Fabrication Technology, Part II - MEMS · Lecture 6 by H.K. Xie 9/8/2003. 2 ... To Microelectronic...

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


Process Flow Examples

Diode Process FlowNMOS Process FlowCMOS Process Flow


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)


Diode Process Flow (cont’d)

N-Si

5. Ion implantation of boron

N-Si6. Strip off photoresist

N-Si7. Apply photoresist


N-Si



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



N-Si


N-Si

14. Oxide etch

N-Si15. Strip off photoresist

N-Si

Al

16. Aluminum deposition(e-beam, or thermal evaporation)


Diode Process Flow

N-Si17. Apply photoresist

N-Si


N-Si19. Aluminum etch

N-Si

SiO2

P+ N+

Al

20. Strip off photoresist

Idealized pictures

P N

Metal (Al)


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.


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.


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.


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


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


Mechanical Properties of Silicon

Ref. K. E. Petersen, “Silicon as a Mechanical Material,” Proc. IEEE 70, pp. 420-457, 1982.


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


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.



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


Isoetch Curves for HNA Etchant System

Ghandi, VLSI Fabrication Principles, p. 481.



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


Bulk MicromachiningWet Anisotropic Etching

Chemical etchingReactant transportSurface chemical reactionProduct removal

Crystallographic dependent etch rate

(110) (111)(100)

Madou, Fundamentals of Microfabrication, p. 148.



Preferential etching of (100) planes

(111) (100) surface

54.7(100)

Si

Example: Nozzle for inkjet head<110>

<110>

(111)

<110>



Etch geometries

Fabrication of channelsRequires wafer-to-wafer bonding

Top (111) (100)Side

(111)(100)

Example: Nozzle for inkjet head



Convex corners[100]

[100] [100]Misalignment



Anisotropic etch of (110) silicon

Madou, Fundamentals of Microfabrication, p. 176.

(110) Si

<111>

Fabrication Technology, Part II - MEMS · Lecture 6 by H.K. Xie 9/8/2003. 2 ... To Microelectronic...

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