WO2003038881A1

WO2003038881A1 - Direct bonding of articles containing silicon

Info

Publication number: WO2003038881A1
Application number: PCT/US2002/033208
Authority: WO
Inventors: Robert Sabia
Original assignee: Corning Incorporated
Priority date: 2001-10-26
Filing date: 2002-10-17
Publication date: 2003-05-08

Abstract

Methods of bonding glass and silicon-containing articles are disclosed. Bonding is achieved without use of adhesives or high temperature fusion. A wide variety of glass and silicon-containing articles may be bonded by the methods of the invention.

Description

DIRECT BONDING OF ARTICLES CONTAINING SILICON

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S. C. § 119(a) of U.S.

Application Serial No. 10/035,564 filed on October 26, 2001.

This application also claims the benefit of priority of continuation-in-part

having U.S. Patent Application Serial No. 10/255,926 filed on September 25, 2002.

FIELD OF THE INVENTION

[0001] This invention relates to direct bonding. More particularly, the invention

relates to methods for direct bonding of a wide variety of articles and objects and devices

produced by such methods.

BACKGROUND OF THE INVENTION

[0002 ] Various methods exist for bonding glass surfaces together. These methods

include, for example, wringing, fusion bonding, adhesive bonding and vacuum bonding.

Bonding without the assistance of polymeric adhesives is a technology of interest for

numerous industries including microelectronics and photonics. Adhesives are sensitive to

thermal treatments and can fail from cycling in relatively moderate thermal environments

(e.g., 0-200 °C). On the other hand, the formation of a direct bond between two glass or

metal surfaces allows for an impermeable seal that has the same inherent physical properties

as the bulk material surfaces being bonded. For bonding of dissimilar materials, the resulting

bond is sensitive only to CTE mismatches between materials, as compared to polymeric

adhesives that typically have a CTE substantially different than at least one of the bulk

substrates. [0003 ] Optical wringing refers to a process of bonding glass surfaces in which

adsorbed surface groups are removed from active bonds on a surface by heating the parts to

temperatures typically above 600° C but below the softening point of the glass. Adsorbed

water and organics will vaporize and the results surface sites become "active." At such a

temperature or after cooling in a clean, low humidity environment, surfaces can be placed in

contact at which point covalent bonds spontaneously form between "active" bonds on each

surface. This is similar to vacuum bonding, except the surface is activated by temperature

rather than by a strong vacuum. A disadvantage of this process is the effect of high

temperatures on polymers that may be associated with the glass article to be bonded, for

example, fiber coatings and fiber array systems that utilize polymeric adhesives to bond the

fiber array together.

[0004] Vacuum bonding involves bringing two clean surfaces into contact in a high

vacuum, thus forming a bond. Provided that the surfaces are flat and clean, a high vacuum

removes adsorbed water and hydrocarbons from the surface while preventing the adsorption

of such species. Surfaces can be cleaved in the vacuum, processed and cleaned before being

placed in the vacuum, or cleaned in the vacuum via ion milling or other plasma techniques.

One disadvantage of this process is the effect of a high vacuum on polymers that may be

associated with the glass article to be bonded, for example, fiber coatings and fiber array

systems that utilize polymeric adhesives to bond the fiber array together. High vacuum

pressure may have a negative effect on these polymers.

[0005] Within the microelectronics field, vacuum bonding has been developed for

sealing of such materials as single crystal silicon, thermal oxide SiO₂ grown on Si, and

various metals, as described in United States patent number 6,153,495. Coefficient of thermal expansions (CTE) mismatch between materials is not an issue because the process

can be applied at room temperature.

[0006] Fusion bonding refers to the process of cleaning two surfaces (glass or metal),

bringing the surfaces into contact, and heating close to the softening point of the materials

being bonded (to the lower softening temperature for two dissimilar materials), thus forming

a welded interface. One example of a fusion bonding process is fusion splicing of optical

fibers. Advantages of fusion bonding include the fact that commercial systems exist for

splicing of fibers and that the process is relatively easy to apply to bulk geometries. One

disadvantage of fusion bonding is that this process typically results in deformation ofthe two

surfaces being bonded due to the flow of softened material, the inability to use this process

for complex geometries where adhesives or other low-temperature materials are used, and

loss of signal transmitted through the interface when fusion bonding is used for signal

transmitting objects such as optical fibers. Furthermore, for bonding of large surfaces, it is

difficult to limit glass softening to the bonding interface. As a result, the entire seal can lose

dimensional tolerances. In addition, the high temperature ranges required to fusion bond

many glass materials are disadvantageous for complex systems that include the use of low-

temperature materials such as adhesives and polymer coatings (e.g., fiber coatings).

[0007] Adhesive bonding is a common process for mounting of fibers in ferules and

for bonding of photonic components such as filters, polarizers, rotators, etc. to each other and

into packages. Some advantages of using such adhesive are that they are readily available,

UV curable, and allow for alignment of components between application of the adhesive and

curing into permanent position. Disadvantages of adhesive bonding include CTE mismatch

especially for low CTE materials such as high purity fused silica, for applications where the

bonded part is exposed to thermal cycling. Another issue is signal loss from transmission through the adhesive when the adhesive is used in the optical path of optical systems.

Although it is possible to utilize an index matching adhesive that has a refractive index

matching the optical component, it is extremely difficult to utilize an adhesive that has a CTE

and refractive index that matches the optical components. In addition, there are concerns

over long-term reliability of packages that incorporate adhesives. Furthermore, bonding of

components with adhesives can require angle polishing (typically 8°) and associated

assembly to prevent back-reflection.

[0008] Another type of bonding process involves chemical bonding. The formation

of a chemical bond between two glass or metal surfaces allows for an impermeable seal that

has the same inherent physical properties as the bulk material being bonded. In literature,

low-temperature bonding technology has been reported for bonding soda-lime-silicate glass

and for crystalline quartz (see, e.g., A. Sayah, D. Solignac, T. Cueni, "Development of novel

low temperature bonding technologies for microchip chemical analysis applications,"

Sensors and Actuators, 84 (2000) pp. 103-108 and P. Rangsten, O. Vallin, K. Hermansson,

Y. Backlund, "Quartz-to-Quartz Direct bonding," J. Electrochemical Society, V. 146, N. 3,

pp. 1104-1105, 1999). Both the Sayah and Rangsten references disclose using acid to contact

the bonding surfaces. Another article, H. Nakanishi, T. Nishimoto, M. Kani, T. Saitoh, R.

Nakamura, T. Yoshida, S. Shoji, "Condition Optimization, Reliability Evaluation of SiO₂-

SiO₂ HF Bonding and Its Application for UV Detection Micro Flow Cell," Sensors and

Actuators, V. 83, pp. 136-141, 2000, discloses low-temperature bonding of fused SiO₂ by first

contacting the bonding surfaces with hydrofluoric acid. While these bonding processes are

useful in certain applications, the bond strength provided by contacting with acidic solutions

is limited and could be improved. [0009] It would be desirable to provide a bonding process that does not have the

disadvantages of fusion bonding, adhesive bonding, and wringing, and offers more reliable

seal integrity than low pH chemical bonding. In addition, it would be useful to provide a

bonding process that was durable, provided high bond strength and could be used on a wide

variety of silicon-containing materials and surfaces.

SUMMARY OF INVENTION

[00010] The invention relates to methods of bonding opposing surfaces of silicon-

containing articles, such as glass articles containing silica. The invention may further find

use in bonding a wide variety of silicon containing materials such as single crystal silicon and

crystalline quartz. According to one embodiment of the invention a method of bonding

opposing surfaces of silicon-containing articles is provided. The method includes providing

reactive termination groups on the opposing surfaces of the articles and placing the opposing

surfaces in contact. According to another embodiment of the invention, the temperature of

the opposing surfaces can be maintained at a temperature below about 300° C, preferably

below about 200° C during the contacting step, resulting in high bond strength and seal

integrity.

[00011] According to another embodiment of the invention, the step of providing

functional groups includes contacting opposing surfaces of the articles to be bonded with a

high pH solution. As used herein, the term high pH means a solution having a pH of about 8

to about 13. Suitable high pH solutions include hydroxide-based solutions such as potassium

hydroxide, sodium hydroxide and ammonium hydroxide. In another embodiment of the

invention, the method may further include cleaning the opposing surfaces with a detergent

and contacting the opposing surfaces with an acid. In still another embodiment, the opposing

surfaces may also be ground and polished prior to contacting the surfaces. According to this embodiment, it may be desirable to provide a bonding surface having a flatness less than 1

micron and a roughness of less than 2.0 nm RMS, preferably less than 1.5 nm RMS.

[00012] In a preferred embodiment of the invention, the pH of the high pH solution is

greater than 8, but less than 14. In a highly preferred embodiment ofthe invention, the step

of contacting the opposing surfaces with the high pH solution is performed after the step of

contacting the opposing surfaces with the acid. Suitable acids for this step may include

hydrochloric acid, nitric acid and sulfuric acid. According to still another embodiment ofthe

invention, the opposing surfaces are rinsed with water and placed in contact without drying

the opposing surfaces. In a preferred embodiment, pressure of at least one pound per square

inch, more preferably, at least two pounds per square inch, is applied to the opposing surfaces

during the step of contacting the opposing surfaces. In another embodiment, it may be

desirable to dry the surfaces to remove adsorbed water molecules and hydroxyl groups and to

draw a slight vacuum, for example, about 10^"3 millibar, to assist in the prevention of an air

gap between the surfaces.

[00013] The method of the present invention is suitable for bonding a wide variety of

silica-containing, glass and oxide-based surfaces. For example, the method could be used to

bond waveguides, optical waveguide preforms, microlens arrays, optical fiber arrays,

photonic components, lenses, ferrules, optical fiber waveguides, and combinations of these

articles. For example, the invention may be utilized to bond two or more optical fiber

waveguide fiber preforms together to provide for an enlarged fiber preform and continuous

fiber drawing process. The invention may also be utilized to bond at least two glass tubes

together that can be drawn into a dual fiber ferrule. The bonding method may be used to

bond glass or silica-containing fibers with ferrules. The invention may also be used in the

manufacture of optical fiber and lens arrays. [00014] The invention provides a simple, low temperature, and inexpensive bonding

method that provides a high bond strength. Bonding can occur at temperatures lower than

300° C, and in some cases lower than 100° C. The resulting seal is complete, impermeable

and does not include an air gap. Additional advantages of the invention will be set forth in

the following detailed description. It is to be understood that both the foregoing general

description and the following detailed description are exemplary and are intended to provide

further explanation ofthe invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

[00015] FIG. 1 is a diagram of termination groups on the surface of a silica-containing

article;

[00016] FIG. 2a is a diagram of adsorbed water molecules and hydroxyl groups on

opposing surfaces after being brought into contact at room temperature;

[00017] FIG. 2b is a diagram of termination groups on the surfaces shown in FIG. 2a after

the surfaces have been heated to drive off adsorbed water molecules and hydroxyl groups;

[00018] FIG. 2c is a diagram of termination groups on the surfaces shown in FIGS. 2a and

2b after the adsorbed hydroxyl groups have been removed; and

[00019] FIG. 3 is flow chart of process steps according to one embodiment of the

invention.

DETAILED DESCRIPTION

[00020] The present invention relates to methods for bonding silicon containing

articles. According to one embodiment of the invention, functional groups are provided on

opposing surfaces of the articles to be bonded. No adhesives, high temperature treatment or

caustic hydrofluoric acid or hydrogen peroxide treatments are required prior to bonding the

opposing surfaces. In one embodiment of the mvention, a surface treatment of a high pH base solution such as potassium hydroxide, sodium hydroxide or ammonium hydroxide is

utilized to provide functional groups on the bonding surfaces of the articles. In a preferred

embodiment, the surfaces are first cleaned using a detergent followed by rinsing with an acid

solution such as a nitric acid solution to remove particulate contamination and soluble heavy

metals respectively.

[00021] Surprisingly, it was found that cleaning and low pH treatment alone did not

result in complete bonds that could be repeatably produced. In several trials, only a portion

of interfaces between articles successfully bonded. However, repeatable and complete bonds

could be provided by contacting the bonding surfaces with a high pH solution. According to

one embodiment of the invention, the surfaces are contacted with a high pH solution, rinsed,

pressed into contact and gradually heated to the desired temperature. The actual temperature

will depend on a variety of factors, including, but not limited to the materials bonded, CTE

mismatch between materials and the presence of polymers. The temperature treatment must

be high enough and for a time sufficient to drive off the adsorbed water molecules and

hydroxyl groups at the bonding interface to allow for the formation of chemical bonds. This

can be done at any temperature below the softening point ofthe glass while preventing loss of

geometrical tolerances. For example, for high purity fused silica, this temperature can be as

high as approximately 1000° C, for Pyrex®, as high as about 650° C, for Polarcor® (a

polarizing glass available from the assignee of the present invention), as high as about 500°

C. For applications where the system includes materials such as polymeric adhesives and

coatings outside ofthe bonding area (for example, the mounting of fibers in a fiber array), the

required temperature is below that which promotes degradation or embrittlement of the

polymer (typically 150-200° C, usually a maximum of around 250° C, but preferably less than 150° C). In cases where a low temperature frit is used to seal and/or adhere components

away from the directly bonded surfaces, the temperature used for direct bonding may need to

be lower, and will depend upon the melting temperature ofthe frit.

[00022 ] For applications that include sealing materials with significantly different CTE

values, sealing must be performed at a low enough temperature (typically less than 100° C)

such that the sealed part will not exhibit high stress when cooled to room temperature. To

enhance bonding, it is highly preferred that the surfaces are flat, as determined by performing

a preliminary cleaning and pressing ofthe dried samples into contact. Resulting interference

fringes can be acquired according to teclmiques known in the art and interpreted to determine

matching flatness. Also, an optical flat or interferometer can be used to evaluate individual

surface flatness. Also, interference fringes between two mating surfaces prior to bonding can

be used to observe and measure conforming flatness.

[00023] Preferably, the bonding process of the present invention consists of machining

each surface to be sealed to an appropriate flatness. Particularly preferred flatness levels are

less than about 1 micron and roughness levels of less than about 2.0 nm RMS, preferably less

than about 1.5 nm RMS. In general, it is desirable to have less than about 1 micron of

conformation between the bonding surfaces, however, less conformance is acceptable if a

higher amount of pressure is applied to the bonding surfaces. After polishing, each surface is

preferably cleaned with an appropriate cleaning solution such as a detergent, soaked in a low

pH acidic solution, and soaked in a high pH basic solution to generate a clean surface with

silicic acid-like terminated surface groups. Such surface groups include ≡Si-OH, and more

reactive groups including =Si-(OH)₂, -Si-(OH)₃ and -O-Si-(OH)₃). In preferred embodiments,

=Si-(OH)₂, -Si-(OH)₃ and -O-Si-(OH)₃) account for the majority of the terminated surface

groups. In certain preferred embodiments, the surfaces are assembled without drying. However, in some embodiments it may be acceptable to moderately dry the bonding surfaces

to remove adsorbed water molecules and hydroxyl groups, especially when using a low

vacuum (e.g., about 10^"3 millibar) to assist in sealing the bonding surfaces without an air gap.

A low to moderate load (at least one PSI) is then applied as the surfaces are heated to less

than 300° C, for example, between 100° C and 200° C, so that adsorbed water evaporates and

silicic acid-like surface groups condense to form a covalently-bonded interface.

[00024] According to one embodiment of the invention, as noted above, it is desirable

to provide a bonding surface that is flat. Under nominal load, relatively thin parts (1-2 mm)

will elastically deform to increase contact area between non-flat surfaces, however such seals

result in low failure strength due to flexure of the thin parts during mechanical testing. An

issue for bonding thin parts under a nominal load is that thin parts may be stressed within the

interface after bonding and subsequent removal of an applied load. Therefore, for thin parts,

it is desirable to maximize flatness and provide parts having uniform thickness to maximize

contact area without the need for applied load. It is preferred to have surfaces finished to 1

micron flatness or better on the surfaces to be bonded.

[00025] Contacting a clean, hydroxyl-terminated silicon-containing surface in a high

pH solution (e.g. pH greater than about 8, or a pH greater than about 9) causes the surface to

slowly dissolve, forming silicate species, such as, for example, SiO₂(OH)₂ ^2", SiO(OH)₃ ^", etc.

in solution. Likewise, the active sites on the reacting surface are terminated by similar ≡Si-O^"

, =Si-(O^")₂, -Si-(O^")₃, and -O-Si-(O^")₃ groups. By lowering the pH ofthe system (e.g., rinsing

in pH neutral Dl-water), the surface termination groups convert to ≡Si-OH, =Si-(OH)₂,-Si-

(OH)₃, and -O-Si-(OH)₃ termination groups (i.e., silicic acid-like surface groups, see Figure 1

for graphical representation). Preferably, a majority of the termination include =Si-(OH)₂ ,—

Si-(OH)₃, and -O-Si-(OH)₃. [00026] After surfaces with Si-O-H termination groups are generated, water molecules

spontaneously adsorb from an aqueous solution onto the silicic acid-like termination groups.

When two such surfaces are brought into contact, these adsorbed water molecules and

hydroxyl groups form similar bonds to both surfaces, thus acting to bridge the surfaces with

hydrogen bonds, as shown in Fig. 2a. With moderate heating, the adsorbed water molecules

are driven off and hydroxyl groups, and hydrogen bonding exists between the silicic acid-like

termination groups on each surface (see Figure 2b). With further, higher temperature heating

these silicic acid-like surface termination groups condense to form covalent bonds between

the two surfaces (e.g.,≡Si-O-Si≡), with water as a byproduct (Figure 2c).

[00027] For silicon-containing surfaces having a high percentage of silica, higher

temperature heating is not necessarily required to form high strength bonds. For silica

systems containing a greater amount of silica, heating below 300° C as part of the sealing

process is usually sufficient to form a high strength bond. On the other hand, samples that

have a lower amount of silica in the glass composition may require heating to higher

temperatures to form a satisfactory bond. For example, Pyrex® glass (containing

approximately 81% silica) and Polarcor™ (containing approximately 56% silica), which are

borosilicate glasses may require additional heating to provide sufficient bond strength for

applications requiring high bond strength. The degree of heating for different bonding

surfaces and glass surfaces will depend in part on the type of surface to be bonded (e.g., a

fiber or a flat surface) and the desired bond strength for a particular application. As noted

above, in systems that include polymeric materials, such as optical fiber waveguides, it is

undesirable to heat the surfaces to the point where the polymeric material is damaged.

[00028] Compared with bonding systems that utilize only a low pH treatment and rely

on hydroxyl terminated surface group consisting only of ≡Si-OH, it is believed the present invention provides more robust bonding between silicon-containing articles for several

reasons. While not wishing to be bound by theory, it is believed that larger silicic acid-like

termination groups allow bonding (both hydrogen and covalent) to occur between surface

groups that extend further away from the surface. Larger surface terminated groups such as

=Si-(OH)₂, -Si-(OH)₃, and -O-Si-(OH)₃ extend further from the surface than ≡Si-OH, and

these larger groups are more susceptible to steric movement which promotes better bonding

between surfaces including these larger groups. Additionally, each surface can be

considerably rougher and still generate bonding due to the length in which the =Si-(OH)₂, -Si-

(OH)₃, and -O-Si-(OH)₃ termination groups extend from the surface. Although termination

groups specific to bulk surfaces being bonded can be formed for a variety of glass formers

(e.g., SiO₂, B₂O₃) and intermediates (e.g., Al₂O₃), application to sealing of glass compositions

that have significantly higher alkali and alkaline earth concentrations is expected to be

difficult. For these types of glass compositions, high pH treatment to form surface

termination groups specific to each constituent that extend from the surface (similar to silicic

acid-like termination groups for silica surfaces) is expected to improve bonding performance

between the surfaces.

[00029] Silicic acid-like termination groups are also more reactive than only ≡Si-OH

groups. In addition, the process of removing adsorbed water molecules and hydroxyl groups

to promote hydrogen bonding between ≡Si-OH and the more reactive =Si-(OH)₂, -Si-(OH)₃,

and -O-Si-(OH)₃ surface groups and condensation of said groups can occur at lower

temperatures (i.e., below 100 °C) or in shorter time periods at equivalent temperatures

compared to hydroxyl-terminated surfaces sites. It is also believed that this process can be applied at lower pressures to attain equivalent or superior strengths compared to low pH

bonding procedures that have been found to require higher pressure.

[00030] A flow chart of the process steps of one embodiment of the invention are

shown in Fig. 3. According to the present invention, the bonding surface of an article can be

provided by grinding the bonding surface flat, lapping to remove grinding damage while

maintaining flatness, and polishing to produce an optically clear surface without subsurface

damage. The samples are then cleaned using a detergent, soaked in a strong acid such as

nitric acid to remove adsorbed hydrocarbons and dissolved rare-earth contaminants (e.g.,

cerium oxide from the polishing step), and finally contacted with a high pH alkali solution

such as ammonium hydroxide. The surfaces are then brought into contact and gradually

heated to approximately 200°C for an extended period of time before cooling and inspecting

the seal. The bond interface is denoted by a lack of interference fringes. If fringes are

observed, they will appear around the bonded area, indicating surface separation due to non¬

conformity ofthe surfaces.

[00031] As noted above, for certain glass compositions, it may be desirable for

additional heat treatment or annealing after placing the surfaces in contact. Whether or not an

annealing treatment is possible or practical depends on the presence of low temperature

constituents of the component or package being bonded (e.g., presence of low-temperature

softening or degrading materials such as adhesives and coatings away from the bond

interface). It must also be considered that excessive heating of certain materials may result in

a loss of dimensional tolerances.

[00032] Another factor to consider during bonding of glass surfaces is the solubility

behavior of the glass being prepared for bonding. During cleaning, solution pH may create

heterogeneous etching rates between the various glass constituents. This can lead to loss of surface quality in terms of increased roughness or generation of a pitted surface. For

example, the ammonium hydroxide soak used to hydrate the glass surface has a pH between

12-13. This is high enough to cause silica in a glass surface to slowly dissolve. Extended

soaking time can lead to a roughening ofthe surface if other constituents ofthe glass dissolve

either faster or slower. Other suitable high pH solutions include hydroxide-based solutions

such as potassium hydroxide and sodium hydroxide. Ammonium hydroxide is a weak base,

and a highly concentrated solution of ammonium hydroxide will not exceed pH of

approximately 13. Comparatively, sodium and potassium hydroxides are strong bases and

can easily exceed pH 14, with 1M concentration for a strong base = pH 14. A I M

concentration of KOH is typically used to clean laboratory glassware. This solution is

effective in removing contaminants by dissolving the glass surface around and under the

contaminant and thus allowing the contaminant to disperse in solution. This level of highly

concentrated solution results in an aggressive attack of a glass surface with a high dissolution

rate, and thus may not be desirable for the present invention. Alternatively, a pH such as 12-

13 will thermodynamically allow for solubility of a glass surface, however, kinetically this

solubility reaction proceeds at a much slower rate than for a pH 14 solution.

[00033] Likewise, the nitric acid solution has a pH near 0, and will preferentially etch

lead from a lead-silicate glass. Thus, modification ofthe cleaning protocol might be required

in terms of soak time and acid and/or alkali concentration for complex glass compositions,

and these modifications can be determined by experimentation for various types of glasses.

[00034] Without intending to limit the invention in any manner, the present invention

will be more fully described by the following examples. EXAMPLES

Sample Preparation

All samples in the examples below had geometries of 2 X ^lA X V* inches. Samples

were prepared by cutting each sample to size plus an additional 0.051" in each direction. The

samples were ground and polished by first grinding 0.040" from each of the four side edges

of the sample to provide a parallel and flat sample. Thereafter, a 7 micron alumina abrasive

was used to lap 0.010" from each of the side edges. Next, all twelve edges of each sample

were chamfered. The faces were then polished with a ceria abrasive (Hastelite grade 919 or

PO) with a polyurethane pad, which removed less than about 0.001" of material. The

surfaces were inspected for surface roughness and flatness.

After a polished surface was obtained, the samples were chemically treated as shown

in Fig. 3. A detergent such as Microclean CA05 was used to clean the samples, and after a

water rinse, the sample was soaked in 10 volume % nitric acid for one hour. The acid-soaked

samples were rinsed again with water, and then the samples were soaked in a 15 volume %

ammonium hydroxide solution for 60 minutes. The samples were rinsed again, and the

bonding surfaces were maintained in a wet condition and bonded under a pressure greater

than about one pound per square inch and at a temperature noted above. In some instances,

samples were further treated by annealing, which is noted in the examples below.

The bars were bonded together in the shape of a cross ("+"), and an Instron-type,

computer controlled mechanical testing unit force measuring device was used to measure the

force to either break the bond between the bars or the sample. Pressure was applied in an

upward direction on the top bar and in a downward direction on the bottom bar. If the bars

separated or delaminated, the measured force was an indication of the bond strength. If the

bars broke, the source of failure in every case was observed to be a flaw of critical size away from the interface, and this was an indication that the bond strength was greater than the

breaking strength ofthe sample.

Example 1

Bonding of High Purity Fused Silica Surfaces

Corning product code 7980 HPFS® bars were bound and the bonding between the

bars at a temperature of 200° C was strong enough so that one ofthe bars failed at 160.9 psi.

Example 2

Bonding of Polarcor™ Surfaces

Polarcor™ is a borosilicate glass. A proprietary polarization process makes the outer

surfaces act as polarizers. Active polarization occurs in the outer 20 - 50 microns of the glass

surface. Polarcor bars were bonded together at a bonding temperature of about 200° C. A

first set of samples resulted in the bond between the bars delaminating at 45.8 psi. A second

set of samples was annealed to about 500° C, and these bars failed in tension at 127.9 psi.

[00035] It will be apparent to those skilled in the art that various modifications and

variations can be made to the present invention without departing from the spirit or scope of

the invention. Thus, it is intended that the present invention cover modifications and

variations of this invention provided they come within the scope of the appended claims and

their equivalents.

Claims

What is claimed is:

1. A method of bonding opposing surfaces of at least two silicon-

containing articles comprising:

providing termination groups selected from the group including =Si-(OH)₂, -

Si-(OH)₃, and -O-Si-(OH)₃, and combinations thereof on the opposing surfaces and placing

the opposing surfaces in contact.

2. The method of claim 1, wherein the temperature of the opposing

surfaces is maintained at a temperature below 200° C during the contacting step.

3. The method of claim 3, further comprising the following steps:

grinding and polishing the opposing surfaces;

cleaning the opposing surfaces with a detergent;

contacting the opposing surfaces with an acid, wherein the acid includes nitric acid;

and,

contacting opposing surfaces of the articles to be bonded with a solution having high

pH greater than 8

4. The method of claim 4, wherein the high pH solution contains a

reagent selected from the group consisting of ammonium hydroxide, potassium hydroxide

and sodium hydroxide.

5. The method of claim 3, further including a step of applying pressure of

at least one pound per square inch during the step of contacting the opposing surfaces.

6. The method of claim 3, further including a step of drying the surfaces

to remove adsorbed water molecules and hydroxyl groups from the surface and utilizing a

low vacuum pressure to prevent an air gap between the surfaces.

7. A method of directly bonding two opposing silicon-containing

surfaces, comprising:

polishing the opposing surfaces;

contacting the opposing surfaces with a detergent;

contacting the opposing surfaces with an aqueous rinse solution;

contacting the opposing surfaces with an acidic solution;

contacting the opposing surfaces with a solution having a pH greater than 8;

and

placing the opposing surfaces in contact.

8. The method of claim 7, further comprising heating the opposing

surfaces to a temperature less than 200° C during the step of placing the opposing surfaces in

contact.

9. The method of claim 7, further comprising a step of applying pressure

of at least one pound per square inch during the step of placing the opposing surfaces in

contact.

10. The method of claim 7, wherein the acidic solution includes nitric acid.

11. The method of claim 7, wherein the solution having a pH greater than 8

includes ammonium hydroxide, potassium hydroxide or sodium hydroxide.

12. The method of claim 16, further including a step of providing

termination groups selected from the group consisting of =Si-(OH)₂, -Si-(OH)₃, and -O-Si-

(OH)₃, and combinations thereof on the opposing surfaces and placing the opposing surfaces

in contact.