Metal bonding. Aida Khayyami, Kirill Isakov, Maria Grigoreva Miika Soikkeli, Sample Inkinen

Transcription

1 Metal bonding Aida Khayyami, Kirill Isakov, Maria Grigoreva Miika Soikkeli, Sample Inkinen

2 Timing (delete before presentation) Introduction (Outline, available bonding techniques, evaluation of metal bondings)-3 min; SLID- 3 min; Eutectic-3 min; Eutectic vs SLID- 1min; Processing (metal deposition, changes in process, bonding systems)-3min; Summary-2 min.

3 Outline Available bonding techniques; Evaluation of metal bonding methods; SLID; Eutectic; Basics; Materials compare and failure tests. Basics; Materials compare failure tests. SLID vs Eutectic (add patent and industrial uses); Possible fabrication process changes; Bonding system and metal deposition; Evaluation of bond quality; Summary

4 Available bonding techniques Drawbacks of anodic bonding: Sodium causes instability of electrical devices; Large voltage can impact: MEMS; Integrated electronic components. Special glass is required. Why Metallic Bonding: Good sealing; Thermal and electrical conductivity; Easier CMOS integration no need for extra contacts.

5 Evaluation of metal bonding methods Factor Criteria Thermocompression Direct SLID Eutectic Bond force <10kN No Bond T large T range No No Heterogeneous integration >3 wafers No Getter activation 375 <T<570 No Used in MEMS production Thermocompression: at some points such companies as Hitachi, Seiko, Canon etc. Direct bonding: V. Dragoi et. all, Aligned Fusion Wafer Bonding for CMOS-MEMS and 3D Wafer-Level Integration Applications by EV Group and Product System Inc. Eutectic: Nasiri fabrication process by Invensense (several patent). SLID: Hasn t been used in MEMS industry so far, but in development.

6 SLID bonding (Au-Sn, Cu-Sn) Solid-liquid interdiffusion Low temperature fabrication method High-temperature-stable bonds

7 SLID process Heating up to the holding temperature below Sn melting point Wafers are brought into contact Heating up to the bonding temperature Holding at the bonding temperature to allow IMC formation

8 SLID: Au-Sn vs Cu-Sn Au-Sn Bonding temperature >320 C Bond thermal stability up to ~520 C Oxidation resistant No reduction in bond strength or conductivity over time Shear strength up to 170 MPa Tensile strength up to 88 MPa Cu-Sn Bonding temperature >300 C Bond thermal stability up to ~700 C Cheaper Cu oxidizes Shear strength up to 275 MPa Tensile strength up to 91 MPa

9 Eutectic bonding(au-si) A metal ring is required only on one side Eutectic temperature is low (363 C) Ease of deposition lack of a native oxide on the Au surface. Long process(2hours) Extensive outgassing and getter activation time(limited productivity) Clean, smooth Si on device wafer needed

10 AuSi eutectic requirements Cap wafer process example Oxide etch top side An underlying SiO2 layer Adhesion layer nm film is sufficient Ti or Cr deposited by evaporation or sputtering Back side alignment/dicing marks Diffusion barrier Pt, Pd, or TiN layers with a thickness of up to 100 nm Actual plating base is deposited in the last part Etching of cavities/oxide removal Plating base deposition/litho Electroplating/removal of base

11 AuSi eutectic reliability When the sizes of the reaction areas are larger than 5 μm, voids will form during the reaction. 5 μm 4.5μm Ti layer helps to achieve void free bonding 7μm 9μm

12 Al-Ge Eutectic Bonding Aluminum-oxide grows almost instantaneously Complex process chain and machine parameters greatly influences the quality of eutectic bond Achieves a nearly void-free bond Scanning Acoustic Microscope (SAM) image of 100 mm diameter wafer pair bonded at 435 C

13 Al-Ge eutectic requirements An underlying SiO2 layer No adhesion layer needed No additional diffusion barrier needed Ge thickness nm Al thickness nm Si N 3 4 Ge Al

14 AuSi vs AlGe eutectic bonds Similar bonding strength and the mechanical behavior Smaller resistivity in case of AlGe structure Both eutectic bondings are hermetic AlGe bonds need no additional adhesion/diffusion layers AlGe bonds need thinner layers of metal than AuSi bond Oxide removal needed on Al, Ge and Si before bonding (not needed on Au) SEM image of an AuSi bonding interface left and an AlGe bonding interface right

15 Possible fabrication process changes Height of bonding (~5 m*) remain same; Frame width of eutectic bonding satisfy anodic bonding condition (~ m*); Additional lithography step for body and cap wafer to metalize sealing area; Possibility to use spacer and special frame design to control bond process. *From Handbook of Wafer Bonding Different seal frame designs with the capability to catch squeezed out liquid eutectic melt during the bonding process in top view

16 SLID vs Eutectic Pros Metals are electroplated cheap No need to care about natural oxides High temperature tolerance of the bond successive bonding is not a problem Cons Time consuming (3-6 hours) Deposition and patterning of both wafers is required Adhesion layers are required Thick metal layers + liquid phase of tin during the bonding + variation in pressure across a wafer = risk of squeezing liquid metal on the device Pros Low bond temperature Low resulting stress High bonding strength large fabrication yield High bonding strength Cons Different CTE of intermediate layer and wafer material Potential oxidation of metal layers Formation of metastable phases after bonding

17 Metal deposition techniques Metal thin film deposition are quite versatile. Deposition Technology Depositing metal/ Bond layer (nm) Comments Sputtering Au or Cu/ 100 nm Ti, TiN, Au/ 20, 60, 500 nm Al/ nm For SLID For eutectic AuSi For eutectic AlGe Evaporation Ti, Pt, Au/ 20, 100, 400 nm Al, Ge/ 100 nm, 57 nm For eutectic AuSi For eutectic AlGe Electroplating Au/ several µm Au 6 µm, Cu 6 µm, Sn 2 µm For eutectic AuSi For SLID CVD Ge/ nm For eutectic AlGe

18 Bonding systems Manufacturers EVG SUSS Microtec AML Ayumi Industry, etc. EVG GEMINI Possibilities Oxide etching with same system (EVG) Anodic, silicon fusion, thermocompression and eutectic bonding possible with same system (EVG GEMINI)

19 Cost Comparison to anodic bonding Number of layers and process steps are increased Eutectic > Anodic Au-Si < Al-Ge Cost of valuable materials (Au, Pt) Eutectic > Anodic Au-Si > Al-Ge Cost of tools Many bonding systems allow the usage of several bonding methods including anodic, eutectic, etc. Frame width and contact area Eutectic bonding < Anodic Dies per wafer Eutectic bonding > Anodic

20 Summary Focus on Al-Ge eutectic and Sn based TLP Bonding as most promising; Both SLID and eutectic are not fully explored and have high potential; Change of metal bonding technique will minimally affect on process; Cost is variable from one to another method, helping save or decrease existing cost.