Wb Course Part 3

Low Temperature Eutectic Bonding ¾There are a range of indium alloys that allow eutectic bonding to be performed at temperatures < 200oC. (Ref 50) ¾This is important for compatibility with materials that cannot be subjected to higher temperatures. ¾ Sufficiently low a temperature to consider using the process for joining dissimilar materials for which a thermal expansion mismatch exists

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Low Temperature Eutectic Bonding (Ref 43)

In Au Phase Diagram AML – Wafer Bonding Machines & Services

The bonding principle of In-Au system: (a) multiplayer Au-In composite on the die converts to In-AuIn2 composite right after deposition; (b) at 157 o C, indium layer melts and turns the composite into a mixture of liquid phase with AuIn2 grains; (c) in 157-180 o C range, more AuIn2 is produced; (d) solidification of the mixture to form a joint below 157 o C. Despite the large mismatch on coefficient of thermal expansion (CTE) between silicon and copper, no die cracking is observed AML – Wafer Bonding Machines & Services

Materials for Adhesive Bonding Epoxies

Thermal or two part curing

UV-epoxies

One of the substrates has to be transparent to UV light)

+ve photoresists UV / thermal curing, selective bonding, weak bond -ve photoresists

UV / thermal curing, selective bonding

Benzocyclobutene (BCB) thermal curing, high yield , selective bonding PMMA

thermal curing, hot melt

Fluoropolymers

thermal curing, chemically very stable bond

Waxes

hot melt, mainly for temporary bond

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Time Scale for Moisture Penetration through various Materials

(Ref 33)

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Typical process steps for adhesive wafer bonding (1/3) 1. Cleaning and drying of the wafers. Remove particles, contaminations, and moisture from the wafer surfaces. 2. Treating the wafer surfaces with an adhesion promoter optional. Adhesion promoters can enhance the adhesion between the wafer surfaces and the polymer adhesive 3. Applying the polymer adhesive to the surface of one or both wafers; patterning the polymer adhesive optional. The most commonly used application method is spin coating. 3a. Polymer patterning (optional) AML – Wafer Bonding Machines & Services

Typical process steps for adhesive wafer bonding (2/3)

4. Soft baking or partially curing of the polymer. Solvents and volatile substances are removed from the polymer coating. Thermosetting adhesives should not be polymerized, or may only be partially polymerized. Thermoplastic adhesives may be completely polymerized, since they can be remelted to achieve bonding. 5. Placing the wafers in the bond chamber, establishing a vacuum atmosphere, and joining the wafers inside the bond chamber. The wafers are joined in a vacuum atmosphere to prevent voids and gases from being trapped at the bond interface. The vacuum atmosphere can also be established after the wafers are joined, as long as trapped gases at the bond interface can be pumped away before the bond is initiated. AML – Wafer Bonding Machines & Services

Typical process steps for adhesive wafer bonding (3/3) 6. Apply pressure to the wafer stack with the bond tool. The wafer and polymer adhesive surfaces are forced into intimate contact over the entire wafer. For thermosetting polymer adhesives, the bonding pressure should be applied before the curing temperature is reached. If thermoplastic polymer adhesives are used, the bond pressure can be applied after the bonding temperature is reached. 7. Remelting or curing the polymer adhesive while applying pressure with the bond tool. The hardening procedure depends on the curing mechanism of the used polymer adhesive. The reflow of the polymer adhesive is typically triggered through elevated temperature. AML – Wafer Bonding Machines & Services

8. Chamber purge, cool down, and bond pressure release.

Polymer adhesive and wafer materials ¾The intermediate polymer adhesive must not release solvents or by-products during the hardening process if the wafer materials are not permeable to gases. ¾Volatile substances get trapped as voids at the bond interface if they evolve from the polymer adhesive after the wafers are joined. The polymer adhesive must provide sufficient wetting of the wafer surfaces and flow or achieve a viscoelastic state during the bonding process. ¾The wafer materials must be compatible with the bonding process e.g., temperatures, UV light transparency.

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Amount /size of particles, wafer surface topography, and adhesive thickness ¾Particle-free surfaces are key to good bonding results. Particles at the wafer surface that are larger than the thickness of the polymer adhesive may cause bonding defects or extended voids. ¾ If the wafer surface topography is high compared to the thickness of the polymer layer, unbonded areas can result. The polymer reflow and the wafer deformation may not compensate for the topographic features on the wafer surface. ¾-The use of very thin 1μm polymer layers more likely results in unbonded areas than the use of thicker polymer layers. Thin polymer layers compensate for surface non-uniformities and particles at the bond interface to a lesser extend. AML – Wafer Bonding Machines & Services

Alignment during adhesive bonding Issue – accurate alignment can be difficult when using thick adhesive layers. The problem arises due the wafers moving relative to each other due to the pre-cured adhesive acting as a lubricating layer. This restricts alignment accuracy to approx 20microns AML have developed special tooling that overcomes this limitation.

The key step is to pin the wafers together at the centre during curing AML – Wafer Bonding Machines & Services

In Situ Observation of Bond Progress

In-situ observation of epoxy spread during aligned bonding

Bonding Tools Controlled wafer contact The system shown enables the aligned wafers to brought into contact without any alignment shift (provided that the platens are set up to be parallel

Wafer bow system

lower chuck

The set up works even with thick adhesive bonding , enabling 1 micron alignment accuracy, whereas conventional mask aligner / transfer to bond chamber processes can only achieve 10-20 micron accuracy.

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Bonding pressure / force ¾The bonding pressure facilitates deformation of the intermediate polymer adhesive and the wafers, and brings the surfaces in sufficiently close contact to achieve bonding. ¾High bonding pressures increase the conceivable deformation of the polymer adhesive and the wafers. However, excessive bonding pressures may cause high stress. Thus, structures that are present on the wafers can be destroyed or the wafers may crack. ¾The bonding pressure that is introduced to the wafer stack should be uniform to avoid differences in the resulting thickness of the intermediate polymer material. ¾Polymer adhesives tend to flow from areas of higher pressure towards areas of lower pressure while they are in a AML – Wafer Bonding Machines & Services liquid phase.

Polymerisation level of the polymer adhesive

¾Thermosetting polymer adhesives should be unpolymerized or partly polymerized prior to bonding. ¾If the amount of polymerization before creating the bond is too high, the polymer adhesive does not deform and adapt sufficiently to the wafer surfaces. ¾For thermoplastic polymer adhesives the amount of polymerization before the bonding is not decisive. They remelt during the bonding process to achieve bonds between the surfaces.

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Wafer thickness ¾Thin wafers are more easily deformed by the bonding pressure to compensate to surface nonuniformities at the bond interface and thus, less stress is introduced at the bond interface.

Polymer curing conditions ¾The bonding temperature and the temperature ramping cycles have to be adjusted to the requirements of the used polymer adhesive.

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Anodic Bonding: Comparison with Adhesive Bonding Advantages: • Higher strength bond • True hermetically sealed cavities achievable (polymer adhesives have significant diffusion rates) • Better long term stability • Zero thickness bondline results in better dimensional control • Better thermal expansion match Drawbacks: • Higher process temperatures • Sealing over thick tracks not possible

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Thermocompression Bonding •Bond formed by plastic deformation of bonding layers •Bond formed with heat + pressure only •Relatively low temperature process (typically in the 250C~320C region) •Thin film intermediary as bonding layer •Au is the principle material used in thermocompression bonding. Cu and other metals can be used. Softer metals allow for less aggressive process conditions, e.g. indium. •Intermediary needs to yield under the available process conditions. •Accommodates larger surface roughness than direct bonding. AML – Wafer Bonding Machines & Services

Thermocompression Bonding 2 •For hermetic seals the surface roughness still needs to be very low •Thin film intermediary means thermal mismatch between bonding layer and substrate is less critical •Good dimensional control of the bond interface / structures in the interface •For large bonding areas, very high forces are required •Bonding has been successful at pressures between 1MPa and 120MPa. Insert SAM example

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Thermocompression Bonding 3 •Trade off between process temperatures and bonding force •High strength bonding requires >30MPa. •Purity of the gold layer is important •A diffusion barrier is required to prevent Si diffusion into the gold layer. •Thermocompression reference – ref 49 SEM images of gold surface without diffusion barrier.

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Thermocompression 4 •Layer thickness can be as low as 0.25um, however thicker layers are required to overcome wafer TTV. •Important to have all bonding surface in contact at low pressure

•Bonding time is not a significant variable. Process can be very short •reference AML – Wafer Bonding Machines & Services

Thermocompression Applications •Gold thermocompression bonding can be performed 260C, therefore many devices can be packaged •In principle a thermocompression bond can form a hermetic seal given smooth surfaces and flat wafers. •Bond can also be used to form interconnections to the device wafer during the encapsulation process. •There is no out gassing during the bonding process so the technique is suitable for high vacuum packaging.

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Examples of Thermocompression Bonds

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Applications of Wafer Bonding Breakdown of bonding applications in industrial devices •

Glass Frit ~43%

•

Anodic 37%

•

Silicon bonding 8%

•

Epoxy 7%

•

Eutectic 5%

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Some products on the market that use anodic bonding • • • •

Pressure sensors – Bosch Accelerometers - SensoNor Gyroscopes - British Aerospace Micropumps - Debiotech

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Devices under development using anodic bonding • Optical MEMS switches

• Microfluidic Devices • Optical Tuneable filters • Microphones • Flow simulators for the oil industry

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Example of Anodic Bonding Application Bosch - Bulk micromachining Pressure sensor Sensor chip Membrane

Bondwires

Reference vacuum

Metall housing

Glass basis Test environment

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Example of SensoNor Product that uses Triple Stack Bonding • 4th generation SensoNor wafer process • 3-stack glass-Si-glass • Hermetically sealed • Buried conductor technology (patent)

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SensoNor - Foundry Service using Bulk Micromachining and Wafer Bonding

The central silicon wafer is structured using bulk micromachining to form beams, cantilevers, nozzles, etc, and the outer glass wafers are hermetically bonded using wafer bonding (in this case, anodic bonding) AML – Wafer Bonding Machines & Services

Product examples Accelerometers •

SA series e.g. 20 ,30 ,50

Tyre pressure sensors •

SP series e.g. 10,11,12,13.. SensoNor

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Silicon Sensing Products; Gyro

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Debiotech’s micropump

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Debiotech’s micropump Cross section showing pump mechanism

Pump chip

-

AML – Wafer Bonding & Services APPLIED MICROENGINEERING - AML Machines - the Design House

Applications of Silicon Direct Bonding

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Application of Direct Bonding Microfluidics (Valves and Pumps)

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Application of Direct Bonding Combine Bonding with DRIE

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Application of Direct Bonding Microcombustor •MEMS for power generation Hydrogen or hydrocarbon fuels •Multi-wafer stack •Aligned wafer bonding

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Application of Direct Bonding Microcombustor

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Smart Cut ™ Process

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Applications of Glass Frit Bonding

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Bosch accelerometer Cross Section of the accelerometer Structure Glass frit bond

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Memscap Precision Pressure Sensor

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Applications of Adhesive Bonding

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Three Dimensional Wafer Integration Using Adhesive Bonding

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Process Sequence for Membrane Transfer Bonding Using Adhesive Bonding Sacrificial Device Wafer

Target Wafer

(a) Contact Pads

Sacrificial Device Wafer

Adhesive Transducer

Target Wafer

Target Wafer

(b)

(c)

Electrical Via Contacts

Target Wafer

Target Wafer

Target Wafer

(d)

(e)

(f)

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Process Sequence for CMOS compatible Bolometers Using Adhesive Bonding Sacrificial Device Wafer SiN

Ti/Pt

SiO2 Sacrificial Device Wafer

ULTRA-i 300

Contact Pads (Al)

Si

Target Wafer (IC)

Target Wafer (IC)

Target Wafer (IC)

(a)

(b)

(c)

Sputtered Al

Electrical Via Contacts MoSi

Target Wafer (IC)

Target Wafer (IC)

(e)

(f)

Target Wafer (IC)

(d)

Electrical Poly Silicon Via Contacts (Al) (Thermistor) Contact Pads (Al) Ti/Pl SiN MoSi Target Wafer (IC)

(g)

Target Wafer (IC)

(h)

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Applications of Low Temperature Bonding • Opto- electronic Integration

• Thermal management

• Engineered Substrates

• Layer transfer

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Applications of Low Temperature Bonding

Layer Transfer •Layer transfer by:–Bonding and Etch Back Using Etch Stop Layer –Etch Release Layer, e.g. porous Si layer –Layer Splitting, e.g. “Smart Cut” technology where H ion implantation causes a defect plane which can be cleanly split away from the bulk material –Lateral Etching

•Important for SOI fabrication and epi layer fabrication ref 19,20, AML – Wafer Bonding Machines & Services

Applications of Low Temperature Bonding Engineered Substrates •Ability to bond at room temperature allows different CTE substrates to be bonded together, e.g. •Particularly important for opto electronics, allows combination of electronics with high performance optical substrates

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Applications of Low Temperature Bonding

Source: MIT

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Applications of Low Temperature Bonding

Source: MIT

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Applications of Low Temperature Bonding

Source: MIT

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Emerging Application: Wafer Level Packaging • Anodic bonding already widely used for first order packaging (i.e. sensitive microstructure already hermetically sealed but bond pads exposed for wire bonding (e.g. SensoNor devices)

• Techniques under development for including vias in the glass, plated feedthroughs and solder bumps such that the diced silicon / glass chips become the complete package.

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Vacuum encapsulation Many MEMS devices require vacuum encapsulation be used – some perform better than others Many bonding methods can Avoid adhesives – issues with outgassing during bonding, and permeation in subsequent use Anodic bonding and glass frit bonding limited to 1 mBar (oxygen evolutin for anodic, general outgassing for glas frit) Metal seals are the most hermetic Eutectic and thermocompression capable of achieving 10-4mBar Maintaining the vacuum level requires use of getters AML – Wafer Bonding Machines & Services

Getters Getters maintain the vacuum level in encapsulated volumes by reacting with / adsorbing evolved gasses For wafer bonding best to use thin film getters Can be deposited and patterned to locate the getters only in the device cavities Service provided by SAES Getters Getters are chemically compatible with standard MEMS cleaning processes inc SC1 and SC2

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Getters Can remove H2O, O2, CO, CO2, N2 & H2 Improves the vacuum level and also extends the life of the device Layer thickness typically 2 micron Activation required at temperature 300 – 500C Selective hydrogen getter also available

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Getter Activation Temp –Time Profile

Commercially Available Wafer Bonding Equipment For aligned bonding, only 3 serious manufacturers in the world • AML • EVG • Suss Microtec (Karl Suss)

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Commercially Available Wafer Bonding Equipment Important difference between AML wafer bonders and other commercially available wafer bonding eqpt.

AML bonders are the only ones to feature in-situ alignment

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Alignment Two approaches taken by bonder manufacturers: prealignment; and in-situ alignment. Method

Pros

Cons

Pre-aligned

•Can prepare wafers in advance •Simplifies bonder (no need for manipulator stage with vacuum feedthroughs) •No lower limit on glass thickness

•Requires separate aligner (usually a mask aligner), and a transfer tool for each wafer pair prepared in advance. •Cannot be sure alignment is maintained during transfer and bonding.

In-situ

•What you see is what you get – can monitor (and change) alignment right up to the moment of bonding •All processes integrated in one machine •Much lower overall cost •Faster throughput

•Increased machine complexity. Needs its own split-field optical system •Upper wafer must be > 200μm thick, or else larger diameter than the silicon

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Important Issues for Bonding Equipment Alignment system is compatible with required processes (visible or IR) Platen flatness (<5μm needed for force uniformity), Parallelism < 10 μm Accurate temperature control (+/- 2oC – needed for some processes) Independent control of upper & lower platens Active cooling using N2(needed for high throughput) Graphite platens and current limiting (for high quality anodic bonding) Uniform force distribution (for eutectic, adhesive and frit bonding) High vacuum – 2x10-6 mBar (needed for some wafer-scale packaging applications, and devices such as bolometers)

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Bonding Environment • The surface chemistry of Si wafers is very important in direct bonding • Initial bond energy shown to decrease with time in vacuum (H2O molecules leave the surface, reduction in range and magnitude of hydrogen bonding) • Variation in surface properties across the wafer can cause variation in bond strength from device to device • AML system maintains large separation between the wafers. Therefore vacuum or process gas pressure is well known where it matters – at the wafer surfaces • Large wafer separation allows rapid and controllable pumping time and therefore minimises time under vacuum

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AWB04 •In-situ alignment +/-1μm • PC controlled system •Up to 2.5kV @ 40mA •Wafers up to 150mm as standard (200mm option) •Max. wafer stack thickness 10mm •Independent control of upper and lower heaters to 560ºC in 1ºC steps •Highly flexible design – easily customised • Force up to 5kN AML – Wafer Bonding Machines & Services

AWB04 •PC control (Automatic & manual modes)

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Chamber Schematic for In-situ bonders

Features of AML Bonding Equipment • Platens held in large separation during heat up and pump down –good for vacuum encapsulation • Top wafer held inverted, clamped at edge, no contact with bonding surface • Top wafer is distorted by central pin to ensure single wavefront propagation

View of chamber lid showing in-situ optics

• Ability for in-situ cleaning / chemical preparation of the wafer surfaces prior to bonding

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AML tooling solutions for Direct Bonding • Two approaches – forced bond propagation and unforced bond propagation • Ideally Si-Si bond is self propagating- minimises stress/bow in the resulting assembly (ref 3). The AML spring pin / edge clamp tool allows control of self bond propagation rate and bond initiation point • If bulk physical properties and / or surface properties of the wafers mean a bond will not self propagate - necessary to force the wafers together. The AML pin chuck bond tool is designed for this requirement • The pin chuck tool allows wafers to be force bonded to a particular curvature – in principle this could help to minimise stress or achieve a flatter post bond assembly (important if multi stack bonding is required) see below AML – Wafer Bonding Machines & Services