranged next to each other, for example, stacked on top of each other, with gravity pressing the upper, typically first, substrate against the second substrate. The orientation above or below is merely descriptive, since the substrates can, of course, assume any orientation in space, and even a side-by-side arrangement should not exceed the scope of protection. The two substrates are typically arranged adjacent to each other with a larger side of their extension.

[0015] If both substrates were completely flat, meaning they had no depressions, elevations, or curvatures at all—which is only theoretically achievable—the first and second substrates would be in full-surface contact. The two substrates would therefore touch at all points on their aligned surfaces. This is generally unattainable and not possible in real-world construction. Rather, substrates are, albeit only to a very small extent, curved, inclined, or provided with depressions or elevations, so that full contact is only achieved in exceptional cases.

[0016] The functional area enclosed by the housing can, in particular, be a cavity configured to accommodate a functional element. The cavity has a bottom surface and side walls provided by the base substrate, and has a cover surface provided by the cover substrate. In this embodiment, the thickness of the side walls corresponds to the contact surface width.

[0017] In other examples of the housing, the functional area can be a functionalized area of ​​the base substrate. Such functionalization can be achieved, for example, by applying a coating and / or by surface structuring.

[0018] The functional area is hermetically sealed by the welded joint. Hermetically sealed is defined in particular as a housing that has a helium leak rate of less than 1 ■ 10 -8mbar l / sec and preferably in the range 1 ■ 10 -10 mbar l / sec to 1 ■ 10 -9 mbar l / sec. The welded joint is created by introducing at least one laser bonding line or laser welding line. The welded joint is preferably created using an ultrashort pulse laser. Typical pulse widths are in the range of 100 fs to 100 ps. A method for creating such a welded joint with one or more laser welding lines is known, for example, from EP 3 012 059 B1.

[0019] The laser welding line has a height HL in a direction perpendicular to its joining plane. The joining plane is the direction in which the adjacent or consecutive beam spots are placed. Typically, laser welding is performed from a top-down perspective, i.e., the substrate stack rests on a surface—such as a table—and the laser is fired from above through at least the topmost substrate layer—or through several substrate layers—to the location of the beam focus. The height HL is therefore measured in the direction of the laser beam, while the width w of the laser welding line is measured perpendicular to the direction of the laser beam.

[0020] The width w of the area altered by the laser beam varies along the depth T of the processed area, i.e., along the laser beam direction. The information provided in this application regarding the width w of the laser welding line refers to the plane of the contact surface between the substrates connected by the laser welding line. In this defined plane, the width w is understood to be the area within which material changes were induced by the laser treatment. Such material changes due to the laser treatment result from heating above the glass transition temperature T gand / or the melting temperature of the materials involved, followed by cooling. This laser treatment bonds the two substrates in the treated area without the need for additional bonding materials. For optically transparent materials, the material change caused by the laser treatment can be detected, for example, by measuring a deviation in the refractive index compared to the untreated material. For this purpose, a cross-section can be examined with a light microscope, for example. In particular, a change in the refractive index of more than 1x10' 5 as a marker for material change and, accordingly, for determining the width w. A micrograph of such a cross-section is shown in Figure 10 and is described in more detail below.

[0021] A variety of materials can be joined together using the laser welding process, whereby at least the substrate facing the laser source should be at least partially transparent to the laser used.

[0022] The generated laser welding lines or laser bonding lines are arranged around the functional area in such a way that, in a contact plane corresponding to the end face of the base substrate facing the cover substrate, a closed area surrounding the functional area is formed by the laser bonding area. In the case of a cuboid-shaped housing, one or more straight laser bonding lines can then be arranged on each of the four sides delimiting the functional area, whereby the laser bonding lines can overlap in the four corners. The one or more laser bonding lines can in particular each run parallel to the side walls. It is also possible to arrange one or more closed laser bonding lines, for example, parallel to an outline of the functional area.

[0023] To hermetically enclose the functional area, at least one laser bond line must be guided seamlessly around the functional area. However, this criterion can also be met by several individually written laser bond lines that overlap at intersection points, thereby forming a closed path around the functional area. Within the meaning of the method, each of these closed paths corresponds to a laser bond line arranged around the functional area, with exactly one such path having a number N=1 and, for example, with exactly two closed paths having a number N=2. If a single such closed laser bond line is sufficient to meet the specified minimum shear force Fmin and exceed the specified minimum length Lmin, the number N of laser bond lines that are guided around the functional area can be selected as N=1.In this case, there are no adjacent laser bonding lines in the sense of the process.

[0024] Preferably, several laser bonding lines are introduced in the form of several such closed paths, with adjacent laser bonding lines that run within the area defined by the respective contact surface width preferably being arranged parallel to one another. Laser bonding lines that are separated from one another by the functional area are not considered adjacent.

[0025] Preferably, a distance H between the centers of two adjacent laser bonding lines with a width w in the range from 1 w to 5 w, preferably in the range from 1.01 w to 2.5 w, and particularly preferably in the range from 1.05 w to 2 w is selected. This prevents the laser bonding lines from overlapping, apart from any intersections between different laser bonding lines.

[0026] By providing a distance between two adjacent, particularly parallel, laser bonding lines of at least the width of the laser bonding lines, the substrate material is processed only once, except for intersections of laser bonding lines, for example, at the four corners around a rectangular functional area. On the other hand, the provided upper limit for the distance results in a particularly compact design of the laser bonding surface. The contact surface width B can thus be selected to be particularly small while still providing sufficient space for the formation of the laser bonding lines.

[0027] The contact area width B is preferably selected in the range from 100 μm to 1000 μm. The exact selection of the contact area width B depends on various criteria, such as the material of the base substrate, the material of the cover substrate, the dimensions of the housing, and / or the type of functional area. If a cavity is provided as the functional area, the contact area width B specifies the thickness of the side walls of the functional area. The contact area width is preferably selected at least large enough to ensure sufficient mechanical stability of the side wall.

[0028] Preferably, the contact surface width B is chosen to be greater than 200 pm, particularly preferably greater than 300 pm, more preferably greater than 400 pm and most preferably greater than 500 pm.

[0029] The larger the contact area width B is selected, the larger the enclosure will be in relation to the enclosed functional area. Accordingly, it is preferable to select a contact area width B of less than 750 pm, particularly preferably less than 500 pm, and most preferably less than 400 pm. The minimum contact area width is limited by the width w of a laser bonding line and is therefore preferably selected to be greater than 30 pm, particularly preferably greater than 50 pm, and most preferably greater than 100 pm.

[0030] The width w of the laser bonding lines is preferably selected in the range from 20 pm to 75 pm, particularly preferably in the range from 30 pm to 60 pm. For example, a width w of 50 pm is selected. This range is optimally selected in order to be able to introduce sufficient energy via the laser for welding the two substrates. It is particularly advantageous if the width w is essentially constant over the entirety of the laser bonding lines. Accordingly, it is preferred if the width w of all laser bonding lines varies by a maximum of 30%, particularly preferably a maximum of 20%, and most preferably a maximum of 10% over the total length Ltotal of the laser bonding lines. Since the width w depends on the position of the focal point of the laser in relation to the contact plane, the use of a laser processing method with precise control of the distance of the laser focus from the contact plane is preferred. A suitable method is known, for example, from EP3012059B1.

[0031] For the most compact housing possible, the fill factor J=Ai / A w formed from a contact area Ai, at which the base substrate and the cover substrate can touch, and a laser bonding area Aw swept over by the at least one laser bonding line with a width w, should be selected as small as possible. Accordingly, it is particularly preferred to select the fill factor J in the range from 1 to 5, most preferably in the range from 1 to 2.

[0032] According to the proposed method, the total length of the laser bonding lines applied, and thus the laser bonding area Aw, is selected to be just large enough to ensure a predetermined resistance of the welded joint to the shear forces acting on it. The term "shear forces" refers in particular to forces acting perpendicular to the bonding plane of the two substrates, which, without a bond between the substrates, would lead to a displacement of the substrates relative to each other.

[0033] The inventors have determined that the shear force resistance of the welded joint increases linearly with the total length of the non-overlapping laser bonding lines. For the non-overlapping criterion, small overlaps, for example at intersection points of laser bonding lines arranged at right angles to each other around a rectangular functional area, can be neglected due to the small area of ​​these intersections. Accordingly, given the minimum shear force Fmin against which the welded joint is to be resistant and an empirically determined constant P for the force increase per unit length, the required minimum length of the sum of the laser bonding lines can be determined using the relationship

[0034] Lmin = Fmin / P can be determined. The total length L gesThe introduced laser bond lines can be determined, particularly in the case of laser bond lines running parallel around the functional area, as an integer multiple of the length of a contour line around the functional area. The slight increase in the actual length of the circumferential laser bond lines due to the fact that the laser bond lines are not overlapping can be neglected due to the narrow width of the laser bond lines. Even small additional bond line segments, which can occur during the production of a large number of enclosures by processing a wafer and subsequently separating the enclosures, can be neglected due to their short length.

[0035] The constant P is specific to the materials of the substrates to be joined and the selected width of the laser bond line and can be easily determined empirically by producing several test specimens, for example, 30 specimens, in which a first substrate made of a cover substrate material is bonded to a second substrate made of a base substrate material with laser bond lines, with the total length L of the laser bond lines being chosen to be the same for the test specimens. The shear force resistance of the test specimens is then determined by applying an increasing shear force to the bond between the first and second substrates, determining the force at which the bond is destroyed, and evaluating a failure probability distribution.The minimum shear force Fmin, to which the welded joint must be resistant, is preferably specified no higher than necessary, so that the welded joint itself, and thus the enclosure as a whole, can be designed as compactly as possible. The specification is preferably based on the mechanical requirements of the enclosure. One criterion, in particular, can be that the minimum shear force is related to other force resistances of the substrates.For this purpose, the minimum shear force Fmin is preferably specified in such a way that a plurality of test specimens are produced in which a first substrate made of a cover substrate material is connected to a second substrate made of a base substrate material with laser bonding lines in such a way that these are designed for a minimum shear force Fmin and when this minimum shear force Fmin is applied, more than 50%, preferably more than 75%, particularly preferably more than 90%, most preferably 95% of test specimens do not break along the contact surface due to failure of the weld connection, but break at other points, in particular at an edge of one or more of the substrates.

[0036] To determine the failure rate, similar to the determination of the empirical constant P, several similar test specimens can be produced, for example 30, in which two substrates have been welded together by introducing laser bonding lines. The total length of the laser bonding lines is selected according to the shear force Fmin to be tested. The test specimens are then increasingly subjected to a shear force. The location at which a test specimen fails mechanically can be easily determined by visual inspection of the test specimen. If the weld fails, the individual substrates are separated again but essentially without further damage. If the number of test specimens that do not fail along the contact area due to failure of the weld is within the specified range, for example more than 75%, then the tested shear force Fmin has been correctly selected.Otherwise, the test is repeated for a higher or lower shear force depending on the result. After the weld joint has been designed, steps for manufacturing the enclosure can follow. These steps may include, in particular, preparing the substrates and, if necessary, cleaning the surfaces of the substrates, placing the substrates on top of one another (where a functional element may be inserted into a functional area), and applying the laser bonding lines.

[0037] During production, it may be provided, in particular, to produce a plurality of enclosures in a single pass. For this purpose, large wafers are first provided instead of substrates already cut to the final enclosure size, stacked on top of each other in layers, and joined together by laser welding. The individual enclosures are then separated by cutting the resulting wafer stack. In such a procedure, it may be provided that the introduced laser bonding lines are written along a grid pattern, with each functional area enclosed by four laser bonding lines forming a rectangle.

[0038] A further aspect of the invention is the provision of a hermetically sealed enclosure. The proposed hermetically sealed enclosure comprises 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 hermetically sealed to one another via at least one laser bonding line, and wherein the functional region is hermetically enclosed within the formed enclosure. Furthermore, it is provided that a ratio J=A / A Wformed from a contact area A, at which the base substrate and the cover substrate can touch, and a laser bonding area Aw swept over by the at least one laser bonding line with a width w on the end face of the base substrate facing the cover substrate in the range from 1 to 10, wherein a contact area width B, measured in the plane of the end face of the base substrate facing the cover substrate as the shortest distance between the functional area and the exterior of the housing, is in the range from 100 pm to 1000 pm.

[0039] The proposed package is particularly compact because the contact area width B is designed so that the laser bonding area Aw fills as large a part as possible of the total contact area Ai.

[0040] In the case of hermetic enclosures obtained by laser welding substrates known from the prior art, it was assumed that a significant portion of the mechanical stability was provided by the largest possible contact area, and that the laser bonding area was essentially required to ensure the hermetic enclosure of the functional area. Due to the typically small laser bonding area Aw compared to the total contact area Ai, the laser bonding area Aw itself was previously assumed to make only a minor contribution to mechanical stability, particularly to resistance to shear forces.

[0041] Furthermore, it was surprisingly found that maximizing the shear strength of the welded joint between the two substrates is not advantageous, as in this case, the enclosure will break uncontrollably at other locations when exposed to high mechanical forces. Therefore, further extending the weld seam does not contribute to improving the overall strength of the enclosure. Furthermore, the "unnecessary" weld seams created in this way require a larger contact area, thus increasing the enclosure's "footprint." Accordingly, it is preferable to specify not only a lower limit but also an upper limit for the shear strength.

[0042] For this purpose, the laser bonding area Aw swept over by the at least one laser bonding line is preferably selected such that the connection between the cover substrate and the base substrate has a failure shear force in the range from 10 N to 1000 N, preferably 50 N to 500 N, particularly preferably in the range from 100 N to 400 N.

[0043] The preferred total length is L ges The laser bonding lines of the housing are determined according to one of the design methods described herein. It is particularly preferred if the specified minimum shear force Fmin, which the laser weld is intended to withstand, corresponds to this failure shear force in the range of 10 N to 1000 N.

[0044] Since the enclosure can be obtained using one of the described methods, features described within one of the methods also apply to the enclosure and, conversely, features disclosed within the enclosure also apply to the methods.

[0045] Preferably, the welding is carried out with a plurality of laser bonding lines, wherein the laser bonding lines have a width w and a distance H between the centers of two adjacent laser bonding lines is selected in the range from 1 w to 5 w, preferably in the range from 1.01 w to 2.5 w and particularly preferably in the range from 1.05 w to 1.5 w.

[0046] The width w of the laser bonding lines is preferably in the range from 20 pm to 75 pm, particularly preferably from 30 pm to 60 pm. For example, the laser bonding lines are 50 pm wide. It is particularly advantageous if the width w is substantially constant across the entirety of the laser bonding lines. Accordingly, it is preferred if the width w of all laser bonding lines varies by at most 30%, particularly preferably at most 20%, and most preferably at most 10%, across the total length L of the laser bonding lines.

[0047] The housing is preferably designed to be as compact as possible. This is achieved by keeping the portion of the contact area not occupied by the laser bonding area as small as possible and, as a result, also making the contact area width B as small as possible. For this purpose, it is preferably provided that the end face of the base substrate facing the cover substrate, which corresponds to the contact area Ai, is covered by at least 20% laser bonding lines, so that J is in the range from 1 to 5. Particularly preferably, at least half of the contact area A is covered by laser bonding lines, with J then being in the range from 1 to 2.

[0048] The cover substrate is preferably formed as a transparent thin-film substrate, wherein the cover substrate has a thickness of less than 200 μm, preferably less than 170 μm, particularly preferably less than 125 μm, and preferably has a thickness greater than 10 μm, particularly preferably greater than 20 μm. This allows the dimensions of the housing to be designed to be particularly compact, even in the stacking direction of the substrates.

[0049] The cover substrate can be provided in the form of a thin-film substrate and welded to the base substrate. Alternatively, a thicker substrate can be thinned by material removal after bonding to the base substrate.

[0050] The cover substrate and the base substrate directly adjoin each other at the contact surface A, so that the connection in the laser bonding area Aw swept by the at least one laser bonding line is free of foreign materials, in particular free of bonding materials such as adhesives, a glass frit, or an absorbent layer. Since no foreign substances were used in the sealing of the housing, contamination of the functional area, for example, by components of an adhesive, is avoided.

[0051] The functional area can be formed as a cavity. Such a cavity is preferably configured to accommodate a functional element, so that one or more functional elements can be accommodated in the cavity of such a housing. The cavity has a bottom surface and side walls provided by the base substrate, and a cover surface provided by the cover substrate. In this embodiment, the thickness of the side walls corresponds to the contact surface width.

[0052] The base substrate can comprise a flat bottom substrate, which forms the bottom surface of a functional area configured as a cavity, and an intermediate substrate, which forms the side walls of the cavity with an end face facing the cover substrate. The bottom substrate and the intermediate substrate are preferably hermetically sealed to one another via at least one laser bonding line. For the design of this welded connection, the method described herein can be applied analogously, and the welded connection can be designed analogously to the connection between the cover substrate and the base substrate described herein.

[0053] Alternatively or additionally, the base substrate can be configured as a functional region in the form of a recess with a bottom surface and side walls, which, together with the cover substrate as the lid surface, forms a cavity. Such recesses can be formed, for example, by grinding or etching.

[0054] In other examples of the housing, the functional area can be a functionalized area of ​​the base substrate. Such functionalization can be achieved, for example, by applying a coating and / or by surface structuring.

[0055] The cover substrate and / or the base substrate preferably consist of a glass, a glass ceramic, silicon, sapphire, or a combination of the aforementioned materials. Borosilicate glasses are particularly suitable as glass materials. The invention also relates to the use of the proposed housing as a housing for a sensor unit and / or a medical implant. In these applications, the hermetic seal and the achievable compact dimensions of the housing are particularly advantageous. With small wall thicknesses and a contact surface width of, for example, 500 μm, the housing is only insignificantly larger than a functional element accommodated therein.

[0056] Furthermore, a sensor unit and / or medical implant comprising one of the housings described herein is provided. Preferably, the housing has a cavity that encloses a functional element of the sensor unit and / or the medical implant.

[0057] An example of a housing comprises a bottom substrate and an intermediate substrate, which together form a base substrate, as well as a cover substrate. All substrates are made of borosilicate glass, which is available, for example, under the name BOROFLOAT 33. The bottom substrate and the intermediate substrate have a thickness of 1.1 mm, and the cover substrate has a thickness of 500 μm. The length and width of the substrates are each 5 mm. By selecting a contact surface width B of 500 μm, a cavity with side walls and a cover wall with a thickness of 500 μm is provided.

[0058] Example of determining the empirical parameter P:

[0059] Samples were produced in which two substrates made of a floated borosilicate flat glass available under the name BOROFLOAT® 33 with a length and width of 5 mm and a thickness of 1.1 mm were placed on top of each other. To verify the assumption that the shear force is linear with the total length of the laser bond lines, laser bond lines with a total length L gesof 20 mm. In a second type 2, laser bond lines with a total length L of 40 mm were inscribed, and in a third type 3, laser bond lines with a total length L of 60 mm were inscribed. The laser bond lines can, in principle, be introduced in any geometry. In the present example, half of the bond line length was inscribed along a first direction and the other half of the bond line length along a second, perpendicular direction, so that a "+" shape was formed. The overlap of the laser bond lines in the center of this cross shape can be neglected due to its small area.

[0060] 30 samples of each of the three types were produced and the shear force at which the bond between the two substrates failed was determined. For this purpose, an apparatus was used in which two superimposed plates can be moved against each other with a defined force, i.e. sheared. Each plate has a depression whose shape and depth correspond to the dimensions of the substrates except for a small tolerance. Accordingly, the side walls of the depressions lie closely against the side surfaces of the respective substrates. To determine the shear force at which a bond between the two substrates of one of the samples fails, a sample was placed between the two plates and the two plates were moved against each other at a rate of 1.5 mm / min, during which force was measured.If the joint fails, the sample no longer resists the displacement, which is detected by an abrupt drop in the measured force. The shear force at which the joint failed is then the highest force measured during displacement or shearing of the two plates. The measurement is repeated for all samples, with the shear force at which the joint between the two substrates of a sample fails being recorded. The measurement results for the cumulative probability KP are plotted in Figure 6 as a log-log plot.

[0061] By adjusting the parameters of a distribution function p(F) for the cumulative failure probability at a shear force of F, a failure shear force Fv can be determined at which the corresponding specimen type fails. The distribution function p(F) is given by where the parameter z specifies the width of the distribution function and can also be determined by parameter fitting.

[0062] For a confidence interval of 95%, the following results for the failure shear force Fv are obtained for the three sample types, which are also shown in Figure 7 as a function of the total length L ges the laser bond lines are shown:

[0063] Type 1: Fv = 107.9 (101.8 ... 114.3) N

[0064] Type 2: Fv = 144.1 (139.6 ... 148.8) N

[0065] Type 3: Fv = 219.6 (206.6 ... 233.6) N

[0066] By fitting an affine function, it is easy to see that the samples without laser welding, i.e. for a laser bond line length of 0, already exhibit a non-zero failure shear force of approximately 45 N. This basic contribution to the shear strength is attributed to the adhesion forces across the optical interface area A cFurthermore, it can be seen that for each mm of laser bond line, the shear force resistance increases by approximately 2.8 N. For the two sample types 1 to 3, an empirical constant P of 2.8 N / mm is determined.

[0067] Since this is a linear relationship, it is sufficient for an empirical determination of the constant P to carry out this measurement on a single sample type for the material pairing to be investigated.

[0068] It is understood that the features mentioned above and those to be explained below can be used not only in the combination specified in each case, but also in other combinations or on their own, without departing from the scope of the present invention.

[0069] Preferred embodiments and embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description, wherein the same reference numerals refer to the same or similar or functionally identical components or elements.

[0070] In schematic form

[0071] Figure 1 is a perspective view of two substrates connected by a laser bonding line,

[0072] Figure 2 is a plan view of a hermetic enclosure,

[0073] Figure 3 is a sectional view of the hermetic enclosure from the side,

[0074] Figure 4 shows a section through laser bonding lines along the welding direction,

[0075] Figure 5 shows a section through laser bonding lines perpendicular to the welding direction, Figure 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 against applied shear force,

[0076] Figure 7 shows a diagram of the characteristic failure force of the laser-welded specimens against the total length of the laser bond lines,

[0077] Figure 8 is a diagram of the determined empirical constant for the bond strength per length,

[0078] Figure 9 is a micrograph of a cross-section of two substrates joined by laser bonding lines,

[0079] Figure 10 shows three examples of fracture patterns for the failure of the weld joint when the failure shear force is exceeded, and

[0080] Figure 11 shows three examples of fracture patterns in which one or both substrates have fractured due to force without prior failure of the weld joint.

[0081] Figure 1 shows a perspective view of two substrates 3, 4 connected by a laser bonding line 2. A first substrate 3 is placed on a second substrate 4 so that the two substrates 3, 4 are in direct contact. The area where the two substrates 3, 4 touch is referred to as the contact area Ai.

[0082] If the surfaces of the two substrates 3, 4 are smooth, the superimposed surfaces have a distance from each other that can no longer be determined optically. This is usually the case with a distance of less than approximately 250 nm. At such small distances, adhesion forces arise between the two substrates 3, 4 even upon application. These adhesion forces occur in an area referred to as the contact area Ac. The contact area Ac is smaller than the total contact area Ai.

[0083] To hermetically seal the two substrates 3, 4 in the area of ​​the contact surface Ac, laser welding is carried out by introducing a laser bonding line 2. Material is melted along the laser bonding line 2 using an ultrashort pulse laser and then cooled again, so that the two substrates 3, 4 are bonded to one another when they are very close to one another, as in the area of ​​the contact surface Ac. In a laser bonding surface Aw created by laser welding, the two substrates 3, 4 are bonded to one another in a materially bonded manner, so that there is no longer any gap between the substrates 3, 4. The width of the laser bonding lines 2 is thin at approximately 20 pm to 75 pm, so that in the full-surface connection of the substrates 3, 4 shown as an example in Figure 1, only a very small part of the contact surface Ac or the contact surface A is additionally welded by laser treatment.

[0084] Surprisingly, it was found that the contribution to the shear force resistance of the connection between the two substrates 3, 4 by the laser bonding area Aw, despite the very small area compared to the total contact area A and the touch contact area Ac in the example shown in Figure 1, is much greater than the contribution of the adhesion forces in the region of the touch contact area Ac. Accordingly, the laser bonding area Aw can be used not only to hermetically seal a gap between the two substrates, but also to increase the resistance of the connection to acting shear forces.

[0085] Figure 2 shows a plan view of an embodiment of a hermetic enclosure 1. The enclosure has a length a, a width b, and a height c (see Figure 3). A functional region 20 in the form of a hollow space or cavity 21 is formed in the enclosure 1, into which a functional element 22, such as a sensor or a transponder, is hermetically encapsulated.

[0086] Typical dimensions of a housing are a = 5 mm, b = 5 mm, c = 2.5 mm, but also larger and flatter (e.g. a = 10 mm, 10 = 5 mm, c = 0.9 mm) or more compact ( a = 3 mm, b = 4 mm, c = 2 mm) are possible.

[0087] To form the housing 1, a cover substrate 14 is placed on a base substrate 10 (see Figure 3) and touches the base substrate 10 at the contact surface A. The contact surface Ai corresponds to an end face 16 of the base substrate 10, see Figure 3.

[0088] The cover substrate 14 is hermetically sealed to the base substrate 10 via a plurality of laser bonding lines 2. The laser bonding lines 2 run parallel to the side walls of the cavity 21, wherein in the example shown, the cavity 21 is rectangular and accordingly has four side walls. In the example, two bonding lines 2 run parallel to each of the side walls of the cavity, wherein the thickness of the side walls corresponds to a contact surface width B. Only those bonding lines 2 are considered adjacent to one another that are not separated from one another by the functional region 20 or the cavity 21. In this example, the laser bonding lines 2 form two closed rectangular paths around the functional region 20.

[0089] The enclosure shown in Figure 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 top of each other and bonded together. This resulted in a wafer stack comprising a plurality of interconnected enclosures 1. The laser bonding lines 2 were each formed across the entire width or length of the wafer stack. The individual enclosure 1, as shown in Figure 2, was obtained by singulating the plurality of enclosures.

[0090] Figure 3 shows a sectional view of the hermetic enclosure 1 of Figure 2 from the side along the section line marked AA in Figure 2.

[0091] The sectional view of Figure 3 shows that the base substrate 10 in this exemplary embodiment consists of a bottom substrate 11 and an intermediate substrate 12. The bottom substrate 11 and the intermediate substrate 12 were joined hermetically via several laser bonding lines 2, analogous to the connection between the cover substrate 14 and the base substrate 10 or the intermediate substrate 11.

[0092] The side walls of the resulting cavity 21 are formed by the intermediate substrate 12, and the bottom of the cavity 21 is formed by the bottom substrate 11. In the example shown, the functional element 22 is arranged within the cavity 21 on the bottom substrate 11.

[0093] Figure 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 joined, with the individual pulses locally overlapping several times, so that a weld seam is created by heat accumulation above the focal points (32). The cross-section of the seam is pear-shaped and is referred to as the welding pear 30. The welding pear 30 represents the area of ​​the substrates 11, 12, 14 that was processed by the respective laser pulse in such a way that the material was heated above the glass transition temperature TG or the melting temperature and the adjacent substrates 11, 12, 14 can be bonded together. The scanning speed, in conjunction with the pulse repetition rate of the ultrashort pulse laser, is selected so that a continuous laser bonding area is created in the area of ​​the laser bonding line 2.The laser beam is focused such that a focal point 32 is placed at a distance T from the connecting plane between the two respective substrates 11, 12, 14. Starting from the focal point 32, the welding bulb 30 with a height HL is then formed by the energy transferred from the laser pulse to the respective substrate 11, 12, 14.

[0094] Figure 5 shows a section through laser bonding lines 2 perpendicular to the welding direction. In this sectional view, it can be seen that the respective laser bonding lines 2 have a width w relative to the connection plane between the respective substrates 11, 12, 14 to be connected, i.e. here once between the base substrate 11 and the intermediate substrate 12 and again between the intermediate substrate 12 and the cover substrate 14. Since the width of the welding heads 30 varies along the height HL of the welding heads 30, the width w of the laser bonding lines 2 can be adjusted accordingly by selecting the depth T of the focal point 32 in relation to the respective connection plane. A distance H between each two adjacent laser bonding lines 2, measured from center to center, is preferably selected such that the laser bonding lines 2 do not overlap. Accordingly, the distance H is greater than or equal to the width w.Furthermore, the aim is to make the housing 1 as compact as possible and, accordingly, to select the contact surface width B, which here corresponds to the width of the side wall of the cavity 21 (see Figure 3), as small as possible. Accordingly, a distance between two laser bonding lines 2 is preferably selected to be a maximum of five times the width w.

[0095] Figure 6 shows a double-logarithmic representation of the cumulative failure probability of laser-welded test specimens in the shear test for three different total lengths of the laser bond lines against the applied shear force in N. A first curve 101 shows the cumulative failure probability for 30 test specimens with a total laser bond line length of 20 mm, a second curve 102 shows the cumulative failure probability for 30 test specimens with a total laser bond line length of 40 mm and a third curve 103 shows the cumulative failure probability for 30 test specimens with a total laser bond line length of 60 mm.

[0096] Figure 7 shows a plot of the characteristic failure force of the laser-welded specimens versus the total length of the laser bond lines. A fitted affine function can 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 area of ​​contact A. c , compare Figure 1 , provided adhesion force.

[0097] Figure 8 shows a diagram of the determined empirical constant P for the bond strength per length for the three curves 101, 102, 103, compare Figure 6. It can be seen that, within the error tolerance, the obtained values ​​for the constant p are independent of the laser bond line length of the respective test specimens.

[0098] Figure 9 shows a micrograph of a cross-section of two substrates 10, 14 joined together by laser bonding lines 2, using the example of substrates 10, 14 made of borosilicate glass. The laser bonding lines 2 are clearly visible due to the refractive index changes that occur during heating and cooling.

[0099] Figure 10 shows three examples (a, b, and c) of fracture patterns for the failure of a welded joint between two substrates when the failure shear force was exceeded. It is clearly visible that the two substrates separated from each other essentially without further damage along the weld seams or laser bond lines. Figure 11 shows three examples (a, b, and c) of fracture patterns in which one or both substrates fractured due to the application of force without prior failure of the welded joint. In each case, it is clearly visible that the fracture lines do not run along the original surfaces of the substrates, but rather the respective substrates themselves were destroyed. Parts of the respective substrates chipped off.

[0100] Although the present invention has been described using preferred embodiments, it is not limited thereto but can be modified in many ways.

[0101] List of reference symbols

[0102] Ai contact area

[0103] Ac touch contact surface

[0104] Aw laser bonding area

[0105] 1 enclosure

[0106] 2 laser bonding lines

[0107] 3 first substrate

[0108] 4 second substrate

[0109] 10 Base substrate

[0110] 11 Soil substrate

[0111] 12 Intermediate substrate

[0112] 14 Covering substrate

[0113] 16 Frontal surface

[0114] 20 Functional area

[0115] 21 Cavity

[0116] 22 Functional element

[0117] 30 welding bulbs

[0118] 32 focus point

[0119] A Cutting line a Length of housing b Width of housing c Height of housing B Contact surface width

[0120] HL height laser bonding line

[0121] T Depth of laser bonding line w Width of laser bonding line

[0122] H Distance between two laser bonding lines

[0123] 101 first curve

[0124] 102 second curve

[0125] 103 third curve

[0126] KW cumulative failure probability

[0127] S shear force p empirical constant

[0128] Fv failure shear force

[0129] L Laser bond line length

Claims

Patent claims Method for designing a laser weld between a base substrate (10) and a cover substrate (14) of a housing (1), wherein the base substrate (10) has a functional area (20), and the cover substrate (14), which is in contact with the base substrate (10), covers the functional area (20), wherein the base substrate (10) and the cover substrate (14) are directly connected to one another in a hermetically sealed manner via at least one laser bonding line (2), so that the functional area (20) is hermetically enclosed in the interior of the housing (1) formed, characterized in that for the connection between the cover substrate (14) and the base substrate (10), a minimum shear force Fmin is specified, which the laser weld is intended to withstand, and that the sum of the lengths L gesof all laser bonding lines (2) is selected to be greater than a required minimum length Lmin of the length of all laser bonding lines (2), where Lmin is determined by dividing the specified minimum shear force Fmin by an empirically determined force per laser bonding line length P Lmin=Fmin / P, and that a contact surface width B, measured in the plane of an end face (16) of the base substrate (10) facing the cover substrate (14) as the shortest distance between the functional area (20) and the exterior of the housing (1 ), is selected such that a ratio J=A / A W formed from a contact surface A, at which the base substrate (10) and the cover substrate (14) can touch, and a laser bonding surface Aw swept over by the laser bonding lines (2) with a width w on the end face (16) of the base substrate (10) facing the cover substrate (14) in the range from 1 to 10. Method according to claim 1, characterized in that a number N of closed paths of laser bonding lines (2) with width w and a distance H between the centers of two adjacent laser bonding lines (2) of at least the width w are arranged around the functional area (20), wherein the number N is determined as the smallest number N for which the total length L gesof all laser bonding lines (2) formed from the number N multiplied by the length of a contour line which delimits the functional area (20) is greater than the minimum length Lmin. Method according to claim 1 or 2, characterized in that the distance H between the centers of two adjacent laser bonding lines (2) with the width w is selected in the range from 1 w to 5 w, preferably in the range from 1.01 w to 2.5 w and particularly preferably in the range from 1.05 w to 2 w. Method according to one of claims 1 to 3, characterized in that the contact surface width B is selected in the range from 100 to 1000 pm. Method according to one of claims 1 to 4, characterized in that the width w of the laser bonding lines (2) is selected in the range from 20 pm to 75 pm, preferably in the range from 30 pm to 60 pm.Method according to one of claims 1 to 5, characterized in that the force per laser bond line length P is determined empirically by producing a plurality of test specimens in which a first substrate (3) made of a cover substrate material is connected to a second substrate (4) made of a base substrate material with laser bond lines (2), wherein the total length Lges of the laser bond lines is selected to be the same for the test specimens, the shear force resistance of the test specimens is determined by applying an increasing shear force to the connection between the first (3) and second substrate (4), the force is determined at. the connection is destroyed and a failure probability distribution is evaluated. Method according to one of claims 1 to 6, wherein the minimum shear force Fmin is specified such that when producing a plurality of test specimens in which a first substrate (3) made of a cover substrate material is joined to a second substrate (4) made of a base substrate material with laser bonding lines (2) in such a way that these are designed for a minimum shear force Fmin and when this minimum shear force Fmin is applied, more than 50%, preferably more than 75%, particularly preferably more than 90%, most preferably 95% of the test specimens do not break along the contact surface due to failure of the welded connection, but break at other points, in particular at an edge of one or more of the substrates (3, 4).Hermetically sealed housing (1) comprising a base substrate (10) which has a functional region (20), and a cover substrate (14) which is in contact with the base substrate (10) and covers the functional region (20), wherein the base substrate (10) and the cover substrate (14) are directly hermetically sealed to one another via at least one laser bonding line (2), and wherein the functional region (20) is hermetically enclosed in the interior of the formed housing (1), characterized in that a ratio J = A / A. Wformed from a contact surface A, at which the base substrate (10) and the cover substrate (14) can touch, and a laser bonding surface Aw, swept over by the at least one laser bonding line (2) with a width w, on the surface of the interface between the base substrate (10) and the cover substrate (14) in the range from 1 to 10, wherein a contact surface width B, measured in the plane of the end face (16) of the base substrate (10) facing the cover substrate (14) is the shortest Distance between the functional area (20) and the outer part of the housing (1) is in the range from 100 μm to 1000 μm. Housing (1) according to claim 8, characterized in that the area Aw swept over by the at least one laser bonding line (2) is selected such that the connection between the cover substrate (10) and the base substrate (14) has a failure shear force in the range from 10 N to 1000 N, preferably 50 N to 500 N, particularly preferably in the range from 100 N to 400 N. Housing (1) according to claim 8 or 9, characterized in that the total length L gesthe laser bonding lines (2) is selected according to a design method according to one of claims 1 to 6. Housing (1) according to one of claims 8 to 10, characterized in that more than one laser bonding line (2) is present, wherein the laser bonding lines (2) have a width w and a distance H between the centers of two adjacent laser bonding lines (2) is selected in the range from 1 w to 5 w, preferably in the range from 1.01 w to 2.5 w and particularly preferably in the range from 1.05 w to 2 w. Housing (1) according to one of claims 8 to 11, characterized in that the cover substrate (14) is designed as a transparent thin-film substrate, wherein the cover substrate (14) has a thickness of less than 200 pm, preferably less than 170 pm, particularly preferably less than 125 pm and preferably has a thickness greater than 10 pm, particularly preferably greater than 20 pm.Housing (1) according to one of claims 8 to 12, characterized in that the cover substrate (14) and the base substrate (10) directly adjoin one another at the contact surface Ai, so that the connection in the. the laser bonding surface Aw swept over by at least one laser bonding line (2) is free of foreign materials, in particular free of connecting materials such as adhesive or glass frit or an absorbing layer. Housing (1) according to one of claims 8 to 13, characterized in that the base substrate (10) has a flat bottom substrate (11) which forms the bottom surface of a functional region (20) designed as a cavity (21), and has an intermediate substrate (12) which forms the side walls of the cavity (21) with an end face facing the cover substrate (14), and in that the bottom substrate (11) and the intermediate substrate (12) are hermetically sealed to one another via at least one laser bonding line (2), or in that a functional region (20) in the form of a depression with a bottom surface and side walls is formed in the base substrate (10), which functional region forms a cavity (21) together with the cover substrate (14) as a cover surface.Housing (1) according to one of claims 8 to 14, wherein the cover substrate (14) and / or the base substrate (10) consists of glass, glass ceramic, silicon, sapphire, or a combination of the aforementioned materials. Housing (1) according to one of claims 8 to 15, characterized in that the width w of the laser bonding lines (2) is in the range from 20 pm to 75 pm, preferably 30 pm to 60 pm, and / or that the width w of all laser bonding lines (2) over the entire length L. ges the laser bonding lines (2) vary by a maximum of 30%, preferably a maximum of 20%, particularly preferably a maximum of 10%. A sensor unit and / or medical implant comprising a housing (1) according to at least one of claims 8 to 16 or obtained after design by a method according to one of claims 1 to 7.

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