MOSCAP Fabrication

7/29/2019 MOSCAP Fabrication

1/34

MOSCAP Fabrication

12th INUP Workshop at

IIT Bombay

12th INUP Workshop at IIT Bombay


2/34

Why MOSCAP ?

The fabrication of the oxide of an MOS structure is one of thecritical steps when fabricating MOSFETs

MOSCAP is used as a diagnostic tool for determining the

quality of the process used

Electrical characterization and monitoring is critical formaintaining gate oxide uniformity

Many electrical characterization techniques have been

developed over the years to characterize gate dielectric quality.

However, the most commonly used tool for studying gate oxidequality in detail is the Capacitance-Voltage (C-V) technique. C-

V test results offer a wealth of device and process information,

including bulk and interface charges and many MOS device

parameters.



3/34

What all we can get from CV of a MOSCAP ?

Oxide (dielectric) thickness Charges in the oxide

Oxide breakdown strength

Conductivity type

Doping concentration

Doping profile in the silicon

Work function differences

Interface trap densities

Properties of electron and hole traps



4/34

Si - body

Top Metal

What is the desired structure

Top view

Cross section view

VgateMOSCAP Structure



5/34

MOSCAP Fabrication

Steps involved

Wafer identification (2 Si wafer)

Four probe measurement (optional)

Cleaning of the wafer

Oxidation

Ellipsometry (optional)

Metal dot deposition

Foaming gas annealing

Back side etching



6/34

Identification of starting Wafer

2 (diameter of the wafer) Si wafer

P type or N type depends on desired structure Front side is shining while back side is rough P type Si (100)

270m

In this experiment: P Type (100) Si wafer having flat in direction

Do the four probe measurement To confirm the type of the wafer

To find the resistivity of the wafer

Four Probe

( ) Planes{ } Eq.Planes

[ ] Directions

< >Eq. Directions



7/34

Cleaning of the Wafer

Various types of cleaning procedures are available RCA cleaning ( Radio Corporation of America) is universally accepted

Most rigorous cleaning procedure, Industry standard

RCA cleaningTwo step procedure: SC1 & SC2

HF (Hydrofluoric Acid) clean To remove any native oxide

Wafer is dipped for about 30 Sec in

2% HF solution

Glass beaker is not used for HF solution

Standard Clean 1 (SC1) To remove the organic contaminatesNH4OH : H2O2 : H2O ( 1: 2:7)

Solution is heated at 750C 800C for 10 minutes

Again HF Clean To remove the oxide formed on the wafer after SC1



8/34

Cleaning of the Wafer (contd.)

Standard Clean 2 (SC2) To remove the heavy alkali ions and cations

HCl : H2O2 : H2O ( 1: 2:7)

Solution is heated at 750C 800C for 10 minutes

Again HF Clean To remove the oxide formed on the wafer after SC1

Other cleaning procedures:

Piranha Cleaning Mixture of H2SO4 and H2O2Removes the organic contaminates

Less rigorous than RCA

BHF etch To remove the oxide

Typical etch rate is about 110nm/min.

SiO2 + 6Hf => H2 + SiF6 + 2H2O



9/34

Thermal Oxidation of the Wafer

Thermal Oxidation

Wet oxidation

Dry Oxidation

Furnace Oxidation

Rapid Thermal Oxidation

You will be doingFurnace Oxidation

Target Oxide Thickness is 10nm



10/34

Metal Dot deposition

Al is deposited by means of evaporation on the front side

Cross Section

Top view



11/34

Back etching and Metallization

After the Al deposition

Strip the back oxide by 2% HF

Do the back metallization



12/34

Oxidation

Stable at high temp >900 C

Good Si/SiO2 interface

Silicon dioxide (SiO2) as a gate dielectric

*SiO2 layer is formed on heating Si in O2 or H2O ambient

The Deal- Grove model of Oxidation

*It works well for predicting the oxide thickness for thermallygrown SiO2 larger than 30nm



13/34

Oxide growth

Oxidation of Si proceed by either Si atom diffusing to theSiO2/gas interface or Oxygen molecules diffusing to Si/SiO2interface

At room temperature there is native oxide of thickness ~ 1-2.5nm due to the presence of oxygen in air

SiO2

Si

O2



14/34

Dry Oxidation

The Si wafer is heated in the presence of oxygen

Reaction

Si + O2 -----> SiO2

It gives a dense oxide hence is suitable for Gate dielectric Thickness < 100 nm

Beyond Deal - grove model requirement



15/34

Thickness limiting factor

For sufficiently thin oxide the

tox (B/A) (t+)

B/A is linear rate coefficient

*It depends on orientation of Si, it is higher for (111)

compared to (100)

For sufficiently thick oxide

tox2 B (t+)

B is parabolic rate coefficient

*It does not depend on Si orientation



16/34

Oxidation rate constant

Ref: Gary S.May, Simon M. Sze ,Fundamentals of Semiconductor fabrication



17/34

Wet Oxidation

Oxidization in the presence of water vapor

Reaction

Si + 2 H2O -----> SiO2 + 2 H2

Due to higher diffusivity of H2O as compared to O2, itsoxidation rate is higher as compared to dry oxidation.

Oxide grown are less dense

Dielectric insulation and Masking



18/34

SiO2 thickness as a function of reaction

time and temperature

Ref: Fundamental of semiconductor fabrication by May, S M Sze



19/34

Oxidation system

*double wall Furnace in which a cooling air stream is flow able to provide fast cool-

down of a load of wafers 12th INUP Workshop at IIT Bombay


20/34

Process requirement

Dry Oxidation

For Thickness ~ 15nm

Time = 2 min

Temperature = 1000 C for all three zone* Ramp up - Constant- Ramp down

High purity O2

N2 for cleaning the inner side of furnace tube

* N2 is an inert gas, it does not react



21/34

Process steps

Set temperature of all zone to 1000 C

Wait till temperature reach the required set point

Place wafer on boat vertically between two dummy wafers

Face of the wafer should be opposite to gas flow

Push the boat in oxidation chamber and close the door

Set the oxygen flow time

Than stop the oxygen flow and ramp it down

Remove the wafer from chamber when process done

Now do the thickness measurement using Ellipsometry



22/34

Thickness Measurement

It is a non-destructive and contact less measurement tool for

the characterization of thin films.

Measuring the change in polarization state of reflected or

transmitted polarized light.

The polarization of reflected light depends on the thickness

and refractive index of the oxide layer



23/34

Procedure to measure thickness

Select model - In thisstep select and the no.

of layers

Simulation of the

measurement

Measuring and editing

the data -Assign optical

functions to each layer

Fitting the SE data to the

model using desired

parameters as a variable

to obtain the best fit.12th INUP Workshop at IIT Bombay


24/34

Basic equation of ellipsometry Light pass through the Polarization State Generator (PSG)

* Sets polarization state for the light beam incident Reflected light is then re-polarized and detected by the

Polarization State Detector (PSD)

Ellipsometry fundamental equation for Measurement

= (rp /rs) = tan () ei

*the amplitudes after reflection are denoted by rp (parallelpolarized ) and rs (perpendicular polarized)

Ellipsometry measures two parameters, which areconventionally denoted by (0- 90) and (0- 360) wheretan () is the amplitude ratio upon reflection, is the phaseshift



25/34

Following Parameters can be extracted

Thin film thickness

Refractive index

Absorption Index (extinction coefficient)

Material composition

Surface & Interface roughness

Uniformity of films and layer stacks

Band gap of material

Reflectance

Transmittance



26/34

References

Stephen A. Campbell, The science and engineering of Microelectronicsfabrication

Gary S. May, Simon M. Sze ,Fundamentals of Semiconductor fabrication

Lecture slide by Prof. R. Rao

H. G. Tompkins and E. A. Irene (Editors), Handbook of Ellipsometry

Manual for Spectroscopic Ellipsometers , SENTECH instruments



27/34

Metallization PVD techniques

Application:

Contacts Interconnects

Sputtering Evaporation

E-beam

Evaporation

Thermal

Evaporation

Metallization

Electric Field

Sputtering

Magnetron

Sputtering

RFSputtering

Reactive

Sputtering



28/34

Metal Sputtering

In sputtering, atoms are physically extracted

out from a target.

1. Using an Electricfield or Magnetron

field or RF field

metal target is

bombarded with

Ar+ ions are

2. Ar+ ions knock offthe Metal atoms.

They lands on the

wafer and

everywhere else

Ar+ Ar+Ar+

Ar+



29/34

Sputtering (vs Evaporation)

Pros: Sputtering can get better uniformity over a large size (from

larger targets).

There can be tighter (and easier) control over alloycomposition.

Pre-surface sputter cleaning of surface and deposition/etching processes to control uniformity are possible.

Con:

Certain sputtering systems (glow discharge plasmas)

require a medium level vacuum that can increasecontamination over evaporation!



30/34

Evaporation (vs Sputtering)

Advantages: Highest purity (Good for Schottkycontacts) due to low pressures.

Disadvantages: Poor step coverage, forming alloyscan be difficult, lower throughput due to lowvacuum.

Evaporation is based on the concept that there existsa finite vapor pressure above any material. Thematerial either sublimes (direct solid to vaportransition) or evaporates (liquid to vapor transition).



31/34

A. Coil heater

B. Dimpled boat

A. Electric heating

B. E-beam heating

Evaporator System

Rotary Diffu-sio

n



32/34

Contact Patterning

Photolithography technique

Shadow masking

Back side etching

Using Buffered HF 1:5, oxide on the back side

of the wafer is etched out before

metallization.



33/34

Forming Gas Anneal

Forming Gas Annealing (FGA) at 400oC for 30

min is performed for passivation.

This will make Ohmic contact for Al contacts.



34/34

Thank you..!!!


MOSCAP Fabrication

Documents

Transcript of MOSCAP Fabrication