# Patent

## Title
Layered bonded structures formed from reactive bonding of zinc metal and zinc peroxide

## Abstract

A system, method, and apparatus for layered bonded structures formed from reactive bonding between zinc metal and zinc peroxide are disclosed herein. In particular, the present disclosure teaches a layered bonded structure wherein two structures are bonded together with a layer including zinc oxide. The zinc oxide is formed through a method that includes processing the two structures by contacting the structures under pressure and applying heat to the structures to promote a reaction with zinc peroxide and zinc metal on one or both of the two structures.

## Background
BACKGROUND
The invention is directed to bonding materials using reactive bonding of zinc peroxide and zinc metal to form zinc oxide.
SUMMARY
The present disclosure relates to a method, system, and apparatus for layered bonded structures formed from reactive bonding between zinc metal and zinc peroxide. In one or more embodiments, the present disclosure teaches a method of forming a bonded layered structure that includes providing a first structure and a second structure, forming layers with zinc metal and zinc peroxide on one or more of the first and second structures, processing the first and second structures to cause oxidation of the zinc metal, and forming another layer with zinc oxide between the first and second structures. In at least one embodiment, the zinc oxide is formed from oxidation of the zinc metal and deoxidation of zinc peroxide. In further embodiments, the oxidation results from oxygen provided by the zinc peroxide, as well as oxygen from ambient conditions.
In one or more embodiments, the present disclosure teaches a bonded layered structure that includes a first structure and a second structure and a layer comprising zinc oxide between the first and second structures, wherein the zinc oxide is formed from oxidation of zinc metal on at least one of the first and second structures. In at least one embodiment, the zinc oxide is formed from deoxidation of zinc peroxide on at least one of the first and second structures.
The features, functions, and advantages can be achieved independently in various embodiments of the present inventions or may be combined in yet other embodiments.



DRAWINGS
These and other features, aspects, and advantages of the present disclosure will become better understood with regard to the following description, appended claims, and accompanying drawings where:
FIG. 1 shows an illustration of a bonding process between two structures to form a layered bonded structure; and
FIG. 2 shows a method of forming a layered bonded structure.



DESCRIPTION
The methods and apparatus disclosed herein provide a system for bonding of layered structures in applications, particularly applications employing layered wafers, such as in semiconductors and other materials such as layered ceramics, layered metals, layered composites. Specifically, this system provides for bonding between layered structures by forming a zinc oxide layer that bonds the structures together. The zinc oxide results from a reaction occurring between zinc metal and zinc peroxide, wherein the zinc metal and zinc peroxide are provided in layers on the structures.
Certain processing techniques require bonding of layered materials with a high-transparency and highly conductive medium, such as zinc oxide, between the layers. In previous methods, bonding processes employed materials such as: 1) thin metallic interface layers; 2) thin polymer film; 3) transparent conducting oxides with direct oxide bonding mechanisms; 4) heavily-doped III-V interface layers; and 5) transparent carbon nanotube films. Methods employing these materials, however, presented various limitations. For example, the layers had to be highly polished or required high levels of energy for processing.
To help minimize such limitations in bonding layers together, the present method takes advantage of the low melting temperature of zinc to initiate oxygen bonding in zinc metal and convert the zinc metal into zinc oxide. The oxygen required for zinc oxide formation comes from zinc peroxide and, if needed, added oxygen gas. The use of zinc metal allows flexibility and higher tolerance in surface topography of the host layered materials prior to bonding, such that this approach presents a high-yield integration scheme.
In the following description, numerous details are set forth in order to provide a more thorough description of the system. It will be apparent, however, to one skilled in the art, that the disclosed system may be practiced without these specific details. In the other instances, well-known features have not been described in detail so as not to unnecessarily obscure the system.
The invention is a processing system and method to bond layered materials using transparent conductive zinc oxide (ZnO) produced through oxidation and reactive bonding of zinc metal (Zn) and zinc peroxide (ZnO2), resulting in a bonded structure formed from the layered materials and the zinc oxide. FIG. 1 shows a schematic illustration of the proposed hybrid zinc oxide bonding approach between a first structure 10 and a second structure 20. In the illustrated embodiment, the first and second structures 10, 20 provided are first and second wafers 10, 20. In other embodiments, the first and second structures may include one or both of a particular device or a handle in a layered unit. In further embodiments, one or both of the structures may be photovoltaic and/or include a solar cell.
The first and second wafers 10, 20 may include materials such as silicon, crystalline silicate, gallium arsenide, indium phosphide, gallium phosphide, aluminum gallium arsenide, and indium gallium arsenide. Because a layer of zinc peroxide is deposited on the first and second wafers 10, 20, as discussed in further detail below, the compatibility of the materials forming the first and second wafers 10, 20 is less impactful, because these materials may not necessarily contact each other during the bonding process. As such, there is greater flexibility to include a wider variety of materials in the first and second wafers 10, 20.
To further describe the bonding process, material containing zinc peroxide is deposited on the first and second wafers 10, 20, forming a zinc peroxide layer 30, as shown in the illustrated embodiment. The zinc peroxide layer 30 may be transparent and/or conductive. In an alternative embodiment, the zinc peroxide layer 30 is deposited on only one of the first and second wafers 10, 20. The zinc peroxide layer 30 may be deposited under atmospheric pressure and thus does not necessarily require vacuum systems or other pressure-altering systems, as with other wafer-bonding applications. Also, the zinc peroxide layer 30 may be deposited using a plasma-enhanced tool or other tool commonly used in the field.
Following the application of the zinc peroxide layer 30, a zinc metal layer 40 is then formed on one or both of the first and second wafers 10, 20. In the illustrated embodiment, the zinc metal layer 40 is deposited over the zinc peroxide layer 30 on both of the first and second wafers 10, 20. In an alternative embodiment, the zinc metal layer 40 may be deposited over one of the first and second wafers 10, 20 that does not have a zinc peroxide layer 30 deposited thereon. The zinc metal layer 40 may have a thickness in the range of 10-100 nanometers (nm) or thicker. As with the zinc peroxide layer 30, the zinc metal layer 40 may be deposited under atmospheric pressure using a plasma-enhanced tool or other tool commonly used in the field.
After the zinc metal layer 40 is deposited on the first and/or second wafers 10, 20, the bonding process of the zinc metal with oxygen, i.e., oxidation of the zinc metal, is initiated. To proceed, pressure is applied to press the first and second wafers 10, 20 together. Consequently, the zinc peroxide and zinc metal layers 30, 40 are pressed together. The amount of pressure applied may be in the range of about 5 to 80 psi and can be as high as the host materials 10, 20 can withstand. In addition to applying pressure to the first and second wafers 10, 20, the first and second wafers 10, 20 are exposed to a temperature in the range of about 100-400° C. The temperature should be at a level sufficient to result in softening of the zinc metal. This may be considered a low temperature range in view of other wafer production methods, although higher temperatures may be applied. This method, however, takes advantage of the low temperatures at which melting and bonding of zinc metal can occur, such that the bonding process may be conducted at lower temperatures than those associated with other materials. This ability to conduct the process at such lower temperature ranges reduces the thermal stress in the bonding material as well as energy associated with the process.
The application of heat and pressure may occur in a time range of about 30 minutes to 3 hours or however long is necessary to achieve the desired effect. As heat and pressure are applied to the first and second wafers 10, 20, additional oxygen gas may also be distributed around the first and second wafers 10, 20 to promote further oxidation of the zinc metal.
The application of the described heat and pressure results in the reactions shown below:

2 Zn+2 O→ZnO  (1)

2 ZnO2→2 ZnO+2 O  (2)

Zn+ZnO2→2 ZnO  (3)

In particular, the application of heat and pressure to the zinc metal layer 40 in the presence of oxygen causes the zinc metal to melt and further results in thermal oxidation of the zinc metal to form zinc oxide, as seen in equation (1). The oxidation rate may be enhanced by oxygen species production from a thermally-induced ZnO2-to-ZnO conversion reaction in the zinc peroxide layer 30, as seen in equation (2). A thermodynamic-based binary phase diagram (not shown) of zinc and oxygen also supports a ZnO2-to-ZnO conversion reaction at the described temperature. The net result from the reaction (1) and (2) is zinc oxide bonding, as shown in equation (3). The zinc oxide bonding process may be conducted until most or all of the zinc peroxide has undergone the reaction leading to zinc oxide. The amount of zinc peroxide remaining may be reduced or eliminated as preferred, depending on the application and function of the first and second wafers 10, 20. To verify the extent to which zinc oxide bonding has occurred, the amount of zinc oxide between the first and second wafers 10, 20 may be determined by detection processes known in the art, such as x-ray diffraction and secondary ion mass spectrometry. Tests may also be conducted for resistance and optical transparency of zinc oxide at specific wavelengths in comparison with known parameters for zinc oxide. The zinc oxide bonding process thus results in a layer of zinc oxide 50 that in turn bonds the first and second wafers 10, 20 together, forming a unitary layered bonded structure 60. Note that complete oxidation of Zn metal film may not be necessary in applications that do not require high optical transparency.
A summary view of the process is shown in FIG. 2. The process is first started (step 100), and the first structure 10 and second structure 20 are provided (step 110). The zinc peroxide layer 30 is formed on at least one of the first and second structures 10, 20 (step 120). A zinc metal layer 40 is then formed on at least of the first and second structures 10, 20 (step 130). The first and second structures 10, 20 are then contacted and processed under pressure and heat to cause oxidation of the zinc metal (step 140). A zinc oxide layer 50 is then formed between the first and second structures 10, 20 that bonds the first and second structures 10, 20 together (step 150). The process may then be ended (step 160).
Notably, this bonding approach for zinc oxide conversion relaxes the surface roughness requirement required by other types of bonding. Specifically, other types of bonding procedures, such as direct wafer bonding in which no additional material is layered on the wafers, require that the bonding surfaces are very smooth with minimal surface irregularities. This may require a significant amount of production time dedicated to treating the bonding surfaces to achieve the desired reduction of surface irregularities through chemical-mechanical polishing and/or other surface treatment steps prior to the bonding process. This bonding process may also provide high yield. In particular, transparent conductive zinc oxide bonding materials, including zinc peroxide, adhere to many different materials, allowing for a variety of material combinations for bonding. Further, the bonded zinc oxide provides for increased mechanical strength with reduced electrical resistance. Zinc oxide also provides optical transparency at certain wavelengths.
One area in particular for which the present bonding process may be utilized is solar cell technology, or semiconductor-bonded technology, which has produced solar cells with high space cell efficiency. In this technology, high cell efficiency depends significantly on the bond quality of gallium arsenide-based solar cells and indium phosphide-based solar cells. Since the present process reduces requirements for surface preparation and material compatibility inherent in other processes, semiconductor-bonded solar cell designs can be produced with additional degrees of freedom, more varieties of materials, and higher efficiencies. For example, on the photovoltaic module level, a solar cell can be bonded to metal or ceramic or light-weight handles. In addition, the ability to accommodate various other solar cell materials, such as crystalline silicate, copper indium selenide, and indium gallium nitride, allows for reduced material and integration costs for high-efficiency solar cell designs. The described bonding process may also be used for any other technology that requires transparent conductive bonding.
Although certain illustrative embodiments and methods have been disclosed herein, it can be apparent from the foregoing disclosure to those skilled in the art that variations and modifications of such embodiments and methods can be made without departing from the true spirit and scope of the art disclosed. Many other examples of the art disclosed exist, each differing from others in matters of detail only. Accordingly, it is intended that the art disclosed shall be limited only to the extent required by the appended claims and the rules and principles of applicable law.

## Claims

1. A method of forming a bonded layered structure, comprising:
providing a first structure and a second structure;
forming a first layer comprising zinc peroxide on at least one of the first and second structures;
forming a second layer comprising zinc metal on at least one of the first and second structures;
contacting the first and second structures such that the first layer contacts the second layer, such contact occurring between the first and second structures;
processing the contacted first and second structures to cause oxidation of the zinc metal; and
forming between the contacted first and second structures a third layer comprising zinc oxide.

2. The method of claim 1, wherein processing the contacted layers comprises applying heat to the contacted layers in a temperature range of about 100-400° C.

3. The method of claim 1, wherein processing the contacted layers comprises applying pressure in the range of about 5 to 80 psi.

4. The method of claim 1, wherein the zinc oxide is formed from oxidation of the zinc metal.

5. The method of claim 4, wherein the oxidation results from oxygen provided by the zinc peroxide.

6. The method of claim 4, wherein the oxidation results from oxygen gas provided to the contacted layers.

7. The method of claim 1, wherein the zinc oxide is formed from a thermally-induced conversion reaction of the zinc peroxide.

8. The method of claim 1, wherein at least one of the first and second structures comprises a wafer.

9. The method of claim 1, wherein at least one of the first and second structures comprises a semiconductor.

10. The method of claim 1, wherein at least one of the first and second structures comprises a solar cell.

11. The method of claim 1, wherein at least one of the first and second structures comprises gallium arsenide.