imaps 2003 presentation by eps

IMAPS 2003 David Stark - Electronics Packaging Solutions, Inc. 1

Unique Metal-to-Glass Bonding Process

for Hermetic Packaging of MicroOptoElectroMechanical Systems

(MOEMS) and Other Applications


HERMETIC

Completely sealed, especially against the escape or entry of air. (airtight)


MicroOptoElectroMechanical Systems (MOEMS) and Other Electro-Optics Devices

Often Require a Transparent Window Assembly

Packaging Options Include:

• Packaging one or more devices in an individual package

• Wafer-level packaging (WLP), where a window or window assembly is attached to a substrate containing multiple devices


AGENDA• Characteristics of Kovar and Glass• Common Glass-to-Metal Sealing Methods

– Compression Seals, Metal-alloy Solder Seals and Solder-glass Seals

• Attributes of a More Efficient Hermetic Window Assembly Process

• Diffusion Bonding Process• Kovar-to-glass Diffusion Bonding Trials• Near-Term Plans• Summary of Technology Development• Approved and Pending Patents


KOVAR’S ALPHA* = INSTANTANEOUS CTE (CTE AT A SPECIFIC TEMPERATURE)

4.05.06.07.08.09.010.011.012.013.014.015.016.017.018.019.020.0

-175

-150

-125

-100 -75 -50 -25 0 25 50 100 150 200 250 300 350 400 435 450 500 550 600 650 700 750 800 850 900 950 1,000

1,050

1,100

CT

E (p

pm/0 C

)

Temperature (Degrees C)

6.6 ppm/0C @ 250

4.3 ppm/0C @ 3000

18.1 ppm/0C @ 7500


KOVAR’S AVERAGE CTE FROM 300C

4.55.56.57.58.59.510.511.5

200 300 400 450 500 600 700 800 900

Avg

. CT

E in

ppm

/0 C

Degrees C


TYPICAL THERMAL EXPANSION-TEMPERATURE CURVE FOR GLASSES

With few exceptions, the length and the volume of glasses increase with increasing temperature (positive coefficient.

• At about room temperature (section A), the curve shows a distinct bend and then gradually increases up to the beginning of the experimentally detectable plastic behavior (section B = quasi-linear region).

•A distinct bend in the extension curve characterizes the transition from the predominantly elastic to the more plastic behavior of the glass (section C = transformation range).


TYPICAL VISCOSITY-TEMPERATURE CURVE FOR GLASS

At temperatures around the Softening Point, glass products deform rapidly under their own weight (forming by "sagging"). The softening point is defined as the temperature at which the glass has a viscosity of 107.6 poise . (ISO 7884-3)


COMMON GLASS-TO-METAL SEALING METHODS

Compression Seals, Metal-alloy Solder Seals and Solder-glass Seals• Assembly Methods

• Advantages and Disadvantages


COMPRESSION SEALS1. Fabricate Frame

A. Create Opening for Glass WindowB. Oxidize Frame

2. Fabricate Window to fit into frame’s opening

3. Fuse window to oxidized wall of frame opening

A. Fixture (constrain) glass in Z-axisB. Heat unit above Tg of glass to allow glass to

expand and flow against frame wallC. Cool unit

• Frame shrinks more than glass window, holding window in compression

4. Grind and polish glass5. Metal plate frame (typically Au over Ni)



4.55.56.57.58.59.510.511.5

200 300 400 450 500 600 700 800 900

Avg

. CT

E in

ppm

/0 C

Degrees C

Typical Compression

Bonding Temperatures


COMPRESSION SEALS (CONT.)

Advantages:• Compression holds glass in place

• Frame contracts more than glass during cooling

Disadvantages:• Numerous process and handling

steps after bonding• High potential to cause scrap

• Very expensive


Window

Frame’s Flange

Window Metallized On

Its Bottom

Solder Preform

Frame’s Seal Ring

SOLDERED GLASS-TO-METAL SEALS USING A PRE-METALLIZED WINDOW AND A SOLDER PREFORM (METAL-ALLOY SOLDER SEALS)


Kovar’s Average CTE From 300C

4.55.56.57.58.59.510.511.5

200 300 400 450 500 600 700 800 900

Avg

. CT

E in

ppm

/0 C

Degrees C

Metal-Alloy Solder Seals


METAL-ALLOY SOLDER SEALS (CONT.)

Advantages:

• Large production base of required process equipment

• Simple solder-bonding procedure• Typically performed in a chain furnace with a

reducing atmosphere• Readily available solder preforms easily die-cut to

size. • Eutectic Au/Sn is most commonly used solder

alloy.


METAL-ALLOY SOLDER SEALS (CONT.)

Disadvantages:• Windows must be metallized to be soldered to frames

• Typically thin films• Application of the thin film metals can cost more

than the glass• The cooled glass is usually under tension, and often

cracks• The solder bonding the frame to the window might

reflow during the welding of the window assembly to the package base, allowing delamination of the window.


SOLDERED GLASS-TO-METAL SEAL USING SOLDER-GLASS (PREFORM OR PASTE)

Kovar Frame’s Flange

Window

Window

Solder-Glass Preform or Paste

Kovar Frame’s

Seal Ring


KOVAR’S AVERAGE CTE FROM 300CA

vg. C

TE

in p

pm/0 C

Degrees C

5.5

8.5

4.5

6.57.5

9.510.511.5

200

300

400

450

500

600

700

800

900Majority of

Solder-Glass Seals

Minority of Solder-Glass

Seals


SOLDER-GLASS SEALED PHOTODIODES


SOLDER-GLASS SEALS

Advantages:• Can bond glass to other glasses, ceramics, or metals

without thermally damaging the materials to be joined

• Usually costs less than prior two bonding methods

Disadvantages:• Requires intermediate material (solder-glass)• Must take care to select right solder-glass and seam

welding parameter to prevent reflowing the solder-glass


ATTRIBUTES OF A MORE EFFICIENT

HERMETIC WINDOW ASSEMBLY PROCESS


ATTRIBUTES OF A MORE EFFICIENT HERMETIC WINDOW ASSEMBLY PROCESS

Simplicity• Direct bonding of the frame to glass without an intermediate joining material

• No solder alloy, solder-glass, etc.

• Ability to use finished glass during the glass-to-metal sealing process

• No post-assembly grinding and/or polishing• Any required coatings could be applied either before or after bonding, including anti-reflective (A/R), ultra-violet (UV) and plated metals• Ability to bond concave or convex glass to the metal frame


PROCESS APPROACH: DIFFUSION BONDING

Diffusion BondingJoints can be made between similar and dissimilar metals, alloys, and nonmetals, through the action of diffusion of atoms across the interface, brought about by the bonding pressure and heat applied for a specified length of time.


PROCESS PROFILE FOR CONVENTIONAL DIFFUSION BONDING

Tem

pera

ture

Time

A B C

The load (pressure) is applied during time zone B.


PROPERTIES OF JOINTS FORMED BY 3 BONDING PROCESSES

Particulars Diffusion Bonding Brazing, Soldering Fusion Welding

Warpage None Light Heavy

Disassembly of joint No Yes NoVibration survival Very high High Low

Corrosion resistance Fairly high Low Satisfactory

Strength That of parent metal or material That of solder Close to that of

parent metal

Bonding Adhesive, diffusion Cohesive, adhesive Cohesive

Susceptibility to solidification cracking

None Weak Strong

Porosity None Blowholes, shrinkage, diffusion

Shrinkage, blowholes


VACUUM DIFFUSION BONDING SYSTEM (VACUUM HOT-PRESS)

System with controller (left), vacuum hot-press chamber (center) and vacuum diffusion pump (right)

Radiant heat box inside the vacuum chamber: The working zone


SEMI-CONTINUOUS VACUUM FURNACES

Valve Valve

Load / Preheat Cooling / Unload

Load UnloadTransfer Transfer

Vacuum or Other

Atmosphere

High Heat

EvacuatePreheatDegas

Hot PressDiffusion Bond

Gas QuenchPost Process


SEMI-CONTINUOUS VACUUM FURNACES

Semi-Continuous Furnace - Processing 2000 lb loads on 65 by 65 inch grids. Load/unload and return conveyor system.

Advanced Vacuum Systems, Inc. Ayer, Massachusetts


Continuous Hot Press

Chugai Ro Co.,Ltd. Osaka, Japan


Glass-to-KovarBonding Trials


MATERIALS FOR OCTOBER 2002 KOVAR-TO-GLASS DIFFUSION BONDING TRIALS

From left to right: • Oxidized kovar frame• Non-oxidized kovar frame• Corning 7056 glass

Kovar Frame:Outside Dimension: 1” SquareWall Width: 0.040”Wall Height: 0.140”Corner Radius: 0.02” ID; 0.06” OD

Corning 7056 Glass Piece:Dimensions: 1.6” Square by 0.135”


CORNING 7056 GLASS PROPERTIES

Characteristic Value

Viscosity

Working Point (104 poise) 1058°C

Softening Point (107.6 poise) 718°C

Annealing Point (1013 poise) 512°C

Transformation Temperature (Tg) N/A

Strain Point (1014 poise) 472°C

Thermal Coefficient of Expansion (0-300°C) 5.15 x 10-6/°C

CTE (25°C to set point 679°C) 5.45 x 10-6/°C

Sheet Thickness 0.135”


CORNING 7056 GLASS PROPERTIES (CONT.)

Element ValueSilicon < 35 %Potassium < 10 %Boron < 10 %Aluminum < 2 %Sodium < 1 %Lithium < 1 %Antimony < 1 %Arsenic < 1 %


Fixturing of the kovar frames and the glass inside the diffusion bonding

chamber.

Kovar frame, pre-oxidized

Base

Kovar frame,not pre-oxidized

Glass

Spacers

AppliedPressureHydraulic

Ram

DIFFUSION BONDING OF GLASS TO KOVAR FRAMES


TYPICAL VISCOSITY-TEMPERATURE CURVE FOR GLASS

At temperatures around the Softening Point, glass products deform rapidly under their own weight (forming by "sagging"). The softening point is defined as the temperature at which the glass has a viscosity of 107.6 poise . (ISO 7884-3)

Diffusion Bonding

Zone



Degrees C

4.55.56.57.58.59.510.511.5

200 300 400 450 500 600 700 800 900

Avg

. CT

E in

ppm

/0 CBonding

Trials: Load Applied between 4500C to

6650C

Corning 7056’s

Softening Point: 7180C

Corning 7056’s Average CTE:

5.15 ppm/ 0C from 0-3000C 5.45 ppm/ 0C from 2500C to 6790C


RESULTS AND CONCLUSIONS FROM CORNING 7056 TRIALSUnable to Achieve Evidence of Diffusion Bonding of the Glass to the Kovar Alloy

Four of the five diffusion-bonding trials produced limited optical distortion of the glass adjacent to the bond area. This distortion was a result of the displacement of the glass away from the frame during creep.

Limiting the creep reduces the width of the optically distorted region on each side of the frame.

Different temperature, pressure and time parameters than those used previously will be required for Corning 7056 glass to achieve not just mechanical bonding to kovar but also the desired diffusion bond.

Glasses with different chemical compositions, viscosities and thermal properties than the Corning 7056 might be required.


SECOND SET OF TRIALS, JANUARY 2003:SCHOTT AF-45 GLASS

Characteristic Corning 7056 Schott AF-45Viscosity

Working Point (104 poise) 1058°C

Softening Point (107.6 poise) 718°C 8830CAnnealing Point (1013 poise) 512°C 6630C

Transformation Temperature (Tg) 6620C

Strain Point (1014 poise) 472°C 6270C

Thermal Coefficient of Expansion (0-300°C) 5.15 x 10-6/°C

Coefficient of Expansion (20-300°C) 4.5 x 10-6/°C

CTE (25°C to set point 679°C) 5.45 x 10-6/°C

Sheet Thickness 0.135” 0.044”


COMPARISON OF GLASSES’ CHEMICAL COMPOSITIONS Element Corning 7056 Molecule Schott AF-45Silicon < 35 % SiO2 49.6 %

Potassium < 10 % K2O

Boron < 10 % B2O3 14.2 %

Aluminum < 2 % Al2O3 11.4 %

Sodium < 1 % Na2O3

Lithium < 1 %

Antimony < 1 % Sb2O3

Barium BaO 24.1 %

Arsenic < 1 % As2O3 0.9 %


KOVAR’S AVERAGE CTE FROM 300C A

vg. C

TE

in p

pm/0 C

Degrees C

4.55.56.57.58.59.510.511.5

200 300 400 450 500 600 700 800 900

Diffusion Bonding Temperature Zone Used For Loading

Schott AF-45 Glass: 7000C to 8000C

Temperature Zone Used For Loading

Corning 7056 Glass:

4500C to 6650C

Corning 7056’s


Schott AF-45’s



TRIAL #1 WITH SCHOTT AF-45 GLASS

Two kovar frames, 1.3” x 3.7” with 0.08” wide surface, diffusion bonded to a 4” square by 0.044” thick piece of Schott AF-45 glass.

Left Photo: Assembly held verticallyRight Photo: Assembly on grid paper


TRIAL #2 WITH SCHOTT AF-45 GLASS

Two 1.3” x 3.7” frames, each on a 2” x 4” piece of glass before bonding

Glass diffusion bonded to frame:• Glass outside frame perimeter cracked during bonding cool-down

•Glass bonded to, and remained on 100% of frame’s surface


CONCLUSIONS FROM SCHOTT AF-45 GLASS BONDING TRIALS

Destructive tests demonstrated that the glass-to-metal seal is permanent

• Dropped bonded assemblies onto concrete from a height of 5 feet, shattering the glass inside and outside the perimeter of the metal frame

• The metal frames still had 100% coverage of glass


CONCLUSIONS FROM SCHOTT AF-45 GLASS BONDING TRIALS (cont.)

1. The thickness of the frames’ oxidation layer may be critical • This variable requires controlled tests

2. Increasing the surface area of the frame in contact with glass improves bonding• Thinner walls act as knife edges forced through butter

3. The CTE of AF-45 glass is too low for reliable diffusion bonded kovar-and-glass window assemblies

4. Balanced construction might be beneficial• Kovar on top and bottom of glass to equalize shear stress on the

bonded glass


AF-45 GLASS SANDWICHED BETWEEN TWO KOVAR FRAMES

Each Frame’s Size: 1.30” x 1.35” with 0.080” wall thicknessGlass Size: 1.8” x 1.8” x 0.044”


SANDWICHED AF-45 GLASS (CONT.)

Initial Samples Held Vacuum To 10-7 Torr• 1 torr = 1 / 760 of an atmosphere at sea level

• 10-7 torr = 1.3 x 10-9 atmospheres•Process optimization would further improve the hermetic seal


OVARALL CONCLUSIONS FROM BONDING TRIALS

Proved the Feasibility of Diffusion Bonding a Metal Alloy (Kovar) Directly to Finished

Glass to Produce Hermetic Window Assemblies

• The bonding process does not affect the optical properties of the windows

•The bonding was performed below the softening temperature of the glass windows


OVERALL CONCLUSIONS FROM BONDING TRIALS (cont.)

The properties of the glass may be critical• Thermal, chemical

• Higher-temperature glass bonding may provide more atomic mobility

The kovar oxidation method and thickness may be critical

Wider frame widths may be desirable for better bonding


PLANS FOR ELECTRONICS PACKAGING APPLICATIONS

Identify other glasses for future tests Better average CTE match to kovar Chemical elements probably similar to those found in the glass-to-kovar

bond of Schott AF-45 Probably higher softening temperatures

Greater atomic mobility at bonding temperatures

Investigate using a different frame material than kovar Better average CTE match to glass for elevated bonding temperatures

Initiate tests of borofloat glasses with an identified frame material for wafer-level packaging (WLP) using diffusion bonding


SUMMARY• Proved feasibility of diffusion bonding metal to glass to

produce hermetic window assemblies• Diffusion-bonded glass-to-metal seals are inherently

stronger, more reliable and more hermetic than any other bonding method

• Diffusion bonding uses fewer process materials and steps than alternative bonding methods– Potential for lower/lowest cost

• Diffusion bonding may be used to join transparent materials other than glass to metals and alloys – Sapphire, quartz and other natural and synthetic materials


STATUS OF PATENTS

Approved Patents# FILING

DATEPATENT APPLICATION

NAMEPATENT

TYPESTATUS

1 22-Mar-2002 HERMETICALLY SEALED MICRO-DEVICE PACKAGE WITH WINDOW

Non-provisional

Issued on Sept 30, 2003

2 26-Apr-2002 HERMETICALLY SEALED MICRO-DEVICE PACKAGE USING COLD-GAS DYNAMIC SPRAY METAL DEPOSITION

Non-provisional, Continuation In-Part

Approved


STATUS OF PATENTS (cont.)

Patents Pending Review# FILING

DATEPATENT APPLICATION NAME PATENT TYPE

1 13-Mar-2003 HERMETICALLY SEALED MICRO-DEVICE PACKAGE WITH WINDOW

International Patent Application filed under the Patent Cooperation Treaty (PCT)

2 30-Jul-2003 HERMETICALLY SEALED MICRO-DEVICE PACKAGE WITH WINDOW

Continuation filing on 1st approved patent

3 15-Nov-2003 WAFER-LEVEL HERMETIC MICRO-DEVICE PACKAGES

Continuation-in- part


STATUS OF PATENTS (cont.)

Provisional Filings# FILING

DATEPATENT APPLICATION NAME

1 27-Jan-2003 METHOD AND APPARATUS FOR PRODUCING HERMETIC WINDOW ASSEMBLIES

2 27-Jan-2003 METHOD FOR FABRICATING A FRAME ASSEMBLY3 14-Mar-

2003METHOD AND APPARATUS FOR PRODUCING HERMETIC INSULATED WINDOW ASSEMBLIES FOR RESIDENTIAL AND COMMERCIAL BUILDING CONSTRUCTION AND OTHER NON-BUILDING CONSTRUCTION APPLICATIONS

4 27-Mar-2003

METHOD AND APPARATUS FOR PRODUCING LAMINATED, STRENGTH-REINFORCED SAFETY WINDOW ASSEMBLIES


FOR INQUIRIES AND/OR PRESENTATION COPIES, PLEASE CONTACT:

David StarkElectronics Packaging Solutions, Inc.

31252 Island DriveEvergreen, CO 80439 USA

Phone: (303) 674-1197E-mail: [email protected]

APPROVED AND PENDING PATENTS The information contained in this presentation has been disclosed in two approved, three pending and four provisional U.S. patents, in addition to an international patent application.

imaps 2003 presentation by eps

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