WO1986003885A1

WO1986003885A1 - Process for enhancing the adhesion of teflon used in advanced space solar cells and in encapsulated semiconductor devices and circuits

Info

Publication number: WO1986003885A1
Application number: PCT/US1985/002307
Authority: WO
Inventors: George J. Vendura, Jr.
Original assignee: Hughes Aircraft Company
Priority date: 1984-12-24
Filing date: 1985-11-25
Publication date: 1986-07-03

Abstract

A process for bonding FEP TEFLON (10) to a solar cell (14), or an anti-reflective coating thereon, which includes the step of plasma etching (16) the bonding surface of a TEFLON substrate to thereby condition the surface by means of various mechanisms. This process enhances the bonding strength and quality of the bond subsequently formed between the TEFLON and the solar cell. These components are then thermo-compression bonded in the second step of the process for a predetermined time, temperature and pressure.

Description

PROCESS FOR ENHANCING THE ADHESION OF TEFLON

USED IN ADVANCED SPACE SOLAR CELLS AND

IN ENCAPSULATED SEMICONDUCTOR DEVICES AND CIRCUITS

BACKGROUND

1. Field of the Invention

The present invention relates generally to the manufacture of solar cells suitable for space appli¬ cations, and more particularly to a process for improving the adhesion between these cells and protective layers therefor.

2. Related Art

In many applications where solar cells are exposed to extreme environments, including the exposure to damaging radiation particles, it is necessary to provide a protective cover, such as a transparent cover glass, which will suitably shield the cells from these sources of potential damage. Thus, it h^s been necessary to provide a suitable bonding material to secure a protective cover glass to the solar cell and simultaneously transmit solar radiation to the cell for conversion to electrical power. One such bonding material is an epoxy resin sold by the Dow Corning Company of Midland, Michigan under the trade name "DC 93500". This latter epoxy material possesses many disadvantages. The material is very expensive and since it is normally applied to the cell in liquid form, it has a high labor cost associated with its application. Other problems are batch to batch adhesive strength variations and limited shelf life. In addition, this epoxy material becomes partially opaque after a certain exposure to the ultra- violet radiation component of the solar spectrum and therefore must be accompanied with an expensive ultra¬ violet filter to insure satisfactory solar radiation transmission.

Recognizing the above disadvantages associated with the use of "DC 93500", workers in this art have proposed the use of fluorinated ethylene propylene (also known as FEP. FEP-A, FEP-C and TEFLON) as a substitute bonding agent between the solar cell and its protective cover glass. Such an approach is disclosed, for example, by Jacob D. Broder et al in an article entitled "The Use of FEP Teflon in Solar Cell Cover Technology" , IEEE 10th Photovoltaic Specialists Conference at page 292, 1973. Using this latter approach, certain thermo compression bonding techniques are used to sandwich the FEP TEFLON between the solar cell and a transmissive cover glass. Other examples of solar cells bonded to FEP TEFLON layers are disclosed in U.S. Patents 3,912,540 and 3,996,067 issued to Jacob D. Broder. While the latter approach using FEP TEFLON did indeed overcome many of the disadvantages associated with cost and transmission quality of the "DC 93500" epoxy material, under certain space and terrestrial conditions it did not provide satisfactory adhesion in the cover glass-TEFLON-solar cell sandwich structure, and in fact the TEFLON has been known to separate from the solar cell in previous spacecraft applications. This separation has resulted in critical power losses, and, in some cases, could result in a total failure of spacecraft mission. Thus, the ability to adequately shield the solar cell from radiation damage by permanently and securely bonding the FEP TEFLON to the solar cell and enabling the solar cell-TEFLON-cover glass sandwich structure to hold together under worst case environmental conditions is extremely important.

THE INVENTION In order to provide such a secure bond between the solar cell and its protective FEP TEFLON and cover glass, and in accordance with the present invention, I have discovered that a substantially enhanced and strengthened bond between the solar cell (or an anti- reflective (AR) coating thereon) and the FEP TEFLON may be achieved by initially plasma etching a major surface or surfaces of the TEFLON substrate using a chosen ion species, such as ionized oxygen. These ion species thus become available to chemically or otherwise react with the atomic surface layers of the TEFLON surface as well as that of silicon solar cell, or its anti- reflective coating, in such a way as to strengthen the mutual adherence between the abutting bonding surfaces of the solar cell and the FEP TEFLON substrate. These mutually abutting and bonded surfaces are brought together under preselected conditions of temperature, pressure and time to create a secure and permanent bond between these surfaces.

In one embodiment of the present invention, only a single surface of the FEP TEFLON is plasma etched, and the application of a quartz or other cover glass to the other surface of the FEP TEFLON is optional. In another embodiment of the invention _f both opposed major surfaces of the FEP TEFLON are plasma etched, and the facing surfaces of the solar cell and the quartz cover glass are also plasma etched, so that all mutually abutting and bonded surfaces of the solar cell- FEP TEFLON-cover glass sandwich structure are bonded at mutually abutting plasma etched surfaces. The present process will be better understood from the following description of the accompanying drawings.

DRAWINGS

FIG. la is a schematic process flow diagram wherein a single plasma etch operation is performed on the FEP TEFLON prior to thermal compression bonding.

FIG. lb, c and d are isometric views showing the bonding of the FEP TEFLON to the solar cell as a completed structure (FIG. lc) or an alternative embodiment of adding a cover glass to the top surface of the TEFLON bonded structure as shown in FIG. Id.

FIG. 2a is a process flow diagram wherein plasma etching of all of the cover glass, FEP, and solar cell components is carried out prior to thermal compression bonding to form the composite and protected solar cell structure.

FIG. 2b and c are isometric views showing the plasma etching of the four mutually abutting and bonding surfaces of the composite protected solar cell structure.

DETAILED DESCRIPTION OF THE INVENTION Referring now to FIG. la, the FEP TEFLON substrate is provided at station 10 and is also identified as material "A" , since the A-type materials listed in Table I below are alternative or substitute materials for the FEP TEFLON in other and different bonding applications. Similarly, materials designated as "B" material at stations 12 and 14 in FIG. la indicate alternatives to the cover glass and solar cell components respectively for different bonding applications.

After the FEP TEFLON substrate from station 10 is exposed on one surface to a plasma etching treatment at station 16 for a predetermined time and temperature as provided in the specific example below, the TEFLON is interposed between the solar cell from station 14 and the cover glass from station 12 in a thermal compres¬ sion bonding press 18 for a predetermined time, tempera¬ ture and pressure to firmly bond the three layers together and provide a physically secure three-layered composite structure.

The process of plasma etching various substrate materials is generally well known to those skilled in the art and for this reason is not discussed in detail herein. However, for a further detailed discussion of this process, reference may be made to a book entitled, Thin Film Processes by J. L. Vossen and Werner Kern, Academic Press, 1978, and particularly Part V-2 thereof. Reference may also be made to an article by R. A. H. Heinecke entitled, "Control of Relative Etch Rates of

Siθ2 and Si In Plasma Etching", Solid State Electronics, Vol. 18, 1975, at page 1146. Both of these references are incorporated herein under the well known doctrine of "incorporation by reference". Referring now to FIGS, lb, c and d, it is seen that the plasma etching treatment is only to the lower, downwardly facing surface of the FEP TEFLON layer 20, as indicated by arrows 22. The solar cell component is designated generally as 24 and has an antireflective (AR) coating 26 thereon to improve the absorbency of beneficial radiation by the solar cell 24, as is well- known in the art. In accordance with the present invention, the complete and protected structure may consist of only the solar cell 24 and FEP TEFLON layer 20, as indicated in FIG. lc, or the complete composite structure may further include a top protective cover glass layer 28, as indicated in FIG. Id. The AR coating will preferably be a layered combination of tantalum pentoxide Ta2θ5 and sapphire, AI2O3, and the cover glass 20 will be either a polished or as-cut quartz sheet. Referring now to FIG. 2a, wherein like reference numerals correspond to like stations in FIG. la, there have been added additional plasma etching steps at stations 30 and 32 so as to provide plasma etching of all mutually abutting bonding surfaces which face each other as indicated isometrically in FIG. 2b. In this embodiment of the invention, both upper and lower surfaces of the FEP TEFLON substrate 10 are plasma etched as well as the facing surfaces of the cover glass material 12 and the silicon solar cell 24. FIG.

2c shows the completed composite solar cell-TEFLON-cover glass structure after these layers have been thermal compression bonded in a bonding press operated at a predetermined temperature, pressure and time as described in more detail in the following example.

EXAMPLE I First, a type K4-3/4 single crystal silicon solar cell having two antireflection (AR) coatings thereon was prepared by first boiling in reagent grade alcohol for approximately 10 minutes, then submerging in a liquid primer solution for 10 minutes to condition the upper AR coating of the cell for better adhesion, and then rinsing in reagent alcohol for 10 seconds. The solar cell was obtained from Spectrolab, Inc. of Sylmar, California, and the primer was the type "A-1100" made by the Union Carbide Company of Danbury,. Connecticut. The silicon solar cell's antireflection coatings consisted of a layer of a2θ5 immediately upon the silicon surface followed by a second, outer layer of A1₂0₃.

Next, a prepolished or as-cut quartz cover glass was cleaned by first placing it in a light etchant consisting of about 100 parts deionized water and 1 part HC1. Then, the same treatment as indicated above for the solar cell was carried out on this component by boiling, priming and rinsing the cover glass in the manner described above.

Similarly, the next step in the process included cleaning the FEP TEFLON substrate by boiling in reagent alcohol for 10 minutes. No further priming and rinsing were required as with the solar cell and cover glass. Then, the FEP TEFLON substrate- was transferred to the chamber of a plasma etch machine wherein one surface of the TEFLON substrate was bombarded with oxygen ions for approximately 7.5 minutes. In this chamber, the FEP TEFLON substrate was treated in an atmosphere of oxygen gas ionized between charged parallel plates operating at about 200 watts of power. A gas flow rate of 30 milliliters per minute was used, and the plasma etch chamber was operated at a pressure of aproximately 800 milliTorr.

Upon completion of the above plasma etch step, the FEP TEFLON substrate was removed from the plasma etcher and inserted between the treated cover glass and the treated silicon solar cell in a thermal compres¬ sion bonding press for about 4 minutes. This thermal compression bonding press consisting of heated parallel plates with major surfaces conforming to the surfaces of the composite glass-FEP-solar cell structure. This structure was exposed to 20 psi pressure and gradually heated from room temperature to approximately 360°C over a period of 3.25 minutes, and thereafter cooled, using liquid nitrogen, down to approximately 150°C over a period of 45 seconds.

The composite bonded structure was then removed from the thermal compression bonder and subsequently transferred to an adhesive wedge-type test station wherein a sharp, razor edge separation tool was used to attempt to separate the quartz cover glass from the silicon solar cell. By means of this procedure, it was determined that the adhesive strength between the cover glass and the silicon solar cell was 3 to 5 times the strength of the prior art Dow Corning 93,500 epoxy adhesive strength, and was 10 to 20 times greater in adhesive strength compared to prior art using FEP but without plasma etching.

EXAMPLE II The above procedure was repeated, but in this case, both parallel major surfaces of the FEP TEFLON substrate 30 and the facing major surfaces of the cover glass 28 and AR coated solar cell 26 were also exposed to the above plasma etching step before being thermal compression bonded to form the composite structure shown in FIG. 2c.

The exact nature and mechanism by which the plasma etching enhances bonding strength described herein is not completely understood. This etching quite clearly has a cleaning effect on the surface of the FEP TEFLON and it probably improves the texture of the surface, and both of these effects undoubtedly improve the bonding quality and adhesion of the TEFLON surface.

However, in addition to the above effects, the impreg¬ nation of the FEP TEFLON surface, as well as other plasma etched surfaces described above, may have the further effect of providing dangling oxygen bonds into the surface atomic lattice of those surfaces.

These dangling bonds may in turn interact chemically at the mutually abutting bonded surfaces in such a manner as to even further enhance the bonding adhesion between the bonded surfaces. In terms of improvement in total device performance, the present invention compared to prior art provides a substantial improvement in the mechanical, optical and electrical properties of the composite structures described. Life testing has established the fact that the enhanced adhesion between bonded layers prevents mechanical degradation and failure which were observed in the above prior art cells similarly tested or used in space applications. Secondly, the enhanced adhesion provided by the present invention prevents delamination regions from forming at the TEFLON-solar cell interface which when observed in the prior art degrade the optical transmission quality of the composite structure and make it partially opaque. Thirdly, the improved mecha- nical and optical properties of the present composite structure tend to insure that its electrical performance will be optimum and longer lived.

In particular, when the composite structures of the present invention were compared to prior art type composite structures, they demonstrated improved resistance to ultraviolet radiation, electron radiation, temperature cycling, and high temperature humidity, and they also exhibited improved solderability and weldability, Obviously, the present invention is not limited to the bonding of plasma etched TEFLON to cover glasses and solar cells and may be used to bond the equivalent materials "A" and "B" as previously indicated in FIGS, la and 2a above and set forth in the TABLE below. TABLE

Thus, polymers which respond to plasma etching in the manner of FEP TEFLON herein and which are otherwise suitable for a composite structure bonding application may be substituted for this specific polymer material within the scope of the present invention. Similarly, it may be desired to use TEFLON or other suitable polymers for protective encapsulants for semiconductor devices, integrated circuits, metals, AR coatings, glasses, insulators and a variety of other thin film structures.

Claims

CLAIMSWhat is Claimed is;

1. A process for enhancing the bonding strength between FEP TEFLON and a solar cell which it protects which includes: a) plasma etching one surface of an FEP TEFLON substrate, and thereafter b) bonding the plasma etched surface of the FEP TEFLON substrate to a surface of a solar cell, or an anti- reflective coating thereon, under preselected conditions of temperature, pressure and time sufficient to cause the atomic surface layers of said solar cell, or its anti- reflective coating, to interact with the atomic surface layers of said TEFLON substrate, thereby providing enhanced adhesion between said FEP TEFLON substrate and said solar cell.

2. The process define in Claim 1 which further includes: a) providing a protective cover glass for bonding to said FEP TEFLON, b) plasma etching the facing surfaces of said cover glass and said solar cell, as well as the opposed major surfaces of said FEP TEFLON substrate, and thereafter c) thermo-compression bonding the opposed surfaces of said FEP TEFLON to the facing surfaces of said solar cell and said cover glass, respectively, whereby all of the bonding surfaces of said FEP TEFLON, cover glass and solar cell interact during thermo-compression bonding to enhance adhesion in the sandwich structure thus formed.

3. A process for enhancing bonding strength between a selected polymer substrate and a second substrate selected from the group of semiconductors, glasses, metals, insulators, and thin films and coatings of same, which includes: a) plasma etching a major surface of said polymer substrate; and b) thermal compression bonding said polymer substrate to said second substrate.

4. The process defined in Claim 3 wherein said polymer substrate and said second substrate are both plasma etched at their mutually abutting and bonding surfaces.

5. The process defined in Claims 3 or 4 wherein said polymer substrate is FEP TEFLON.

6. The process defined in Claim 5 wherein said FEP TEFLON substrate is cleaned and primed prior to the plasma etching thereof.

7. The process defined in Claims 5 or 6 wherein said second substrate is cleaned and primed prior to the plasma etching thereof.

8. A composite TEFLON-solar cell device wherein the solar cell is secured to a TEFLON substrate by the process of: a) plasma etching one surface of an FEP TEFLON substrate, and thereafter b) bonding the plasma etched surface of the FEP TEFLON substrate to a surface of a solar cell, or an anti- reflective coating thereon, under preselected conditions of temperature, pressure and time sufficient to cause the atomic surface layers of said solar cell, or its anti- reflective coating, to interact with the atomic surface layers of said TEFLON substrate, thereby providing enhanced adhesion between said FEP TEFLON substrate and said.solar cell.

9. The device of Claim 8 which further includes a cover glass which is thermal compression bonded to the outer surface of said TEFLON substrate.