LECTURE 5
SUMMARY OF KEY IDEAS
Etching is a processing step following lithography: it transfers a circuit
image from the photoresist to materials form which devices are made
or to hard masking or sacrificial material, such as SiO2.
General etch requirements:
1. Obtain desired profile (sloped or vertical)
2. Minimal undercutting or bias
3. Selectivity to other exposed films and resist
4. Uniform and reproducible
5. Minimal damage to surface and circuit
6. Clean, economical, and safe
There are two main types of etching used in IC fabrication: wet
chemical etching and dry or plasma etching.
Wet etching has usually excellent selectivity since chemical reactions
are very selective but it is also isotropic. Selectivity comes from
chemistry; directionality usually comes from physical processes, such
as ion impacts in plasma etching systems.
Wet etching is done in liquid solution of aggressive chemicals that
react with solids, such as silicon, oxides, nitrides, and metals. The
reaction products are usually in liquid form and are removed with the
solution.
Dry etching is done in ionized gas at low pressure.
GAS PHASE AND VACUUM
Many processing steps in micro-fabrication and semiconductor technology
are conducted in gases at atmospheric pressure or at lower pressures.
Materials in gas phase are also used to form thin film, etch, and dope
semiconductors. Gasses at considerably lower than atmospheric pressures
are refereed to as vacuum. Ideal vacuum is not achievable: different levels
of vacuum only correspond to different pressure ranges and thus the
molecular, or atomic, densities. The ideal gas law is a good model for
understanding properties of gasses at low pressure and normal and elevated
temperatures.
PV  no RT  no kN AT
where P is gas pressure, V volume, no is number of moles, k Boltzmann’s
constant,
NA Avogadro’s number, and T absolute temperature.
The molecular density n is given by the ratio of the number of molecules and
volume.
n
no N A
P

V
kT
Thus n is proportional to pressure and inversely proportional to absolute
temperature.
The gas molecules are in random motion and the absolute temperature is the
measure of their kinetic energy. Distribution of molecular velocity is given
by Maxwell-Boltzmann equation:
298.15K (25 C)
Thermal random motion of gas molecules results in their mutual collisions
and scattering. They travel only certain distance before they collide and
scatter changing direction of their motion. The distances between collisions
vary randomly and the mean distance or mean free path  depends on both
molecular density and the molecular size (cross section). In terms of gas
parameters:

kT
2 Pd 2
For air at room temperature a handy equation gives  = 5x10-3/P, where  is
in centimeters and P is in Torrs. Thus in “high vacuum” of 10-6 Torr  = 50
meters.
Since the distance between molecular collisions in the high vacuum region is
longer than the dimensions of the vacuum chamber, the molecules move in
straight lines being deflected only by the chamber walls.
Every surface in the chamber, as well as the walls is subjected to impacts of
the molecules. These impacts are creating the gas pressure. The flux of the
molecules on any surface (the number of impacts per unit area per unit time)
is equal to the product of the velocity component normal to the surface and
the mean molecular density:
_
  v normal n  P 2mkT
where m is the mass of the molecules in mass units (not the molecular mass).
When P is given in Torrs and  in cm-2 s-1 then
  3.51 10 22 P MT
where M is molecular or atomic mass.
In film deposition process the flux of the molecules of atoms of interest is
often called arrival rate. If all arriving molecules or atoms stick to the
surface and form a film, one can consider the time that it takes to form one
molecular layer on the surface (monolayer):  = /Ns, where Ns is molecular
or atomic surface density of the film.
The values of these basic quantities for air at room temperature age given in
the table below.
BASICS OF VACUUM
Examples of basic molecular parameters in different vacuum ranges
(for air at room temperature)
CONDITIONS
TYPICAL
PRESSURE
Molecular
Density
MEAN FREE
PATH
P [Torr]*
cm-3

TIME TO
FORM A
MONOLAYER
m
Atmospheric
Pressure
760
2.4x1019
66 nm
3 ns
Low Vacuum
1
3.2 x1016
50 m
2.4 s
Medium
Vacuum
10-3
3.2 x1013
50 mm
2.4 ms
High Vacuum
10-6
3.2 x1010
50 m
2.4 s
Ultra High
Vacuum
10-10
3.2 x106
500 km
6.4 h
* 1Torr = 133.3 Pa
1Pa = 1N/m2
Plasma
P = ~1Torr
~500V
Power supply
1 mTorr < P < 10 Torr
Plasma density ne/n 10-7 – 10-3
Neutrality ni = ne
E ≈ 0 (except near boundary)
Plasma etching combines both chemical and physical effects. One or
the other can dominate depending on the apparatus geometry,
voltage, frequency, magnetic field as well as gas pressure and its
chemical composition.
Physical effects are due to ion impacts. Inert gasses such as argon
cause only physical effects (sputtering). Using reactive gasses results
in chemical effects (reactions) but also in physical effects, such as in
Reactive Ion Etching (RIE) process. When the effects of ion impacts
are minimized purely chemical etching can occur (Plasma Etching).
The diagram below shows schematically contributions of physical
and chemical processes to etching in relation to the gas pressure in
the reactor and the ion energy. The direction of the arrows shows
increase of a given parameter.
Selectivity is the ability to etch only a selected material and not
another (such as silicon dioxide but not silicon).
Anisotropy is a measure of directionality of the etching (vertical vs.
horizontal).