U. Gösele and Q.-Y. Tong

Max-Planck-Institute of Microstructure Physics, D-06120 Halle, Germany and School of Engineering, Duke University Durham, NC 27708, USA

Tentative Table of Contents

1. Introduction

1.1. Short history of bonding materials

1.2. What is wafer direct bonding?

1.3. What is it good for (driving forces)?

1.4. How does it work?

1.5. Typical problems and solutions

2. Silicon surface

2.1. Surface morphology and chemical properties

2.2. Micromechanical properties

2.3. Thermaldynamic properties

2.4. Electrical properties

3. Basics of interactions between flat surfaces

3.1. Van-der-Waals interactions

3.2. Interface energies and their measurement

3.3. Flatness criterion for avoiding unbonded areas

4. Influence of particles, surface steps and cavities

4.1. Investigation methods for unbonded areas

4.1.1 transmission

4.1.2 X-ray topography

4.1.3 Acoustic microscopy

4.1.4 Magic mirror method

4.1.5 Interface etching

4.2.5 Transmission Electron Microscopy

4.2 Bubble diameter as a function of particle size

4.3 Influence of surface steps and cavities

4.4 Hydrophilic versus hydophobic wafer surfaces

5. Initiation of wafer bonding at room temperature

5.1 Basic conditions for room temperature(RT) bonding

5.2 Cleaning of wafer surfaces

5.3 Activation of wafer surfaces

5.4 Room temperature bonding in conventional cleanroom

5.4.1 Manual bonding

5.4.2 Bonding fixtures and machines

5.5 Room temperature bonding with micro-cleanroom setup

5.6 Initiation and propagation of bonding front

5.7 Thick wafer bonding

5.8 Debonding

6. Temperature treatment of bonded wafer pairs

6.1 Bonding strength as a function of time

6.2 Bonding strength as a function of temperature

6.3 Low temperature wafer bonding

6.4 Temperature-dependent interface bubbles

6.4.1 Phenomenology: IR-type and X-type bubbles

6.4.2 Cause of bubbles

6.4.3 Thermodynamics of bubble formation

6.4.4 Elimination of bubble formation

6.5 Structural development of thin interface oxides

6.5.1 Influence of rotational wafer misorientation

6.5.2 Influence of oxygen content (CZ versus FZ silicon wafers)

7. Thinning Procedure

7.1 Mechanical thinning

7.1.1 Chemical-mechanical polishing

7.1.2 Refinement by local thinning

7.1.3 Polish-stop approach

7.2 Silicon chemical etching

7.2.1 Aqueous alkaline etchants

7.2.2 Etch selectivity of aqueous alkaline Silicon oxide p++ silicon Carbon-doped silicon Germanium-doped silicon Nitrogen-implanted silicon

7.2.3 Silicon anisotropic etching

7.2.4 Silicon isotropic etching

7.3 Layer transfer

7.3.1 Layer transfer by bonding and etch-back B and B/Ge etch-stop Carbon-implanted etch-stop Oxygen-implanted etch-stop SiC etch-stop for transfer of SiC layer Porous silicon etch-release layer

7.3.2 Layer transfer by bonding and layer-splitting

7.3.3 Layer transfer by bonding and lateral etching

8. Electrical properties of bonded interface

8.1 Electrical properties of bonded hydrophilic and hydrophobic Si/Si interface

8.1.1 Charges in bonded Si/Si interface region

8.1.2 Boron contamination at bonding interfaces

8.1.3 Current transport through bonded Si/Si unipolar and pn junctions

8.1.4 Recombination centers at bonded interface

8.1.5 Origins of interface traps

8.2 Electrical properties of bonded Si/SiO2 and SiO2/SiO2 interface

8.2.1 Charges in bonded Si/SiO2 interface region

8.2.2 Negative charges in bonded SiO2/SiO2 region

8.2.3 Leakage current of bonded oxide layer

8.2.4 Radiation-hardness of bonded Si/SiO2 structure

9. Basics of stress and strain in bi-layer structures

9.1 Origins of stress in bonded wafers

9.2 Thermal stress distribution in bonded materials and at bonded interface

9.3 Stress relief mechanisms

9.4 Stress reduction methods

10. Bonding of dissimilar materials

10.1 Bonding of wafers of dissimilar materials

10.1.1 GaAs/Si bonding

10.1.2 InP/Si bonding

10.1.3 Quartz crystal/Si bonding

10.1.4 Si/quartz glass bonding

10.1.5 Si/glass bonding

10.1.6 Si/sapphire bonding

10.2 Bonding layers for direct bonding of dissimilar materials

10.2.1 CVD silicon oxide

10.2.2 CVD polysilicon

10.2.3 CVD silicon nitride

10.2.4 CVD InP and other materials

11. Stresses in bonded wafers

11.1 Stresses caused by surface nonuniformity

11.2 Stress in bonded GaAs/Si structures

11.3 Deformation of bonded SOI pairs

11.4 Bonding of pre-stressed wafers

12. Bonding of structured wafers

12.1 Design considerations of patterned wafers

12.2 Bonding of wafers with cavities

12.2.1 Influence of hydrocarbon contamination

12.2.2 Bonding in different atmospheres

12.3 Bonding alignment for structured wafers

13. Mainstream applications

13.1 Bonding applications in VLSI

13.1.1 Competing technologies (SIMOX, ZMR and SOS)

13.1.2 Radiation-hard CMOS

13.1.3 DRAM s

13.1.4 Low power, low voltage CMOS

13.1.5 High temperature CMOS

13.1.6 Body contacts of SOI MOSFETs

13.2 Bonding applications in high voltage and power devices

13.2.1 Dielectric isolation

13.2.2 High voltage devices

13.2.3 High power devices

13.2.4 Buried metal structures

13.3 Bonding applications in micromechanics

13.3.1 Surface sacrificial layers

13.3.2 Microchannel systems

13.3.3 Microvalves and microinstrumentation

13.3.4 Pressure sensors and accelerometers

13.3.5 Electro-acoustic devices

13.4 Bonding application in optoelectronics

13.4.1 Integrated optical waveguides

13.4.2 Lasers and photodetectors

14. Emerging and future applications

14.1 Surface protection of semiconductor wafers

14.2 New material combinations

14.2.1 Infrared window protection using Si on ZnS bonding

14.2.2 Buried C60 layers

14.3 Bonding interface as gettering layer

14.4 X-ray masks

14.5 X-ray optics with curved membranes

14.6 Micro-vacuum-tubes

14.7 Thick wafer bonding for cooling application

14.8 Non-linear optics via multiple GaAs or ZnS bonding

14.9 3-D devices

14.10 SOI microwave MMIC

14.11 Research applications

14.11.1 Room-temperature UHV bonding

14.11.2 Grain boundary generation

14.11.3 Determination of interface energies

14.11.4 Electrical and mechanical properties and interactions of monolayers of polymers and biomolecules

14.11.5 Self-stressed semiconductor structures

14.11.6 Investigation of surface contamination

14.11.7 Investigation of surface topology

14.11.8 Investigation of helium quantum effect

15. New developments

includes anything in terms of essential information which appears after finishing the rest of the book and which would be difficult to incorporate.

Table of constants

list of mechanical, thermal and physical properties of commonly used materials for wafer bonding

list of commonly used constants

Sample: Chapter 7

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