Method for permanent bonding of wafers

Abstract

A method for bonding of a first contact surface of a first substrate to a second contact surface of a second substrate according to the following steps: forming a reservoir in a surface layer on the first contact surface, at least partially filling the reservoir with a first educt or a first group of educts, contacting the first contact surface with the second contact surface for formation of a prebond connection, and forming a permanent bond between the first and second contact surface, at least partially strengthened by the reaction of the first educt with a second educt contained in a reaction layer of the second substrate.

Claims

1. Method for bonding of a first contact surface of a first substrate to a second contact surface of a second substrate, said method comprising: forming a reservoir in a surface layer of the first contact surface; at least partially filling of the reservoir with one or more first educts; forming a prebond connection by contacting the first contact surface with the second contact surface; and forming a permanent bond between the first and second contact surface, said permanent bond at least partially strengthened by reaction of the first educt with a second educt contained in a reaction layer of the second substrate.

2. Method as claimed in claim 1, wherein the method further comprises plasma activation to form the reservoir, wherein reduced species of the ions present in the plasma process are located in the reservoir.

3. Method as claimed in claim 2, wherein the reduced species are selected from the group consisting of: O.sub.2, N.sub.2, H.sub.2, and Ar-ions.

4. Method as claimed in claim 1, wherein between the second contact surface of the second substrate and the reaction layer of the second substrate there is a growth layer comprised of native silicon dioxide.

5. Method as claimed in claim 4, wherein the growth layer has an average thickness A between 1 angstrom and 10 nm before the formation of the permanent bond.

6. Method as claimed in claim 1, wherein formation and/or strengthening of the permanent bond takes place by diffusion of the first educt into the reaction layer of the second substrate.

7. Method as claimed in claim 1, wherein the formation of the permanent bond takes place at a temperature between room temperature and 200 Celsius.

8. Method as claimed in claim 1, wherein the permanent bond has a bond strength of greater than 1.5 J/m.sup.2.

9. Method as claimed in claim 1, wherein during the reaction of the first educt with the second educt a reaction product with a greater molar volume than the molar volume of the second educt is formed in the reaction layer of the second substrate.

10. Method as claimed in claim 1, wherein the surface layer adjoining the first contact surface of the first substrate is comprised of an amorphous material and the reaction layer of the second substrate is comprised of an oxidizable material.

11. Method as claimed in claim 1, wherein the reservoir is formed in a vacuum.

12. Method as claimed in claim 1, wherein the step of at least partially filling the reservoir includes one or more of the following steps: exposing the first contact surface to a fluid comprising deionized H.sub.2O and/or H.sub.2O.sub.2, and exposing the first contact surface to N.sub.2 gas and/or O.sub.2 gas and/or Ar gas and/or forming gas comprising 95% Ar and 5% H.sub.2.

13. Method as claimed in claim 1, wherein the reservoir is formed with an average thickness between 0.1 nm and 25 nm.

14. Method as claimed in claim 1, wherein immediately before formation of the permanent bond the average distance between (i) the reservoir in the surface layer adjoining the first contact surface and (ii) the reaction layer of the second substrate is between 0.1 nm and 15 nm.

15. Method as claimed in claim 1, wherein the permanent bond has a bond strength which comprises 2 times a bond strength of the prebond connection.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a first step of the method as claimed in the invention immediately after the first substrate makes contact with the second substrate,

(2) FIGS. 2a and 2b show other steps of the method as claimed in the invention for forming a higher bond strength,

(3) FIG. 3 shows another step of the method as claimed in the invention which follows the steps according to FIG. 1, FIG. 2a and FIG. 2b, with substrate contact surfaces which are in contact,

(4) FIG. 4 shows a step as claimed in the invention for formation of an irreversible/permanent bond between the substrates,

(5) FIG. 5 shows an enlargement of the chemical/physical processes which proceed on the two contact surfaces during the steps according to FIG. 3 and FIG. 4,

(6) FIG. 6 shows another enlargement of the chemical/physical processes which proceed on the interface between the two contact surfaces during the steps according to FIG. 3 and FIG. 4 and

(7) FIG. 7 shows a diagram of the production of the reservoir as claimed in the invention.

(8) The same components/features and components/features with the same action are identified with the same reference numbers in the figures.

DETAILED DESCRIPTION OF THE INVENTION

(9) In the situation which is shown in FIG. 1 shows only a section of the chemical reactions which proceed during or immediately after the prebond step between a first contact surface 3 of a first substrate 1 and a second contact surface 4 of the second substrate 2. The surfaces are terminated with polar OH groups and are accordingly hydrophilic. The first substrate 1 and the second substrate 2 are held by the force of attraction of the water bridges between the OH groups present on the surface and the H.sub.2O molecules and between the H.sub.2O molecules alone. The hydrophilicity of at least the first contact surface 3 is increased by plasma treatment of the first contact surface 3 in a preceding step.

(10) A reservoir 5 in a surface layer 6 consisting of thermal silicon dioxide has been formed as claimed in the invention by plasma treatment. Plasma treatment with O.sub.2 ions with ion energies in the range between 0 and 2000 eV yields an average thickness R of the reservoir 5 of roughly 15 nm, the ions forming channels or pores in the surface layer 6.

(11) Likewise the reservoir 5 is filled with H.sub.2O as the first educt prior to the step shown in FIG. 1 and after plasma treatment. Reduced species of the ions present in the plasma process can also be located in the reservoir, especially O.sub.2, N.sub.2, H.sub.2, Ar.

(12) The contact surfaces 3, 4 therefore still have a relatively wide gap, especially dictated by the water present between the contact surfaces 3, 4. Accordingly the existing bond strength is relatively small and is roughly between 100 mJ/cm.sup.2 and 300 mJ/cm.sup.2, especially more than 200 mJ/cm.sup.2. In this connection the prior plasma activation plays a decisive part, especially due to the increased hydrophilicity of the plasma-activated first contact surface 3 and a smoothing effect which is caused by the plasma activation.

(13) The process which is shown in FIG. 1 and which is called prebond can preferably proceed at ambient temperature or a maximum 50 Celsius. FIGS. 2a and 2b show a hydrophilic bond, the SOSi bridge arising with splitting of water by OH terminated surfaces. The processes in FIGS. 2a and 2b last roughly 300 h at room temperature. At 50 C. roughly 60 h. The state in FIG. 2b occurs at the indicated temperatures without producing the reservoir.

(14) Between the contact surfaces 3, 4, H.sub.2O molecules are formed and provide at least partially for further filling in the reservoir 5 to the extent there is still free space. The other H.sub.2O molecules are removed. In the step according to FIG. 1 roughly 3 to 5 individual layers of OH groups or H.sub.2O are present and 1 to 3 monolayers of H.sub.2O are removed or accommodated in the reservoir 5 from the step according to FIG. 1 to the step according to FIG. 2a.

(15) In the step shown in FIG. 2a the hydrogen bridge bonds are now formed directly between siloxane groups, as a result of which a greater bond force arises. This draws the contact surfaces 3, 4 more strongly to one another and reduces the distance between the contact surfaces 3, 4. Accordingly there are only 1 to 2 individual layers of OH groups between the contact surfaces 1, 2.

(16) In the step shown in FIG. 2b, in turn with separation of H.sub.2O molecules according to the reaction which has been inserted below, covalent bonds in the form of silanol groups are formed between the contact surfaces 3, 4 which lead to a much stronger bond force and require less space so that the distance between the contact surfaces 3, 4 is further reduced until finally the minimum distance shown in FIG. 3 is reached based on the contact surfaces 3, 4 meeting one another:
SiOH+HOSiSiOSi+H.sub.2O

(17) Up to stage 3, especially due to the formation of the reservoir 5, it is not necessary to unduly increase the temperature, rather even at room temperature to allow it to proceed. In this way an especially careful progression of the process steps according to FIG. 1 to FIG. 3 is possible.

(18) In the method step shown in FIG. 4, the temperature is preferably increased to a maximum 500 Celsius, more preferably to a maximum 300 C., even more preferably to a maximum 200 C., most preferably to a maximum 100 C., most preferably of all not above room temperature in order to form an irreversible or permanent bond between the first and the second contact surface. These temperatures which are relatively low, in contrast to the prior art, are only possible because the reservoir 5 encompasses the first educt for the reaction shown in FIGS. 5 and 6:
Si+2H.sub.2O.fwdarw.SiO.sub.2+2H.sub.2

(19) At the aforementioned slightly increased temperatures H.sub.2O molecules diffuse as the first educt from the reservoir 5 to the reaction layer 7. This diffusion can take place either via a direct contact of the surface layer 6 and growth layer 8 which are formed as oxide layers, or via a gap 9 or from a gap which is present between the oxide layers. There, silicon oxide, therefore a chemical compound with a greater molar volume than pure silicon is formed as a reaction product 10 of the aforementioned reaction from the reaction layer 7. The silicon dioxide grows on the interface of the reaction layer 7 with the growth layer 8 and thus deforms the layer of the growth layer 8 formed as native oxide in the direction of the gaps 9. Here H.sub.2O molecules from the reservoir are also required.

(20) Due to the existence of the gaps which are in the nanometer range, there is the possibility of bulging of the native oxide layer 8, as a result of which stresses on the contact surfaces 3, 4 can be reduced. In this way the distance between the contact surfaces 3, 4 is reduced, as a result of which the active contact surface and thus the bond strength are further increased. The weld connection which arises in this way, which closes all pores, and which forms over the entire wafer, in contrast to the products in the prior art which are partially not welded, fundamentally contributes to increasing the bond force. The type of bond between the two amorphous silicon oxide surfaces which are welded to one another is a mixed form of covalent and ionic portion.

(21) The aforementioned reaction of the first educt (H.sub.2O) with the second educt (Si) takes place in the reaction layer especially quickly or at temperatures as low as possible to the extent an average distance B between the first contact surface 3 and the reaction layer 7 is as small as possible.

(22) Therefore the pretreatment of the first substrate 1 and the selection of the second substrate 2 which consists of a reaction layer 7 of silicon and a native oxide layer as thin as possible as a growth layer 8 is decisive. A native oxide layer as thin as possible is provided as claimed in the invention for two reasons. The growth layer 8 is very thin so that it can bulge due to the newly formed reaction product 10 on the reaction layer 7 toward the surface layer 6 of the opposite substrate 1, which surface layer is made as an oxide layer, predominantly in regions of the nanogaps 9. Furthermore, diffusion paths as short as possible are desired in order to achieve the desired effect as quickly as possible and at a temperature as low as possible. The first substrate 1 likewise consists of a silicon layer and an oxide layer produced on it as a surface layer 6 in which a reservoir 5 is formed at least partially or completely.

(23) The reservoir 5 as claimed in the invention is filled at least accordingly with the amount of the first educt which is necessary to close the nanogaps 9 so that an optimum growth of the growth layer 8 can take place to close the nanogaps 9 in a time as short as possible and/or at a temperature as low as possible.

REFERENCE NUMBER LIST

(24) 1 first substrate 2 second substrate 3 first contact surface 4 second contact surface 5 reservoir 6 surface layer 7 reaction layer 8 growth layer 9 nanogaps 10 reaction product 11 first profile 12 second profile 13 sum curve A average thickness B average distance R average thickness