Dielectric Properties of Ceramics

1
Dielectric Properties of Ceramics

• EBB 443
• Dr. Sabar D. Hutagalung
• School of Materials Mineral Resources
  Engineering, Universiti Sains Malaysia

2
Introduction

• Dielectric materials high electrical
  resistivities, but an efficient supporter of
  electrostatic fields.
• Can store energy/charge.
• Able to support an electrostatic field while
  dissipating minimal energy in the form of heat.
• The lower the dielectric loss (proportion of
  energy lost as heat), the more effective is a
  dielectric material.
• Another consideration is the dielectric constant,
  the extent to which a substance concentrates the
  electrostatic lines of flux.

3
Dielectric Constant

• The capacitance, C, of a capacitor formed by two
  parallel plates of area A spaced d apart with the
  area between the plates filled with dielectric
  material with a relative dielectric constant of e
  is

4
Dielectric Loss

• For a lossy (imperfect) dielectric the dielectric
  constant can be represented by a complex relative
  dielectric constant
• The imaginary part of this complex dielectric
  constant, e at a frequency, ? is equivalent to a
  frequency-dependent conductivity, s(?), given by

5
Dielectric Loss

• e" is also known as the loss factor.
• The small difference in phase from ideal
  behaviour is defined by an angle d, defined
  through the equation
• tan d is known as the loss tangent or dissipation
  factor.
• A quality factor, Q, for the dielectric is given
  by the reciprocal of tan d.

6
Dielectric Loss
Equivalent circuit diagrams (a) capacitive cell,
(b) charging and loss current, (c) loss tangent
for a typical dielectric
7
Dielectric Loss

• From Q ? ?oAV/d CV
• If V being sinusoidal, total charge Q may be
  written as
• Current flow on discharge of the capacitive cell
  in time, t
• For a real dielectric the current I has vector
  components IC and IR
• I IC IR

8
Dielectric Loss

• From magnitude of these currents, also we can
  define a dissipation factor, tan ?, as
• Quality factor Q is

9
Alternating Current Theory

• Impedance of a resistance R
• Impedance of a capacitance 1/i?C
• Mean power, P, dissipated over a cycle in a lossy
  capacitor with plates of area A separated by a
  distance d

10
Dielectric Strength

• Dielectric materials are insulators (conduction
  cannot generally occur).
• However, under certain conditions, dielectric
  materials can break down and conduct a
  significant current.
• Generally, the lattice of a dielectric has
  sufficient strength to absorb the energy from
  impacting electrons that are accelerated by the
  applied electric field.
• However, under a sufficiently large electric
  field, some electrons present in the dielectric
  will have sufficient kinetic energy to ionize the
  lattice atoms causing an avalanching effect.
• As a result, the dielectric will begin to conduct
  a significant amount of current.

11
Dielectric Strength

• This phenomenon is called dielectric breakdown
  and the corresponding field intensity is referred
  to as the dielectric breakdown strength.
• Dielectric strength may be defined as the maximum
  potential gradient to which a material can be
  subjected without insulating breakdown, that is
• where DS is the dielectric strength in kV/mm,
• VB the breakdown voltage, and d the thickness.

12
Current-voltage characteristic up to breakdown
for a typical dielectric materials
13
Dielectric Strength

• Dielectric strength depends on
• material homogeneity,
• specimen geometry,
• electrode shape and disposition,
• stress mode (ac, dc or pulsed) and
• ambient condition.

14
Capacitors
Tantalum capacitor
15
Capacitors

• The basic formula for the capacitance of a
  parallel-plate capacitor is
• To increase C, one either increases ?, increases
  A, or decreases d.
• Early capacitors consisted of metal foils
  separated by wax (? 2.5), mica (? 3 - 6),
  steatite (? 5.5 - 7.5), or glass (? 5 - 10).
• The use of titania provided a significant
  increase (? 170), was followed by
  perovskite-based, such as BaTiO3 (? 1000).

16
Capacitors
C "capacitance"    q /DV Units  Coulomb/Volt            Farad (F)-----------------------------The capacitance of a capacitor is constant if q increases, DVincreases proportionately.                    Michael Faraday           (1791-1867)
17
Capacitors
18
Capacitors

• DRAM chips currently utilize capacitors with
  Si3N4 or SiO2 as dielectric materials.
• The electrodes are made of doped Si or poly-Si.
• Capacitors can be fabricated onto IC chips.
• They are commonly used in conjunction with
  transistors in DRAM.
• The capacitors help maintain the contents of
  memory.
• Because of their tiny physical size, these
  components have low capacitance.
• They must be recharged thousands of times per
  second or the DRAM will lose its data.

19
Q CV Q charge (Coulomb) C capacitance
(Farad) V potential difference (Volt) d
separation/thickness (meter) ?o permitivity of
vacuum 8.854x10-12 C2/m2 or F/m ?r
dielectric constant
20
Multilayer Ceramic Capacitor

• The multilayer ceramic capacitor (MLCC)
• where N is the number of stacked plates.
• Ideally, the dielectric should have a low
  electrical conductivity so that the leakage
  current is not too large.

21
Multilayer Ceramic Capacitor
Ceramic surface-mount capacitors.
Cut-away view of multilayer ceramic capacitor.
22
High-K Dielectric

• The bit count of MOS DRAM devices is continuously
  increasing. However, as bit count goes up,
  capacitor cell area goes down.
• The capacitance per cell must remain in the 25-30
  fF range, which means the capacitance density
  must increase.
• One approach for DRAM manufacturing is to replace
  the traditional silicon nitride silicon oxide
  with a higher dielectric constant (k) such as
  tantalum pentoxide (Ta2O5), Hf-oxide (HfO2) and
  Zr-oxide (ZrO2).

23
The roadmap of capacitor with DRAM technology.
D.-S. Yoon et al. / Progress in Materials Science
48 (2003) 275371
24
High-K Dielectric

• High-k dielectric films are anticipated to be
  required for certain applications with low power
  and leakage current specifications.
• High-k materials should be compatible with
  conventional industry standard MOSFET process
  flows using a poly-Si gate electrode.
• HfO2, ZrO2, and Ta2O5 as high-k gate-dielectrics.

25
HfO2/Poly-Si high-k transistor
26
ZrO2/Poly-Si high-k transistors
27
Typical material stack used in aTa2O5 DRAM
capacitor
28
A Review of High High-k Dielectrics

• Gate dielectric materials having high dielectric
  constant, large band gap with a favorable band
  alignment, low interface state density and good
  thermal stability are needed for future gate
  dielectric applications.
• Ultra high-k materials such as STO (SrTiO3) or
  BST (BaSrTiO3) may cause fringing field induced
  barrier lowering effect.

29
A Review of High High-k Dielectrics

• High-k gate dielectrics have a number of
  difficulties
• (1) crystallization upon heating,
• (2) dopant penetration,
• (3) fixed charge,
• (4) low channel mobility and
• (5) uncontrolled oxide formation at the
  Si/high-k interface.

30
High-K Problems
31
High-K and PolySi are Incompatible
32
Phonon Scattering in High-K
33
The Gate Stack
Expected performance trends for complementary
metal oxidesemiconductor (CMOS) transistor
technologies. The unrelenting reduction in
transistor size and the associated decrease in
gate delay for (a) an NMOS transistor and (b) a
PMOS FET are evident.
Schematic illustration of important regions in a
CMOS FET gate stack
34
EOT- equivalent oxide thickness
Schematic image of MOS transistors in the year
2003 and 2013.
35
Physical and electrical thickness of high-k gate
dielectric (ideal). SiO2 equivalent thickness
EOT is smaller than high-k physical thickness.
36
The depletion region of thickness Wd forms
adjacent to the poly-Si/oxide interface.
37

• For example, if the capacitor dielectric is SiO2,
  teq 3.90eo (A/C), eo 8.85x10-3 fF/mm, thus a
  capacitance density of C/A34.5 fF/mm2
  corresponds to teq 10 Å.
• A dielectric with a relative permittivity of 16
  results in a physical thickness of 40 Å, to
  obtain teq 10 Å.

38
Comparison of (a) stacked and (b) single-layer
gate dielectrics in a hypothetical transistor
gate stack. Either structure results in the same
overall gate stack capacitance or equivalent
oxide thickness, teq 10 Å.