Wb Course Part 1

Wafer Bonding 18th June 2009 Course Tutor: Tony Rogers, Applied Microengineering Ltd, UK

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Course Schedule 09:00

Introduction of Tutor and Participants

09:15

Wafer Bonding: Theory and Background (anodic bonding).

09:45

Practical issues for anodic bonding (types of glass, flatness control, compatible materials, temperature limitations, interconnect methods, alignment etc.)

10:45

Break

11:00

Wafer Bonding: Theory and Background (Si direct bonding)

11:45

Comparison of various wafer bonding techniques (anodic, silicon direct, glass frit, adhesive, eutectic).

12:45

Lunch

13:45

Wafer bonding techniques – continued

14:15

Applications of wafer bonding

14:45

Break

15:15

Review of commercially available bonding equipment

16:15

Q+A session

16:45

Close

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Types of Wafer Bonding Wafer bonding processes include: • Anodic bonding (ref’s 1-12, 48) • Direct (fusion) bonding (ref’s 19, 34-36, 44-46)

• Glass frit bonding (ref 28) • Eutectic bonding (ref 29,30,31,50) •Solder bonding (ref 25) • Adhesive bonding (ref 19,20,33) •Thermo-compression bonding (ref 19,20,49)

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Energy Content of various Bond Types Bond type

Energy content (kJ/mol)

Ionic bonds

590 – 1050

Covalent bonds

563 – 710

Metallic

113 – 347

Van der Waals (intermolecular) bonds:

4 – 42 (H2)

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Energy Content Vs Interatomic Distance for various Bond Types Therefore wafers need to be polished to (
Ref 33. AML – Wafer Bonding Machines & Services

Anodic Bonding Theory & Background

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Anodic Bonding Theory & Background •Process discovered in 1969 by Wallis & Pomerantz (1)

•Also known as Electrostatic Bonding, Field-Assisted Bonding, or Mallory Bonding

•Primarily used for bonding Silicon to Glass

•Other material combinations have been demonstrated (2, 3) AML – Wafer Bonding Machines & Services

Features of Anodic Bonding •Bonding temperature is below the softening temperature of the glass •Thermal expansion of the two materials needs to be well matched •Materials must be polished to less than 5 nm and flat (4) •Process temperatures typically in the range 300 –500oC •Applied voltages between 100V and 2kV •Glass needs to contain mobile ions AML – Wafer Bonding Machines & Services

Description of the Bonding Process (1 of 5) •The two wafers are heated to the required temperature •The wafers are brought into contact •Voltage applied (glass negative) •Ionic movement in the glass due to electrolysis (5) •Depletion of mobile ions at the Si-glass interface •Voltage drop in the depleted layer produces large electrostatic attractive force at the interface

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Description of the Bonding Process (2 of 5) •Wafers are pulled into intimate contact •Oxygen liberated at the silicon:glass interface (6,7) •Anodic oxidation of the silicon occurs leading to high strength Si-O-Si chemical bond (7,8) •Bond is irreversible (9) •Composition of the glass is now inhomogeneous (10, 11) •Bond is hermetic

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Description of the Bonding Process (3 of 5) Anode

-

+

-

+

-

Glass

Silicon

+ +

-

Cathode

O

Na +

-

+

-

+

-

+

Depletion layer (high field)

Bulk (low field)

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Description of the Bonding Process (4 of 5)

ref. (4)

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Description of the Bonding Process (5 of 5)

ref. (4)

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Bonding current and the Sodium depleted layer (source – SensoNor) - Thermal activation energy for the sodium ions: 0,97 +/- 0,14 eV 3

Bond temp 400 OC

1000 V

2.5

800 V

2

600 V 400 V

1.5 1

3.5

I [mA]

I [mA]

3.5

3 2.5 2 1.5 1

375 °C 350 °C 300 °C

0.5

0.5 0 0

20

40

60

80

100

0 0

50

The depth of the depleted layer [µm]

1.2 1 0.8 0.6 0.4 0.2 0 0

200

400

600

800 1000

The applied voltage [V]

100

150

200

Time [sec.]

Time [sec.] The depth of the depleted layer [µm]

Bonding at 1000 V

400 °C

1.2 1 0.8 0.6 0.4 0.2 0 300

325

350

375

400

The bonding temperature [°C]

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The bonding pressure for “bulk” bonding(source – SensoNor) Bonding pressure ~20000 atm with bonding on bare silicon at 800V Bonding at 1000 V and 400 degC

Bonding at 400 degC 40

25

[103 atm]

0.1µm oxide 15 10

increasing oxide thickness

5 0 0

0.02 0.04 0.06 0.08

0.1

Wafer separation air gap [µm]

The electrostatic pressure [103 atm]

20

The electrostatic pressure

without oxide

without oxide 30 20

0.1 µm SiO2 10

0.3 µm SiO2 0 0

500

1000

The applied voltage [V]

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Strength of the Anodic Bond •Difficult to measure because when the bonded sample is pulled apart, the break occurs in the glass and not at the Si-glass interface

•Values for pull tests are typically 30- 40Mpa (glass fracture) (12)

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Standard Bonding - Typical conditions Glass: 100mm diameter borofloat 33, 0.5 mm thick, could have some metallisation (eg Ti/W:Au) Silicon: 100mm diameter, 500µm thick, could have etched features (type (p or n), resistivity, orientation not important) Temperature: 370°C Voltage: 800 V Time (that high voltage is applied): 10 minutes Pre-clamping pressure: 100N (greater pressure exerted by electrostatic forces anyway, but pre-pressure helps with I vs t reproducibility) Alignment accuracy: +/- 5µm (limited by TCE runout) Total cycle time: 30 minutes AML – Wafer Bonding Machines & Services

Benefits of Anodic Bonding

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Benefits of Anodic Bonding •Low bonding temperature giving more process flexibility (e.g. can bond to metallised wafers without suffering diffusion of the metals into the wafer) •Thermally matched, low stress bond producing stable mechanical dimensions over a wide temperature range •Procedure can be optimised for producing flat assemblies •No measurable flow of the glass occurs, hence sealing around previously machined grooves, cavities etc. without any loss of dimensional control AML – Wafer Bonding Machines & Services

Benefits of Anodic Bonding •Since glass is an electrical insulator, parasitic capacitances are kept extremely small •Hermetic seals – the bonding process can readily be performed in vacuum, allowing hermetically sealed cavities to be formed (or the sealing-in of a special gas mixture) •Glass transparency at optical wavelengths enables simple, but highly accurate, alignment of pre-patterned glass and silicon wafers. The transparency can also be exploited via optical addressing, and to see inside microfluidic devices

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Benefits of Anodic Bonding •High yield process – tolerant to particle contamination and wafer warp •Low cost wafer scale process for wafer scale packaging (note: bonding can be done at the chip level when required) •High bond strength – higher than the fracture strength of the glass •Multi-stack bonding is possible •Anodic Bonding Review Paper ref 48 AML – Wafer Bonding Machines & Services

Practical Issues for Anodic Bonding

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Control of Bonding Current (1 of 2) • Anodic bonding has conventionally been done under voltage limited conditions • In this mode there is typically a high current spike (~40mA) at the start • With a voltage of ~1kV this spike dissipates ~40W at the interface • At the start of the bond only selected wafer areas are in intimate contact • Therefore Joule heating is large and concentrated, causing hot spots AML – Wafer Bonding Machines & Services

Control of Bonding Current (2 of 2) •The hot spots change the local stress and result in device to device variation • The use of current limiting results in a gentle increase in voltage, and more uniform, lower power dissipation and hence better device to device repeatability • This has been demonstrated to significantly improve the yield in the manufacture of MEMS gyroscopes for which stress can significantly affect temperature sensitivity • Current limiting also results in fewer rejects due to voltage breakdown in the glass. AML – Wafer Bonding Machines & Services

Anodically Bonded Structures some Design Issues Diaphragms When bonding to wafers with thin diaphragms, these can be electrostatically attracted to the glass during bonding and become permanently bonded. Possible solutions are to coat the glass in the cavities with gold, or to back etch the silicon to thin the diaphragms after bonding.

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Bonding of wafers with flexible structures (source – SensoNor) V cathode V cathode Vd ~ V

depleted layer

glass glass

silicon

0V Non-bonding overload protection

silicon

Pel screen electrodeglass press contact

0V • Electrostatic collapse • Bonding for structure

silicon

Non-bonding overload protection by use of shield electrodes AML – Wafer Bonding Machines & Services

Anodically Bonded Structures some Design Issues Access to the silicon surface (e.g. for bond pads) This can be achieved either by pre-machining through holes in the glass or by using additional depth-controlled cuts during the dicing process.

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Suitable Glasses Need good match of TCE to silicon – hence borosilicate glasses Manufacturer

Designation

Comments

Corning

7740

Used to be the standard glass for anodic bonding, but Corning no longer make it

Corning

7070

Has high bonding temperature and low softening point.

Hoya

SD1, SD2

Expensive. Suffers from severe staining and electrical breakdown. This can be greatly reduced by backside metallisation.

Schott

Borofloat 33 Float glass. Available in a wide variety of thicknesses. Std low cost glass for anodic bonding – all wafer sizes

Pilkington

CMZ

Specifically made for solar cells. Available only in thin sections. Expensive.

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Glass Properties to Consider • Thermal Expansion Coefficient • Volume Electrical Resistivity • Viscosity (Strain Point) • Chemical Resistance • Bulk Modulus (Stiffness)

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Silicon – Glass Differential Expansion

Ref (13)

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Electrical Resistivity of Glass • The glass electrical resistivity is important when bonding to wafers with oxide or nitride coatings, especially when there are also micromachined cavities defined in these coatings This is because the percentage of the applied voltage that is dropped across the coating / cavity depends on the ratio of the coating resistance to the glass resistance at the bonding temperature.

• Too high a voltage across thin insulating layers and cavities can cause electrical breakdown

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Resistivity

ref. (13) AML – Wafer Bonding Machines & Services

Glass Viscosity Considerations • It is necessary to bond at a temperature well below the strain point of the glass to achieve bonding around micromachined cavities whilst preventing any measurable flow of the glass, which could compromise dimensional control. • For 7740 glass (strain point 510oC), with a normal bonding temperature of ~400oC, glass flow is not a problem. However, for 7070 glass (strain point 456oC) with a normal bonding temperature of ~450oC, glass flow becomes an important consideration. AML – Wafer Bonding Machines & Services

Wafer Stiffness The stiffness of the wafers can be measured in terms of the flexural rigidity which is given by: F = Eh3/(12(1-ν2) Where E = Bulk Modulus h = wafer thickness ν = Poisson Ratio

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Flexural Rigidity of Various Glass and Silicon Wafers Glass Type

Thickness Flexural (mm) Rigidity (Nm)

Stiffness Ratio Glass / 0.5mm <111> Si

Stiffness Ratio Glass / 0.5mm <100> Si

7740

1

5.45

2.69

3.68

7740

0.5

0.68

0.33

0.46

7070

1

4.47

2.20

3.02

Si <111>

0.5

2.03

-

-

Si <100>

0.5

1.48

-

-

Si <100>

0.1

0.012

-

-

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Wafer Flatness Considerations • Silicon wafers are not necessarily flat. Standard SEMI specifications for Si wafers allows a warp / bow of +/-25μm • AML’s experience with glass wafers is that these tend to be much flatter than the Si (typical bow <5 μm) • When the wafers are bonded, the bow in the Si may be removed resulting in stress at the interface – depending on the relative stiffnesses of the two wafers.

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Flatness Control Two sources of bow – thermal; compositional Can minimise thermal bow by selecting the optimum temperature for thermal mismatch (provided this is hot enough to allow bonding to take place). Composition of glass is permanently altered by the flow of current during anodic bonding. Excess of Na+ at free surface, depletion of Na+ at bond interface. This causes the free surface to go into compression. This needs to be taken into account for final flatness.

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Compatible Materials Interbond Metallisation – Al no good (oxidation of the Al)

Bonding to oxides (thermal and LPCVD – OK, PECVD – NO)

Bonding to nitrides (LPCVD OK, PECVD - NO)

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Compatible Materials

ref. (13)

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Temperature limitations Generally must have > 300°C (to get sufficient ionic conduction)

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Interconnect Methods Can use ordinary metal tracks if hermetic seal not needed, or if only 1 lead across the seal

If need hermetic seal, can use thin Ti/W:Au (up to 50nm), or use the buried via technique (SensoNor) AML – Wafer Bonding Machines & Services

Interconnect Methods Vertical feedthroughs These can be either in the glass or in the silicon, and are perhaps the most versatile and reliable method of providing electrical connection across the seal.

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Interconnect Methods Example of a vertical feedthrough in glass

ref. (16)

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Surface Quality More tolerant of particle contamination than fusion bonding, because electrostatic force assists movement of bonding front. Permitted surface roughness a few nm, versus a few Å for fusion bonding. Permitted bow up to 50µm for a standard thickness wafer. Fusion bonding requires a heavy clamping force if there is significant bow.

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Rucking up This problem can occur with very thin substrates, especially where there are few cavities built into the glass or silicon. When bonding is initiated in several places at once (happens when using a large area electrode), bond fronts will converge in places on the wafer, and an unbondable bubble will appear. Can be prevented by using a single central electrode – but bonding will be slower. Can arrange to involve one or more additional rings of electrodes as bond front progresses, but simplest solution is to use a graphite electrode that produces uniform bonding over the whole interface

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Oxygen Generation The process causes oxygen ions to accumulate at the bonding surface of the glass (7) Where there is no silicon to bind to these, oxygen gas is liberated. The shallower the cavity it is liberated into, the greater will be the pressure. This pressure can be considerable. A useful number is the product of pressure and cavity depth (approximately 1bar.micron) This pressure can vary across the wafer, according to how early during the bonding process any particular cavity has become sealed. AML – Wafer Bonding Machines & Services

Oxygen Generation Things to do about this: •Make cavities as deep as possible, or provide a channel to a large volume reservoir •Leave cavities unsealed, and seal as part of the packaging process. •Incorporate a getter inside the cavity

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Effect on CMOS Devices High electrostatic field can have damaging effects on CMOS devices: “ … the electrostatic bonding between silicon and glass affects the electrical characteristics of the silicon. The presence and the movement of the positive and negative ions through the glass, the formation of a thin oxide layer at the interface and the related charging effects, cause changes in the resistivity of the silicon underneath the glass, in the breakdown voltages and the leakage currents of the p-n junctions.(15)”

AML is currently developing a technique that will allow the use of anodic bonding with CMOS devices, provided that the CMOS circuit is designed for compatibility with the anodic bonding process. AML – Wafer Bonding Machines & Services

Bonding to Bulk Micromachined Wafers • Micromachining of both the Si and glass wafers is possible prior to bonding • All standard wet and dry etching techniques can be used for the Si • Glass machining can be achieved via: ¾Ultrasonic machining ¾Laser processing ¾Water-jet machining ¾Powder blasting ¾Chemical etching AML – Wafer Bonding Machines & Services

Bonding to Surface Micromachined wafers • Bonding to silicon that has surface layers of thermal oxide and LPCVD nitride has been demonstrated • PECVD layers can be problematic due to the presence of hydrogen, which makes the oxygen unavailable for bonding to the silicon • Hermetic sealing can be achieved over metallisation tracks provided that the step height is <50nm (13). • Readily oxidisable metals such as aluminium should be avoided (13).

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Thin Film Bonding Can bond two silicon wafers together, using a thin sputtered, evaporated, or sol-gel deposited glass film. General problem is there is usually not much thickness of glass. Therefore limited supply of Na+ and O 2- ions Sputtering is a slow process, but should result in a layer with the same composition as the sputtering target. Evaporation is another method that can be used Sol-gel is currently under investigation as a possible method

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Thin Film Bonding General issues with thin-film bonding: • Flatness control • Thickness control (electrical breakdown) • Roughness control

•Therefore we recommend that unless there are good specific reasons for using this technique then it should be avoided

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Multi-stack Bonding • Can build up more complex sandwiches of glass and silicon. • Both silicon:glass:silicon & glass:silicon:glass are possible • Both can be performed using a two-stage bonding process • Often find that 2nd bond is more difficult to make than the first, usually because a finite amount of bow has been introduced, and the assembly is now also stiffer than just the silicon. With AML bonders we have a simple process for both AML – Wafer Bonding Machines & Services

Multi-stack Bonding Si-glass-Si •This can be done in two stages; i.e. one silicon wafer is bonded to the glass. Then the 2nd bond is made •With AML bonders we have a simple process for doing this 2nd bond • The bonded wafer pair is mounted on the upper platen, glass side down, and the 2nd bond done as normal • It is often necessary to use slightly higher temperature / voltage to overcome the bow of the first bond • Alignment can be achieved using IR optics, or visible if there are appropriate holes machined in the Si AML – Wafer Bonding Machines & Services

Multi-stack Bonding glass:Si:glass •This can be done in two stages; i.e. one glass wafer is bonded to the silicon. Then the wafers are turned over, and the other glass wafer is bonded on. • The problem is making electrical connection to the central Si wafer •With AML bonders we have a simple process for doing this • Alignment can be achieved for all three wafers provided the Si wafer is patterned on both sides • Care needs to be taken not to bond the lower glass to the platen! AML – Wafer Bonding Machines & Services

Multi-stack Bonding in One Step With appropriate design and switching of the electrode HT supplies, the operation can be carried out in one step. This minimises the problem of bow. More difficult to achieve alignment with one-step bonding (especially if both glass wafers need to be aligned to the silicon)

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Silicon Direct Bonding Process Hydrophylic bonding

Hydrophobic bonding AML – Wafer Bonding Machines & Services

Schematic of hydrophilic direct bond formation, before annealing (ref 19) AML – Wafer Bonding Machines & Services

Silicon Direct Bonding Process

•Bring clean, polished surfaces into intimate contact

• Forms reversible low strength bond •Heat to form stable / permanent bond •Results in nearly perfect high-strength bond AML – Wafer Bonding Machines & Services

Bonding Kinetics – first contact • Attractive forces – hydrogen bonding / van der Waals force • Repulsive forces – strain energy of wafer bow / warp • Attractive force across bonded area must counteract repulsive force of the induced strain in wafers as the surfaces are brought into intimate contact • Initial hydrogen bond converted to higher strength bond by heat / time / pre bond activation treatment AML – Wafer Bonding Machines & Services

Direct Bonding Video

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Crack Propagation (Si-Si direct bond, post 450C anneal)

Video of crack propagation

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Strained Attraction • Surface bow and waviness can be overcome by elastic deformation of the wafers. Surface roughness can be overcome if the range of the hydrogen bonding attraction is large enough (e.g. water molecule triplets (ref 19 p.59)) • Bonds can self propagate from an initial contact point if the range and magnitude of the attractive forces is high enough to overcome the roughness, waviness and bow of the wafers • Wafers can be forced into contact. If the bond area is made large enough the bond may not delaminate (ref 34)

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SiO2-SiO2 Hydrophylic Direct Bonding SiO2 Surface

SiO2 Surface

•Pre-Bond Hydration–SiO2 terminates in Si dangling bonds that react with water •Heating (200ºC) – Hydrogen bond bridging develops between OH groups •Post-Bond (>300ºC) –Hydrogen bonds are replaced with Si-O-Si bonds • Further anneal at ~1000oC for full strength AML – Wafer Bonding Machines & Services

Si-Si Hydrophobic Direct Bonding To prepare wafers for hydrophobic bonding it is necessary to remove the surface oxide. This is normally done using an HF dip After the dip the wafers can optionally be rinsed in DI water. If this is done then the mechanism for bond front propagation is essentially the same as for the hydrophylic case

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Si-Si Hydrophobic Direct Bonding Silicon Surface

•Pre-Bond Hydration –dangling Si bonds react with water •Heating (200ºC) – Hydrogen bond bridging develops between OH groups Silicon Surface

•Post-Bond (>300ºC) – Hydrogen bonds are replaced with Si-O-Si bonds •Completion (>500ºC) – Si-O-Si bonds are replaced with Si-Si

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Si Hydrophylic Direct Bonding – Process info Hydrophilic: RCA Clean 25°C - 110°C: Hydrogen bonds 110°C - 200°C: Si + 2H2O → SiO2 + 2H2 200°C - 700°C: Si — OH + OH — Si → Si – O – Si + H2O >700°C: Increasing contact area; further hydrogen removal AML – Wafer Bonding Machines & Services

Si Hydrophobic Direct Bonding – Process info Hydrophobic: (HF Dip) 25°C - 400°C: van der Waals 400°C – 700°C: H desorption >700°C: H completely removed: bond strength equal to bulk Si

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Si-Si Direct Bonding – Bond Strength

Bond strength vs. anneal temperature for hydrophilic / hydrophobic bonding. AML – Wafer Bonding Machines & Services

Si-Si Hydrophobic bonding For applications whereby the bond interface needs to be oxide-free then it is best to omit the water rinse after HF dip The bond propagation is the driven by fluoride bonding rather than hydrogen bonding

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Wafer Bond Strength SiO2-SiO2 Bond Strength • increases with increasing temperature

• does not depend on oxide thickness • not a strong function of annealing time for temperatures below 1200ºC • increases with annealing time for temperatures above 1200ºC • Easier to form pre bond, lower requirements of wafer surface w.r.t. Si-Si bond

Si-Si Bonds Strength • very strong -bonds performed at 500ºC are equivalent to SiO2-SiO2 bonds performed at temperatures > 1000ºC • Si-Si bonds performed at > 600ºC can be too strong to measure • More difficult to form pre bond / higher requirements of wafers AML – Wafer Bonding Machines & Services

Quality of a Si Direct Bond Silicon Surface Oxide Interface

Silicon Surface

•Nearly perfect bond •Fractures do NOT follow bond AML – Wafer Bonding Machines & Services

Si-Si Bonds using UHV Direct Bonding If Si wafer are heated up to >850C in UHV -8 Torr), Surface (<2×10Silicon then a super clean Si surface can be achieved, where OH groups, H2O and H is Oxide Interface removed from the surface to leave highly reactive Si. Two wafers can be bonded under these conditions to give a true Si-Si hydrophobic direct Silicon Surface bond. Alternatively a gas can be allowed into the chamber to terminate the Si with selected molecules. This technique can allow the bond to be tuned for specific applications. AML – Wafer Bonding Machines & Services

Imperfectly Bonded Wafers

Particulate

•Void Formation (mm-scale) –insufficient wafer flatness –surface contamination –particulates •Extreme Cleanliness Needed!! AML – Wafer Bonding Machines & Services

Si-Si Direct Bonding – Wafer Specifications ¾ Ra Surface Roughness: <1nm (ideally < 0.5nm Ra) ¾ Flatness / TTV: <2μm ¾ Bow: <25 μm (single peak)

Other Requirements ¾ Recessed alignment marks ¾ Class 10 cleanliness

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Wafer Cleaning

Schematic of wafer surface before cleaning AML – Wafer Bonding Machines & Services

Wafer Cleaning _issues with Organic contamination / H2O inclusion •Depending on the latter annealing stages, organic contamination can cause significant defects in the bond. If a high temeprature anneal step is performed (over 300C) then organics can result in void formation. Cleaned Si surfaces can easily trap organics, especially if hydrophobic. •A UV organic removal cleaning step immediately before bonding can be recommended. •Interface H2O can also cause voiding at elevated temperature – need to optimise surface H2O to minimise voiding, but allow bond propagation. •Appropriate anneal temperature ramp rate can minimise H2O voids (ref 47)

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Wafer Cleaning _issues with Organic contamination / H2O inclusion

IR image of pre bond. Some particle related voids.

IR image of post 800C annealed bond. Voids caused by organic contamination generating gas during the anneal

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Bonding Criterion • Successful bonding requires intimate contact of wafers • Flatness deviations prevent bonding • Moderate flatness deviations can be accommodated through elastic deformation wafer bow

surface waviness

surface roughness

ρ <10 A

10-100 μm 100-1000 μm 100 mm

Bonding is a competition between surface and strain energy. AML – Wafer Bonding Machines & Services

Surface Roughness – Ra can be misleading Same Ra. Overall bond strength is a function of the contact area therefore the greater the surface contact then the higher the bond strength

Hard to bond surface

Easy to bond surface

•Bearing Ratio should be considered in conjunction with Ra AML – Wafer Bonding Machines & Services

Example of a “easy to bond” surface – Bearing Ratio / Firestone Abbot Curve

Example of a “hard to bond” surface– Bearing Ratio / Firestone Abbot Curve

Additional Factors etch pattern

mounting

100 mm 100 mm (Ref’s 35,36)

particles

~1 μm

~1 mm ~1 mm AML – Wafer Bonding Machines & Services

Modeling Approach • Surface forces expressed in terms of work of adhesion • Balance between surface and strain energy • Each flatness deviation increases the strain energy Criterion: inBonding the bonded stack

(Ref’s 35,36)

dU E ≤W dA AML – Wafer Bonding Machines & Services

Axisymmetric Analysis of Bowed Wafers

(Ref’s 35,36)

silicon wafers, E=150GPa, ν=0.22, ρ=10m

w

Strain energy accumulation rate:

where:

advance ratio, R=b/a initial curvature, κo=1/ρo final curvature, κf = 1/ρf

Wafer thickness is critical Must have one compliant wafer

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Effect of Etch Patterns

(Ref’s 35,36)

• Reduce bonding area and energy available to deform wafers • Shallow features – reduce surface energy – do not effect stiffness

• Deep features – reduce surface energy –reduce stiffness AML – Wafer Bonding Machines & Services

Effect of Etch Patterns

(Ref’s 35,36)

Comparison of model results with actual bond. AML – Wafer Bonding Machines & Services

Low Temperature (Plasma) Activated Direct Bonding

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Plasma Activated Direct Bonding • This is an important development in the last few years and has been subject of extensive R&D • Enables full strength Si:Si (SiO2:SiO2) at temperatures as low as 200oC • The mechanism is not fully understood but linked to “dangling bonds” • Very short plasma exposures needed (~30s) makes in-situ process viable • Opportunity for removing expensive, difficult to dispose wet chemicals • Makes direct bonding more attractive process (temperature compatible) • Enables otherwise non-compatible materials to be bonded (eg GaAs:Si, quartz:Si etc.) AML – Wafer Bonding Machines & Services

Plasma Activation / Low Temperature Direct Bonding

Comparison of activated & Non-activated Bond Strength vs Anneal temperature AML – Wafer Bonding Machines & Services

Plasma Activation Process Conditions • Wafers are exposed to brief low power plasma • Typical RIE plasma power of ~50W-200W, • Exposure time ~20 sec to 200 sec • Increased plasma exposure roughens surface • Water dip post exposure - can improve quality but compromises bond strength vs. anneal curve • Oxygen is most common plasma, also N2 and Ar.# • H2 plasma can be used to create hydrophobic surface

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Plasma Bonding Notes • Strong bonds can be achieved at anneal temepratures of 200C

• Bond interface will be porous – • Therefore may not be suitable for hermetic sealing

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Surface Activation Using Radicals (RAD Activation) • Exposing wafer surfaces to Plasma can cause roughening of the surface = narrow process window (ref 44) • Exposing wafer surface only to the Radical components of the plasma achieves bond strength without roughening wafer surface.

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Radical Activation ¾AFM results showing Surface Characteristics vs Activation Time Wafer 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Process

Pressure

Time 2

Plasma

65mTorr

5 10 2

RAD (Air) 60mTorr

5 10

None

-

-

Plasma

200mTorr

2

Sa Phase (˚) 0.51 1 0.84 0.52 0.42 0.34 0.58 0.42 0.16 0.18 0.19 -

Sa Topographic (nm) 0.17 0.18 0.75 0.29 0.26 0.24 0.14 0.1 0.13 0.14 0.12 -

AML data

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Radical-Co-axial Source Gas supply

Source

Earthed screen

Source

Only radicals in this region

DC source

Wafers for in-situ activation R. pump

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Radical Activation Tool

Experimental set-up using glass vacuum chamber – showing electrical discharge confined between the ring electrodes AML – Wafer Bonding Machines & Services

Radical Activation XPS spectra showing comparison of silicon wafer before and after activation by Oxygen Radicals Spectra of untreated and treated silicon

Spectra of untreated and treated silicon Centre

6000

6000

5000

5000

4000

4000

Counts per second

Counts per second

Untreated

3000

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1000

0 1000

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Binding energy /eV

200

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Binding energy /eV

Shows reduced carbon peak post exposure

200

0

Plasma Activation / Low Temperature Direct Bonding •A drawback with plasma activated bonding is the long term stability of the interface. •Electrical measurements have been made on plasma activated wafers which have been stored for 5 years. •These show the capacitance across the interface, and the resistance of the interface changes with time.

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Direct Bonding of Substrates Other Than Si • Low

tempature activated bonding means dis-similar substrates can be joined (CTE mismatch less important) • Surafce roughness/ flatness requirements the same (or better) than Si case, hydrophilic / hydrophobic considerations apply •Important substrates are Quartz – Quartz; Pyrex – Pyrex (pharmasutical); Si – Pyrex (replaces anodic bond) and Si – Sapphire (SOI)

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Dissimilar substrates • Important emerging technology – engineered substrates, e.g. Si on sapphire, Si on Quartz – improved CMOS / optoelectronic performance. (Ref 45,46) • Different CTE materials need to be joined = requirement for low temperature bonding / smart cut process in order to minimise CTE induced strain. • Need to effect the smart cut process without causing bulk material fracture / bond delamination. • After smart cut a high temperature anneal can be used to remove / diffuse interface species (OH groups). AML – Wafer Bonding Machines & Services

Dissimilar substrates • Contacting wafers at intermediate elevated temperature can increase the working range and allow smart cut temperature to be achieved without fracture / delamination. • May also need to contact at high temperatureif the bonded pair is going to need further processing – eg if the TCE mismatch allows a 200C temperature range after bonding and the device needs to work at RT but will require further processing at 400C, the best to contact the wafers at 200C • Note that it is necessary to maintain some water on wafer surfaces and so contacting at higher temperatures may not result in bondfront propagation

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Alignment of Si-Si Bonded Wafers Can use IR optics, “front & back” camera systems (eg EVG), or use wafer movement to enable optical path to both bonding surface (eg Suss) Double side polished wafers, with back-side alignment marks are necessary for the “front & back” method AML – Wafer Bonding Machines & Services

IR Alignment of Si-Si Bonded Wafers

• Accuracy of IR alignment depends on the nature of the Si wafers • Double side polished wafers- resolution of alignment features is dependent on imaging system and wavelength of light, 2μ accuracy possible • Single side polished wafers- resolution limited by the roughness of the back surface, typically 10-20 μ features can be resolved AML – Wafer Bonding Machines & Services

Bond Inspection Techniques • • • • •

3 main techniques for inspecting bond quality Optical transmission, using visible and IR light SAM (Scanning Acoustic Microscopy) XRT (X Ray Topography) TEM (Transmission Electron Topography) limited use but best resolution

ref 19,20

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Optical Transmission • Photon energy hν ≥ Bandgap Eg, photons can break bonds in material and easily be absorbed • If ν < Eg/h then photons can travel through the material • Standard exponential absorption relationship with material type and material thickness , Iz = I0exp(αZ) • Photons can be absorbed by free carriers (hence metals are opaque). Heavily doped materials do not transmit. AML – Wafer Bonding Machines & Services

Material Bandgap vs. Minimum Transmissive Wavelength 2 m a te r ia l m in b a n d g a p (e V )

material bandgap (eV) min wavelength (um) Ge 0.67 1.85 Si at 600K 1.03 1.20 Si 1.12 1.10 Ge at 600K 1.28 0.96 InP 1.35 0.91 AlGaAs 1.42 0.87 GaAs 1.43 0.86 AlAs 2.16 0.57 a SiC 2.2 0.56 b SiC 2.93 0.42 GaN 3.4 0.36 ZnS 3.6 0.34 C 5.48 0.22 AlN 6.2 0.20 SiO2 8 0.15

1.5

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min tramsmission wavelength (um)

• As material temperature increases min transmission wavelength increases • CCD cameras only effective to ~1.1μm so Si at 300C not easy to image through AML – Wafer Bonding Machines & Services

7

8

9

Optical Transmission • For 1.1μ light only voids with a depth of λ/4 can be imaged. • Depth of void can be estimated from number of fringes (N) using D = N×(λ/4 ) • Typically a 1mm void can be seen. Smaller features can be seen using an IR microscope • Non destructive technique • Cheap and quick to setup and get results • Poor resolution relative to other techniques AML – Wafer Bonding Machines & Services

Scanning Acoustic Microscopy (SAM) • Acoustic waves, typically 10-200MHz focused on bond interface, reflected waves measured • For 160MHz scan, resolution of 10μm • Attenuation increases with increasing frequency = better resolution but smaller signal • Sample must be in liquid coupling medium, e.g. DI water. Therefore no use for weakly bonded substrates • Results can be confusing if voids are not homogeneous • Better resolution than optical transmission but more expensive and slower

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X-Ray Topography (XRT) • Offers best resolution (2μ) but only of use for crystalline solids • X-ray beam diffracted according to- n λ=2dsinθ (θ = Bragg angle, d = lattice spacing) • Gives info on voids plus elastic and plastic lattice distortion • Very slow technique, difficult to set up. • Very expensive systems

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Transmission Electron Microscopy (TEM) • Need to thin samples to 1000~3000A • Very high resolution (2A)

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