Oxide and interface trapped

charges in MOS-C

17.11.2016

Máté Jenei
mate.jenei@aaalto.fi
 Outline

• Introduction
• Tools for measurement
• 4 types of oxide impurities
–
Fixed Oxide Charge (FOC)
–
Mobile Oxide Charge (MOC)
–
Oxide Trapped Charge (OTC)
–
Interface Trapped Charge (ITC)
• Fundamental techniques for characterization
• Homework
 About me

• First year PhD student

• Charge capturing in silicon quantum dot single-electron
pumps
Expectation Reality … :(
• My motivation:

Differential tunneling rate log(Hz)

Differential current (A)
tox = 4 nm, Toperation < 200 mK & f > 1 GHz
 Introduction
accumulation flatband depletion inversion

Metal-Oxide-Semiconductor

If we have imperfections:
• Stronger 1/f noise
• Oxide turns to be amorphous
• Field effect mobility changes

Description is applicable to all insulator-semiconductor systems,

but we aim for learning SiO2-Si (p-type).

1) Interface Trapped Charge

2) Fixed Oxide Charge
3) Oxide trapped charge
4) Mobile Oxide Charge

Image - Philipp Hehenberger: Advanced Characterization of the Bias Temperature Instability (Dissertation)
 Capacitance in MOS
Applying VG >0
oxide on p-type substrate
gi
re
e
rg
c ha
e-
ac
sp

p-type substrate
φS – surface potential (total bending of Ei) Cox – oxide capacitance
Vox – oxide voltage
VFB – flatband voltage Cp – hole capacitance
Cn – electron capacitance
Cb – space-charge density bulk charge
Cit – interface trap capacitance
 Quantifying oxide imperfections
• Q – charge per unit area (C cm-2)
• N – number of charges per unit area (cm-2)
• D – charge density (cm-2 eV-1 )
• ρ – number of charges per volume (cm-3)

Metal – Semiconductor work function difference

 Capacitance in MOS

4th floor lab Hewlett Packard

4192A LF impedance analyzer

• Frequency range:
5 Hz – 13 MHz
• Vpp = 2 V
• Vbias = -40 V to +40 V
 Capacitance measurement
• High frequency:

G – conductance of scr and oxide

Two phases: on phase RG,
and 90° phase component RC,
C and G can be extracted

• Quasi-static:
can be measured using op-amp,
not so efficient, strong 1/f noise linear ramp on gate voltage

constant
 C – V curves in theory

• High frequency ac voltage on Vg (10 kHz – 1 MHz)

• Low frequency ac voltage on Vg

ΦF – Fermi potential
Ûs – sign of Us
Vox – oxide voltage
VFB – flatband voltage
NA – acceptor doping
density
ni – intrinsic carrier
density
LDi – intrinsic Debye
length

• Rapidly changed dc bias → no inversion charge generation

→ Deep depletion
 1) Fixed Oxide Charge (Qf, Nf)

Positive charges near the Si-SiO2 interface

Origin: created during oxidation
Indistinguishable from Interface Trapped Charges
→ measurable after annealing
Cure : 1. Use high temperature oxidation → reduces Qf
2. Anneal after oxidation with N2 or Ar

Deal triangle: effect of annealing on Qf

 How to measure FOC?
Remove other oxide charges

Measure high frequency high frequency C – V curve

Compare with theory curves

How to know φMS?

Indirectly from photoemission measurements

An other approach:
No need for work function!
Just measure at different tox
 2) Mobile Oxide Charge (Qm, Nm)

Ionic impurities in oxide such as Na+, Li+, K+, H+

●
Lithium: vacuum pumps oil
●
Potassium: chemical-mechanical polishing
●
Sodium: most common contaminant
Origin: negative ions and heavy metals

Mobility: Electric field in oxide → drift velocity of

mobile ions through the oxide

Transit time :

Mobility :
 Measuring Mobile Oxide Charge:

• Bias-Temperature Stress: High temperature (150 – 250 °C) and VG applied

electric field E=106 V/cm for 5-10 min → VFB shifts (repeated with opposite
bias polarity)
Direct charge measurement, Area independent

•
Triangular Voltage Sweep: High temperature (200 – 300°C)
low-frequency C – V curve is measured → charge flow through the oxide →
slow V ramp makes sure current is due to mobile charges
•
Other methods: sodium detection or neutral impurities - SIMS
 3) Oxide Trapped Charge (Qot, Not)

Positive or negative charges due to holes or electrons in the oxide

Origin: Ionizing radiation, avalanche injection or other mechanisms
(even operation)
No cure
Not routinely measured

Oxide trapped charge distribution:

• Etch-off: etching one layer → C – V curve is measured → repeat it
→ spacial distribution (destructive)
• Photo I-V: electron injection to oxide → depends on the barrier and
injecting surface distance & barrier height
→ monitor VFB and measure
 4) Interface Trapped Charge (Qit, Nit)

Positive or negative charges in SiO2 – Si interface

Origin: Structural defects, oxidation- induced defects, metal
impurities or radiation caused bond breaking (hot electrons)
Symptoms: electrostatically coupled to the conducting channel in
MOSFET
Cure: low-temperature (~450°C) hydrogen annealing
 Interface Trapped Charge
1) Low frequency methods: traps and minority carrier inversion
charges are responding to ac signal
●
Compare low frequency C – V curve with
theory predicted trap free curve
➔
Capacitance in depletion-inversion case:

➔ Trap density (Dit = Cit/q2)

●
Compare low frequency C – V curve with
high frequency C – V curve
➔
Measure it at high frequency, but not too high – series resistance effect may arise
 Interface Trapped Charge
2) High frequency methods:
●
Terman method:
High frequency C – V measurement (no Cit), where dc gate voltage is swept slowly.
The additional depletion or inversion charges induces extra charges in the semiconductor
→ hf curve “stretches out”
●
Gary-Brown method:
Measuring C – V at reduced temperature (77K) → interface trap time increases → traps are not
responding to the room temperature ac anymore

●
Jenq method:
MOS is biased to accumulation at Troom → cooling down to 77 K, bias is swept to inversion → again
back to accumulation → The hysteresis between the two curve is propotional to the average interface
trap density
 Interface Trapped Charge
3) Conductance method ITC: capture and emission of carriers, loss is
represented by Gp, CS and Cit are combined to Cp

Measure Gp at different f and VBL → Dit

can be extracted
model simplified measured

No CS dependency in conductance
 Summary

Mobile Oxide Charge Oxide Trapped Charge Interface Trapped Charge

Bias Triangular Etch-off Photo I-V Low frequency High Conductance
temperature Voltage Sweep C – V curve Frequency method
stress C – V curve

Easy to High sensitivity Spatial Non- Large surface easy to High

measure for mobile distribution of destructive and potential range measure sensitivity, no
Pro oxide charges impurities better → better Dit need for CS
accuracy estimation estimation

Other oxide Oxide leakage Destructive Not efficient Current Requires Limited surface
Con charges current for thin method measurement cooling potential range
appear oxide is required

Sensitivity 109 cm-2 109 cm-2 3*1010 cm-2eV-1 3*1010 cm-2eV-1 109 cm-2eV-1
 Homework
Consider an MOS capacitor with tox = 40 nm and VFB = 0.
(a) Now consider a similar device except the oxide is contaminated with
mobile ions. These are very peculiar mobile ions. The upper half of the
oxide (the side nearest the gate) contains a uniform density of positively
charged ions with ρm1 = 0.04 C/cm3. The lower half of the oxide (the side
nearest the substrate) contains a uniform density of negatively charged ions
with ρm2 = −0.06 C/cm3. Determine VFB for this case.
(b) The device undergoes a bias-temperature stress at elevated
temperature with positive gate voltage and all charges move.
Determine VFB for this case. (Problem 6.5)