Method for joining substrates

Abstract

The invention relates to a method of joining substrates. It is the object of the invention in this respect to join substrates of substrate materials together without having to exert an increased effort for a coating with additional coating processes to be carried out and to be able to achieve a good quality of the join connection in so doing. In the method in accordance with the invention a pretreatment of at least one join surface of a substrate to be joined is carried out in low pressure oxygen plasma prior to the actual joining. On the joining, a contact force acts on the substrates to be joined in the range 2 kPa to 5 MPa and in this process a heat treatment is carried out at an elevated temperature of at least 100° C. and at under pressure conditions of a maximum of 10 mbar, preferably <10.sup.−3 mbar.

Claims

1. A method of joining substrates comprising: applying a pressing force in the range of 2 kPa to 5 MPa on substrates to be joined; heat treating the substrates at an elevated temperature of at least 100° C. and under pressure conditions of a maximum of 10 mbar; pretreating at least one joining surface of at least a first of the substrates to be joined prior to the applying of the pressure and the heat treating; and smoothing at least a portion of the joining surface of the first of the substrates prior to the applying of the pressure and the heat treating such that, after the smoothing, at least the portion of the joining surface has a planarity or a deviation with a specified peak to valley value, wherein the specified peak to valley value is less than 1 μm per 100 mm of diameter of the portion of the joining surface if a thickness of the first of the substrates is greater than 5 mm, and wherein the specified peak to valley value is less than 10 μm per 100 mm of diameter of the portion of the joining surface if the thickness of the first of the substrates is less than 5 mm, and wherein the pressing force and the heat treating joins the substrates.

2. The method in accordance with claim 1, wherein the substrates comprise a glass that is predominantly at least one of SiO.sub.2, a glass ceramic, a crystalline material with a high oxide portion, aluminum oxide, SiC, SiSiC, ZnSe, Si, Ge, and GaAs.

3. The method in accordance with claim 1, further comprising: cleaning the joining surface by alternate rinsing with aqueous NH.sub.4OH solution and H.sub.2O.sub.2 solution, each of the NH.sub.4OH solution and the H.sub.2O.sub.2 solution having a respective concentration in the range of 2 mass % to 4 mass %, subsequent to the cleaning, rinsing the joining surface in distilled or deionized water while simultaneously exposing the substrates to be joined to sound waves in the megahertz range, and subsequent to the rinsing, drying the substrates.

4. The method in accordance with claim 1, wherein the pretreating comprises applying a low pressure oxygen plasma to at least the portion of the joining surface.

5. The method in accordance with claim 4, wherein the low pressure oxygen plasma has a power density in the range 0.5 W/cm.sup.2 to 10 W/cm.sup.2.

6. The method in accordance with claim 4, further comprising, subsequent to the applying of the low pressure oxygen plasma, rinsing the joining surface in distilled or deionized water while simultaneously exposing the substrates to sound waves in the megahertz range and subsequent to the rinsing, drying the joining surface.

7. The method in accordance with claim 1, further comprising forming a surface layer of a metal, a semiconductor, or a dielectric material on at least the portion of the joining surface prior to the applying of the pressure and the heat treating.

8. The method in accordance with claim 7, further comprising forming an alternating multilayer system having individual layers of materials having a different index of refraction.

9. The method in accordance with claim 7, further comprising forming at least one layer of a non-stoichiometric oxide on at least one of the substrates.

10. The method in accordance with claim 1, wherein the heat treating is carried out under pressure conditions less than 10.sup.−3 mbar.

11. The method in accordance with claim 2, wherein the glass comprises at least one of yttrium vanadate, yttrium aluminum garnet, and sapphire.

12. The method in accordance with claim 1, wherein, after the smoothing, the specified peak to valley value is better than 40 μm per 100 mm of diameter of the portion of the joining surface if the thickness of the first of the substrates is less than 1 mm.

13. The method in accordance with claim 1, wherein, after the smoothing, at least the portion of the joining surface has a surface roughness of less than 3 nm root mean square.

14. The method in accordance with claim 1, wherein, after the smoothing, at least the portion of the joining surface has a surface roughness of less than 1 nm root mean square.

15. A method of joining substrates comprising: providing or receiving a plurality of substrates to be joined, the substrates comprising a glass comprising at least one of SiO.sub.2, a glass ceramic, a crystalline material with a high oxide portion, an aluminum oxide, SiC, SiSiC, ZnSe, Si, Ge, and GaAs; smoothing at least a portion of a joining surface of a first of the plurality of the substrates such that, after the smoothing, at least the portion of the joining surface has a surface roughness less than 1 nm root mean square, and, after the smoothing, at least the portion of the joining surface has a planarity or a deviation with a specified peak to valley value, wherein the specified peak to valley value is less than 1 μm per 100 mm of diameter of the portion of the joining surface if a thickness of the first of the plurality of the substrates is greater than 5 mm, and the specified peak to valley value is less than 10 pm of per 100 mm of diameter of the portion of the joining surface if the thickness of the first of the plurality of the substrates is less than 5 mm; forming a surface layer of metal, a semiconductor, or a dielectric material on at least the portion of the joining surface; cleaning the joining surface by alternate rinsing with aqueous NH.sub.4OH solution and H.sub.2O.sub.2 solution, each of the NH.sub.4OH solution and the H.sub.2O.sub.2 solution having a respective concentration in the range of 2 mass % to 4 mass % subsequent to the cleaning, rinsing the joining surface with distilled or deionized water while simultaneously exposing the plurality of the substrates to sound waves in the megahertz range; subsequent to the rinsing, drying the joining surface; subsequent to the drying, plasma-treating at least the portion of the joining surface with low pressure oxygen plasma having a power density in the range 0.5 W/cm.sup.2 to 10 W/cm.sup.2; subsequent to the plasma-treating, applying a pressing force in the range of 2 kPa to 5 MPa on the plurality of the substrates; and heat treating the plurality of the substrates at an elevated temperature of at least 100° C. and under pressure conditions of less than 10.sup.−3 mbar while applying the pressing force on the plurality of the substrates, wherein the pressing force and the heat treating joining the plurality of substrates.

16. The method in accordance with claim 4, wherein the low pressure oxygen plasma is applied over a time period of at least 60 seconds.

17. The method in accordance with claim 16, wherein the applying of the low pressure oxygen plasma comprises multiple successive plasma action over the time period.

18. The method in accordance with claim 16, wherein the applying of the low pressure oxygen plasma comprises a continuous process over the time period.

19. The method in accordance with claim 2, wherein the crystalline material with the high oxide portion comprises at least one of a vanadate, a garnet, KTiOPO.sub.4, LiB.sub.3O.sub.5, or LiNbO.sub.3.

20. The method in accordance with claim 15, wherein the crystalline material with the high oxide portion comprises at least one of a vanadate, a garnet, KTiOPO.sub.4, LiB.sub.3O.sub.5, or LiNbO.sub.3.

Description

(1) There are Shown:

(2) FIG. 1 an example of substrates prior to the joining; and

(3) FIG. 2 a further example of substrates prior to the joining.

(4) FIG. 1 shows two substrates prior to the joining (in cross-section, not to scale). The substrates are two solid glass plates 1, 2, wherein the plate 2 is provided with a coating 3 and the coating 3 was superficially oxidized by O.sub.2 plasma treatment and hereby has a modified composition with an oxide portion in the layer 4 at the surface. An application is possible as a dichroitic filter, laser reflector, holding element (chuck), mechanical platform, among many others.

(5) The two glass plates 1 and 2 can be made of fused silica, ULE or BK7 glass. A coating 3 of a metal (e.g. nickel, titanium, chromium, silver) or a metal alloy in the form of a thin film having a layer thickness in the range 10 nm to 100 nm is formed on the surface of the glass plate 2.

(6) A metal oxide (NiO, TiO.sub.2, Cr.sub.2O.sub.3, Ag.sub.2O) corresponding to the metal can be formed directly on the surface 4, i.e. exactly at the surface at which it is required for the joining, by an oxygen treatment in low pressure plasma. On joining, a chemical bond with the surface of the glass plate 1 is achieved by the formed oxide.

(7) At least one oxide layer can also be formed instead of the coating 3 of metal. This/these can be layers of HfO.sub.2, indium tin oxide (ITO), MgO, Nb.sub.2O.sub.5, SiO, SiO.sub.2, TiO.sub.2, Ta.sub.2O.sub.5or ZrO.sub.2. If a plurality of layers are formed, they can form an optical multilayer system in which alternating layers are formed alternately from an oxide with higher and lower indices of refraction. So-called interference filters for optical applications can thus be manufactured in this manner. The layer thicknesses of the individual layers can be coordinated with specific wavelengths while taking account of the respective indices of refraction as λ/4 layers. SiO.sub.2, for instance, has an index of refraction of approx. 1.5 and TiO.sub.2 has an index of refraction of approx. 2.4 for wavelengths of electromagnetic radiation around 500 nm, that is in the middle spectral range of visible light.

(8) The forming of layers can be achieved using PVD and CVD processes known per se.

(9) It can be advantageous for optical or mechanical reasons to configure a multilayer system having one or more layers in which the respective oxide is not stoichiometric and is preferably hypostoichiometric. With TiO.sub.x and SiO.sub.x x can thus be <2. An oxygen treatment with low pressure plasma can effect an oxidation to higher valency in which the thermodynamically more stable stoichiometric oxides TiO.sub.2 or SiO.sub.2 are obtained directly at the surface 4. The joining conditions and the strength of the join can thereby be improved.

(10) Non-stoichiometric oxide layers with an oxygen deficit can result in application-specific optical advantages, e.g. in an increased absorption in the wavelength range of visible light, but good transmission at wavelengths of electromagnetic radiation from the spectral range of IR light. This applies, for example, to SiO.sub.x, where 1<x<2. In addition, the layer properties for the joining can be improved since these layers are more porous and/or their Young's-modulus (E) is smaller than the Young's modulus of the substrate material. Such a layer can be “more adaptable”.

(11) If different materials are used for the substrates to be joined together, their thermal coefficients of expansion should be taken into account. With different thermal coefficients of expansion, mechanical stresses can occur in the joining zone during the thermal treatment and the pressing if this is not sufficiently taken into account. A non-permanent connection of substrates could thereby be achieved. For this purpose, however, the dimensioning of the joining surfaces, the Young's modulus, the geometry of the substrates, the bending stiffness and the maximum temperature in the heat treatment also have an influence so that it should be monitored for respective cases.

(12) In trials with circular disks of terbium gallium garnet (Tb.sub.3Ga.sub.5O.sub.12) which had a diameter of 10 mm with a height/thickness of 2 mm, it was shown that they could be joined without problem to circular disks of sapphire having an outer diameter of 12 mm with a height/thickness of 2 mm at a maximum temperature of 200° C. The thermal coefficient of expansion of therbium gallium garnet (cubic crystal system, expansions the same in all directions) lies at 9*10.sup.−6K.sup.−1 and that of sapphire in the plane of the surfaces to be joined together (perpendicular to the optical axis of the substrates connected to one another) at 6*10.sup.−6K.sup.−1. It follows on from this that at correspondingly low temperatures which are used for the joining substrates of different materials can also be joined and the connection is permanently firm in so doing.

(13) FIG. 2 likewise shows two substrates prior to the joining (exploded representation, not true to scale). The substrates in this example are two solid glass prisms 1, 2, wherein both prisms 1 and 2 can be provided with different layers 5, 6 (or can also be not coated) and are modified microscopically by

(14) a low pressure O.sub.2 plasma treatment or also another plasma treatment (N.sub.2 plasma, AR plasma, . . . ) on their surfaces. A use as a beam splitter is possible here.

(15) The prism shape selected in this example for the substrates represents a special embodiment which can in particular be tailored to beam splitter cubes. In this respect, optical multilayer systems are frequently applied, such as have already been mentioned above, for example, to facilitate an optical filter effect.

(16) However, other geometries such as circular disks, hemispheres or substrates with concavely or convexly curved surfaces can also be connected to one another instead of the prisms 1 and 2. With substrates of SiC, the joining can be achieved when the surfaces to be connected to one another satisfy the named geometrical surface conditions and a sufficient plasma treatment has been carried out prior to the joining with oxygen as the plasma gas.

(17) The formation of the volatile/gaseous carbon oxides (CO and CO.sub.2) as well as of the non-volatile/solid silicon oxides (SiO and SiO.sub.2) can be achieved by the reaction of the oxygen plasma with the SiC directly at the surfaces. In throughflow operation or with a multiple treatment with the plasma, the carbon can largely be removed from the surface regions and can be replaced by a thin silicon oxide layer. The hydrophilic joining process can advantageously be carried out using formed silicon oxide layers on the surfaces to be joined.

(18) In an analog form, substrates from other materials such as SiC with silicon or silicon with Pyrex glass (e.g. type 7740 from Corning) can thus be connected to one another by joining.