fabrication technology, part ii - mems · lecture 6 by h.k. xie 9/8/2003. 2 ... to microelectronic...
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EEL5225: Principles of MEMS Transducers (Fall 2003)1
EEL5225: Principles of MEMS Transducers (Fall 2003)
Fabrication Technology, Part II
Agenda:Process flow examples (diodes)Micromachining--OverviewBulk micromachining
Reading: Senturia, pp. 45-49. Next time: Senturia, pp. 29-44.
Lecture 6 by H.K. Xie 9/8/2003
EEL5225: Principles of MEMS Transducers (Fall 2003)2
Process Flow Examples
Diode Process FlowNMOS Process FlowCMOS Process Flow
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Diode Process Flow
N-Si
1. Starting material: n-type (100) silicon N-Si
SiO2
N-Si
2. Grow thermal oxide
photoresist
3. Apply photoresist
N-Si
4. Lithography (mask1)
EEL5225: Principles of MEMS Transducers (Fall 2003)4
Diode Process Flow (cont’d)
N-Si
5. Ion implantation of boron
N-Si6. Strip off photoresist
N-Si7. Apply photoresist
8. Lithography (mask2)
N-Si
EEL5225: Principles of MEMS Transducers (Fall 2003)5
Diode Process Flow (cont’d)
N-Si
9. Ion implantation of phosphorus
N-Si
10. Strip off photoresist
N-SiP+ N+11. Drive-in diffusion
N-Si
12. Apply photoresist
EEL5225: Principles of MEMS Transducers (Fall 2003)6
Diode Process Flow (cont’d)
N-Si
13. Lithography (mask3)
N-Si
14. Oxide etch
N-Si15. Strip off photoresist
N-Si
Al
16. Aluminum deposition(e-beam, or thermal evaporation)
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Diode Process Flow
N-Si17. Apply photoresist
N-Si
18. Lithography (mask3)
N-Si19. Aluminum etch
N-Si
SiO2
P+ N+
Al
20. Strip off photoresist
Idealized pictures
P N
Metal (Al)
EEL5225: Principles of MEMS Transducers (Fall 2003)8
NMOS Process Flow
Ref. R. C. Jaeger, Intro. To Microelectronic Fabrication, p. 7.
a) Grow oxide and deposit nitride
b) Pattern nitride and implant boron for device isolation
c) Grow oxide using nitride as oxidation mask (LOCalOxidation of Silicon (LOCOS)).
d) Grow gate oxide and deposit polysilicon gate material.
EEL5225: Principles of MEMS Transducers (Fall 2003)9
NMOS Process Flow (cont’d)
e) Deposit oxide and pattern contact holes. Deposit and pattern metal.
f) Sinter metal for ohmic contacts.
Ref. R. C. Jaeger, Intro. To Microelectronic Fabrication, p. 7.
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CMOS Process Flow
a) Implant and diffuse p-well.b) LOCOS isolationc) Grow gate oxide, deposit
and dope polysilicon, and pattern polysilicon.
d) Implant boron for source and drain of p-channel MOSFETe) Implant arsenic for source and drain of n-channel
MOSFETf) Deposit oxide, pattern contact holes, deposit aluminum,
pattern metal, and sinter contacts. Ref. R. C. Jaeger, Intro. To Microelectronic Fabrication, p. 9.
EEL5225: Principles of MEMS Transducers (Fall 2003)11
Micromachining -- Overview
MicromachiningSpecialized techniques for fabricating mechanical structuresKey processes:
machining
SurfaceMicro-
machining
BulkMicro- Wafer
Bonding LIGA
Suspended mass structure. Ref. Ristic, ed., Sensor Technology and Devices, p. 80.
Lucent Technologies’ 2D scanning mirror for optical switching
Analog Devices’ accelerometer
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Micromachining -- Overview
Original Starting Substrate--SiliconMechanical
– Essential perfect elasticityno mechanical hysteresis
– High modulus of elasticitysimilar to steel
– High tensile yield strengthstronger than steel
– Low densitysimilar to aluminum
– Hard materialsimilar to quartz
Electrical– Exhibits change in resistance with applied stress
Piezoresistance effect discovered in silicon in 1954 by Smith at Bell Labs.
Why predominantly silicon?Dominant substrate for microfabrication of CMOS IC’sWill discuss other types of substrates later
EEL5225: Principles of MEMS Transducers (Fall 2003)13
Mechanical Properties of Silicon
Ref. K. E. Petersen, “Silicon as a Mechanical Material,” Proc. IEEE 70, pp. 420-457, 1982.
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Bulk Micromachining
Wet Isotropic EtchingWet Anisotropic EtchingDry Anisotropic Etching
BulkMicro-
machining
Fabrication of mechanical structure via removal of bulk semiconductorRequirements:
Control of etch-rateControl of sidewall geometrySelectivityMasking materials
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Bulk MicromachiningWet Isotropic Etching
Goal of silicon bulk micromachining
Etch silicon
Chemistry of silicon etchingSi + 2h+ → Si+2 (oxidation of Si)Si+2 + 2(OH)-→ Si(OH)2 (reaction
with hydroxyl group)Si(OH)2 → SiO2 + H2
SiO2 + 6HF → H2SiF6 + 2H2ONote: Holes for oxidation are provided by oxidizing agent or electrically.
Structural layer
Mask
1 3
Silicon substrate
2
D. R. Turner, “On the mechanism of chemically etching Geand Si,” J. Electrochem. Soc 107, p.810, 1960.
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Bulk MicromachiningWet Isotropic Etching
HNA isotropic etchantHF:Nitric:Acetic (HF:HNO3:HC2H3O2)Holes supplied by HNO3 (strong oxidizer)HF: oxide removal
– [HF] >> [HNO3]Dependent on [HNO3] oxidation reactionDifficult to initiate
– [HNO3] >>[HF]Dependent on oxide removal via [HF]Polishing etch
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Isoetch Curves for HNA Etchant System
Ghandi, VLSI Fabrication Principles, p. 481.
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Bulk MicromachiningWet Isotropic Etching
Masking materialsThermal SiO2
– Etches at 300-800 angstroms/minSilicon nitride
– Etches at 10-100 angstroms/minPhotoresist
– Rapidly attacked by HNO3
Fabrication of sharp tip via isotropic etch
Silicon substrate
Sharp tip
Structural layerSilicon nitride
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Bulk MicromachiningWet Anisotropic Etching
Chemical etchingReactant transportSurface chemical reactionProduct removal
Crystallographic dependent etch rate
(110) (111)(100)
Madou, Fundamentals of Microfabrication, p. 148.
EEL5225: Principles of MEMS Transducers (Fall 2003)20
Bulk MicromachiningWet Anisotropic Etching
Preferential etching of (100) planes
(111) (100) surface
54.7(100)
Si
Example: Nozzle for inkjet head<110>
<110>
(111)
<110>
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Bulk MicromachiningWet Anisotropic Etching
Etch geometries
Fabrication of channelsRequires wafer-to-wafer bonding
Top (111) (100)Side
(111)(100)
Example: Nozzle for inkjet head
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Bulk MicromachiningWet Anisotropic Etching
Convex corners[100]
[100] [100]Misalignment
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Bulk MicromachiningWet Anisotropic Etching
Anisotropic etch of (110) silicon
Madou, Fundamentals of Microfabrication, p. 176.
(110) Si
<111>