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ECE6397 MEMS, NEMS, and NanoDevices. Introduction to Microfabrication. Overview of Microfabrication.

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Ece6397 mems nems and nanodevices

ECE6397MEMS, NEMS, and NanoDevices

Introduction to Microfabrication

Overview of microfabrication
Overview of Microfabrication

  • MEMS technology was implemented directly from IC fabrication. Si was the leading material and the same processes have been used. Many others and other materials are included to meet specificity of MEMS.

Overview of Microfabrication

go to see video of fabrication

Thermal Oxidation and SiO2 Interface

MEMS use oxides of various thicknesses

Applications in microeletronics

SiO2 grows on Si (also @ RT); enables very easy IC formation; ensures stability and reliability.

Lower thermal budget

1 – 2 nm

New dielectrics   to avoid tunneling. (high K)

Low K dielectrics

Plummer et al.

Historical Development and Basic Concepts

Oxide growth using O16 and O18 isotopes identifies the mechanism.

Neutral O2 and H2O and/or OH are dominant species in oxidation, not atoms or ions O, O- , O2-,

Volume of SiO2 is 30% larger than Si.

(1.3)3 ~ 2.2 volume of the oxide cannot be accommodated in Silicon

Plummer et al.

Effect of Volume Mismatch in Si/SiO2 System; Recessed LOCOS

[email protected]

2.2X volume expansion -> 45%xox=xSi



Silicon Consumption During Oxidation (LOCOS)

Nonplanar structures form due to Si consumption - stress between Si and SiO2

Bird’s Beak formation

Stress at the Si/Si3N4 interface

Plummer et al.

Structure of Silica Glass

Short range order maintained

Amorphous material

Non-bridging oxygen in fused Silica (not present in crystalline SiO2)

Si can be replaced by deposits. B,P,As or Sb = network modifiers.

  • large compressive stress (5*109 dynes/cm2) exists in SiO2. High temperature can relief stress by viscous flow.

  • Large difference in the thermal expansion coefficients of Si and SiO2.

Silicon in tension  refer to curvature

Properties of OXIDES such as stress, porosity are important in MEMS

Plummer et al.

SiO2/Si System: Structure and Charges

When charges are important in MEMS? In capacitive sensors (impedance measurements)

Amorphous/crystalline – interface is flat. (TEM)

Roughness  with  growth rate and  T.

Detect density at the interfaces is ~ 109 – 1011 cm-2.

Fixed charge ~ 109 – 1011 cm-2 is + and does not change in device operation.

Interface charge =traps due to dangling Si bonds  change in operation QpQit – both related to unoxidized Si atoms.

Reduce charges since they degrade device operation   T , H2 anneal..

Plummer et al.

Oxide Charges and Their Annealing

Much more important in Si devices than in MEMS

Increasing surface roughness increases charges

Plummer et al.

Manufacturing Methods and Equipment

Typical for ICs and for MEMS

Vertical furnaces are also used.

3 zones

+ 0.5 ° C

Dry or wet


Ramping of T from/to 800 °C ( 10 °C/sec)

Add HCl or TCA for gettering purpose (metals, Na +)

Plummer et al.

Models and Simulation

First -Order Planar Growth Kinetics - Linear Prabolic Model

Deal and Groove Model

Transport to Si = Diffusion through the oxide

Reaction at the Si surface

In steady state F1=F2=F3

Transport of the oxidant to the oxide surface.

Plummer et al.

Deal-Grove Model for Wet and Dry Oxidation

Slow growth rates

Dry oxidation - used up to 100-200 nm

Fast growth rates.

Wet oxidation - used for thicker oxides


MEMS usually use thick oxides

Plummer et al.

Orientation Effects in Oxidation

Orientation effects are important in MEMS

(100), (111), and Polysilicon

Density of atoms (bonds) in (111)>(100)

No effect of orientation for thick oxides

Strong effect of orientation for thin oxides

Simulated oxide growth

Related to stress

Plummer et al.

2D SiO2 Growth Kinetics

Difference in volume -> problems when expansion is restricted (SiO2 confined)

  • Experiments by Kao et al.:

  • Retardation at sharp corners (2X for 500 nm SiO2)

  • Retardation larger @ low T (no effect @ 1200 °C)

  • Interior (concave) corners oxidize slower than exterior (convex) but both slower than flat Si

  • Reasons

  • Crystal orientation

  • Diffusion of oxidant through amorphous SiO2 is the same -> no dependence on direction

  • Stress (volume difference): SiO2 under large compressive stress -> affect both oxidant transport and reaction at the Si surface

Plummer et al.

Oxidation of Non-Planar Structures

Stress retards oxidation; @high T viscoelastic flow relaxes stress

Oxide viscosity=f(stress, T)

no stress

Stress included

Stress @T> Stress @T

Large stress can bend thin mechanical structures in MEMS: Cantilevers etc.

History of Stress VERY IMPORTANT

Stress in an oxide depends on growth T. In sequential processing, transient will appear in the next step @ higher T from the original stress (=higher at lower T) which sets the oxide growth rate below that at high T (lower stress).





Plummer et al.

Segregation of Dopants at the Si/SiO2 Interface

Highly doped layers are used in MEMS so the role of oxidation on dopant distribution is important

Plummer et al.

Oxidaion Enhanced Didiffsion

Oxidation can change dopant diffusion - affect layer thickness.

Substrate Doping Effects

Concentration Enhanced Oxidation (CEO)

Low T

High T

CEO stronger for N+ than P+

Plummer et al.

Beginning of Integrated Circuits in 1959

Kilby (TI) and Noyce (Fairchild Semiconductors)

Photolithography used for Pattern Formation

Plummer et al.


Exposure system gives sharp contrast

Will develop

Resist has to respond with changes

Plummer et al.

Wafer Exposure Systems

25-50 wafers/hr

Degradation of patterns by diffraction

Basic properties and characterization of results

Contrast allows distinguishing light and dark areas on the mask. DUV resists have better contrast and better sensitivity because of chemical amplification.

Affected by processing conditions also


Plummer et al.

  • Photoresists

  • Negative (older –resolution limited by swelling)- more soluble when not exposed.

  • Positive – more soluble when exposed. Important parameters:

  • Sensitivity – how much time is reqd. for changes [mJcm-2] (Ex. 100 mJcm-2 for g line and i-line resists, newer down to 20 mJcm-2 .

  • Resolution

  • Robustness to etching.

  • Photoresists for g-line and i-line: a hydrocarbon inactive resin, a (hydrocarbon) photoactive compound(PAC), and a solvent: PAC replaced by a Photo Acid Generator (PAG) to act as a chemical amplifier.

Plummer et al.

Historical Development and Basic Concepts of Doping


  • Development (40 years) in predeposition

  • Solid-phase diffusion from glass layer.

  • Gas phase deposition at high temperatures (B2H6, PH3, AsH6)  reproducibility; good only for solid sol. (too high Ns)

  • Replace predeposition by ion implantation; good for bigger devices but difficult for small ones (TED)

  • Return to diffusion

Evolution of the Fabrication Process: The Planar Design of Bipolar Transistors

Beginning of Silicon Technology and End of Ge devices

Implementation of a masking oxide to protect junctions at the Si surface

Oxidation possible for Si not good for Ge

Lithography to open window in SiO2

Boron diffusion



Phosphorus diffusion through the oxide mask

Oxidation and outdiffusion

Plummer et al.

Dopant Diffusion Bipolar Transistors

Concept of Sheet Resistance of doped layers.

Higher doping lower the resistance



s[/sq.] 4 point probe orvan der Pauw



Sheet resistance

In MOSFETs Rcontact + Rsource + Rext < 10% Rchen

s but keep xj small to avoid DIBL (conflicting requirements

Plummer et al.

Junction Formation – Process Choice Bipolar Transistors

Plummer et al.

Dopant Solid Solubility Bipolar Transistors

Concentrations above SS limits result in inactive coplexes (defects,precipitates)

Metastable electrical activation

Practical concenrations for active P and As

As complexes

Plummer et al.

Intrinsic Diffusion coefficients of Dopants in Silicon Bipolar Transistors

Arrhenius fit

Fast Diffusers

Slow Diffusers

@ High dopant concentrations the diffusion is enhanced.

Plummer et al.

Successive Diffusion Steps Bipolar Transistors

Dt is a measure of thermal budget

T1 followed by T2:

Equiv. time

Transient Enhanced Diffusion (TED) and Concentration Enhanced Diffusion (CED) when D increases with C and/or crystallographic/point defects (can be related to damage)

Plummer et al.