Overview of Plasma Processing in Microelectronics Fabrication
99
PRINCIPLES OF PLASMA PROCESSING
Course Notes: Prof. J. P. Chang
PART B1: OVERVIEW OF PLASMA PROCESSING IN
MICROELECTRONICS FABRICATION
I. PLASMA PROCESSING
Plasma processing is the most widely used
chemical
process in microelectronic industry for thin
AMD-Athlon
film deposition and etching. Its application extends
to surface cleaning and modification, flat panel
AMD-K6-2
display fabrication, plasma spary, plasma microAm486
discharge and many other rapidly growing areas.
The fundamental understanding of plasma processes
is now sufficient that plasma models are emerging as
tools for developing new plasma equipment and
process, as well as diagnosing process difficulties.
0.35µ (1,200,000) 0.25µ (9,000,000)
0.18µ (37,000,000)
In addition, plasma diagnostics are now being
35mm2
78mm2
120mm2
implemented as process monitors, endpoint
Courtesy of AMD (SDC Director John Caffal)
detectors, and process controllers to improve
Fig. 1. The evoluation of microprocessors processing flexibility and reliability.
shown with the device critical dimension and
Take plasma etching processes for example,
the packing density.
high density plasma reactors have been developed to
address the challenges in patterning features less
than 0.25 µm with high aspect ratios (Fig. 1). The
challenges include maintaining etching uniformity,
etching selectivity, high etching rate, and reducing
the substrate damage. Various high density plasma
sources such as transformer coupled plasma (TCP)
TCP Coil
Dielectric
and electron cyclotron resonance (ECR) reactors
window
have been developed to achieve high fidelity pattern
transfer for manufacturing of ultra large scale
13.56 MHz
integrated (ULSI) electronic devices (Fig. 2).
TCP Coil
Dielectric
In a transformer coupled plasma (TCP)
window
Gas inlet
reactor, a spiral planar inductive coil is mounted on
a dielectric window on the reactor. Plasma is
Chamber
Plasma
Plasma
generated by coupling the oscillating radio
Wafer
sheath
frequency (rf) magnetic field (13.56 MHz)
Bottom
13.56 MHz
Electrode
inductively. Plasma sheath, a dark space between
plasma and the electrodes, is developed due to the
Fig. 2. A schematic diagram of a high density different mobility of electrons and ions. The bottom
electrode can be powered by a separate rf source to
transformer coupled plasma (TCP) reactor.
control the ion bombardment energy. The energetic
ions and reactive neutrals produced are highly
reactive, thereby facilitating surface (and/or gas
phase) reactions with lower activation energies, and
enhance greatly the reaction kinetics.
In plasma processes for the fabrication of
microelectronics, DC or rf glow discharges are used
100
Part B1
Thin film deposition
Thin film
Substrate
UV
Photoresist coating
& development
II. APPLICATIONS IN MICROELECTRONICS
Plasmas are used in several major
microelectronics processes: sputtering, plasmaenhanced chemical vapor deposition (PECVD),
plasma etching, ashing, implantation, and surface
cleaning/modification, each is described below and a
few process steps are shown schematically in Fig. 3:
Mask
Photoresist
Thin film etching
plasma
Photoresist ashing
plasma
Fig. 3. Photolithography process flow.
Table 1. Typical operating ranges of a glow
discharge used in microelectronic fabrication.
Property
to etch, deposit, sputter, or otherwise alter the wafer
surfaces. These plasmas produce highly reactive
neutrals and ions at low temperatures by the
introduction of energy into the plasma through its
free electrons that in turn collide with the neutral gas
molecules.
Range
Pressure
0.001-10 torr
Electron density
Low density
109-1011 cm-3
High density
1011-1013 cm-3
Average electron energy
1-10 eV
Average neutral/ion energy 0.025-0.05 eV
Ionized fraction of gas
Low density
10-7-10-5
High density
10-3-10-1
Neutral diffusivity
20-20,000 cm2/s
Free radical density
5-90%
Power dissipation
0.1-10 W/cm2
Deposition:
- Semiconductor (silicon)
- Metal (aluminum, copper, alloys)
- Dielectric (silicon dioxide, silicon nitride,
metal oxides, low-k dielectrics)
Etching:
- Semiconductor (silicon)
- Metal (aluminum, copper, alloys)
- Dielectric (silicon dioxide, silicon nitride,
metal oxides, low-k dielectrics)
Ashing:
- Photoresist removal
- Photoresist trimming
Implantation:
- Dopant ion implant (B+, P+, As+ …etc.)
Surface Cleaning / Modification:
- Contamination removal
- Modification of surface termination
In each the plasma is used as a source of ions and/or
reactive neutrals, and is sustained in a reactor so as
to control the flux of neutrals and ions to a surface.
The typical ranges of properties for a glow discharge
used in microelectronic fabrication are as shown in
Table 1.
In sputtering processes (Fig. 4), ions are
extracted from a plasma, accelerated by an electric
field, and impinge upon a target electrode composed
of the material to be deposited. The bombarding
ions dissipate their energy by sputtering processes in
which the surface atoms are ejected primarily by
momentum transfer in collision cascades. The
ejected atoms are deposited upon wafers that are
placed within line-of-sight of the target electrode,
thus facilitating the vapor transport of material
Overview of Plasma Processing in Microelectronics Fabrication
Tasolid + Ar+ → Tagas → Tafilm
Ta
Ar
Si
Fig. 4. Sputtering deposition process
Si (OC 2 H 5 ) 4 + e − → Si (OC 2 H 5 ) 3 (OH ) + C 2 H 4 + e −
O 2 + e − → 2O + e −
O + Si (OC 2 H 5 ) 3 (OH ) → Si (OC 2 H 5 ) 2 (OH ) 2 + C 2 H 4 O
e-
+ e
+
+
e-
+
e-
e-
+
+
+
E
e+
}
sheath
feature
Si
Silicon
Fig. 5. A plasma-enhanced chemical vapor
deposition process.
101
without appreciably heating either the target
electrode or the wafer on which the film is
deposited.
Plasma enhanced chemical vapor deposition
uses a discharge to reduce the temperature at which
films can be deposited from gaseous reactants
through the creation of free radicals and other
excited species that react at lower temperatures
within the gas-phase and on the surface (Fig. 5).
The quality of the deposited film often can be
improved by the use of the plasma ion flux to clean
the surface before the deposition begins and by
heating during processing. In addition, the ion flux
can alter the film during deposition by cleaning,
enhancing the mobility of adsorbed species, etc.
In plasma etching, as shown in Fig. 6, the
plasma produces both highly reactive neutrals (e.g.,
atomic chlorine) and ions that bombard the surface
being etched. The neutrals react with the surface to
produce volatile species that desorb and are pumped
away. Ion bombardment often increases the etching
rate by removing surface contaminants that block
the etching or by directly enhancing the kinetics of
the etching. Ultra large scale integration (ULSI)
requires the etching of films with thickness
comparable to their lateral feature dimensions.
Directional plasma etching processes must be used
to pattern such features to obtain the necessary
fidelity of pattern transfer. Wet etching processes
(which use aqueous acids or bases) and chemical
based plasma etching processes are typically
isotropic, and produce undercutting of the pattern at
least equal to the film thickness.
An ideal plasma etching process requires
perfect pattern transfer by anisotropic (directional)
etching of polysilicon (the portion not protected by
the photoresist), and no etching of either photoresist
or silicon dioxide upon ion bombardment (infinite
selectivity).
This typically requires highly
directional ions and minimal spontaneous etching of
polysilicon by reactive neutrals.
In reality, many non-ideal factors including
transport of reactive species into the feature and
interactions of reactive species with the surface
affect etched profiles. For example, ions undergo
collisions across the sheath, bear a finite angular
distribution, and affect the etching anisotropy. As
the aspect ratio (depth/width) of the feature
102
Part B1
e
bulk plasma
e-
+ e
e-
+
+
Cl+
Cl
ee+
+
e-
+
_
Cl2
+
+
}
+
E
SiCl2
sheath
mask
SiCl4
+
poly-Si
Cl
→ SiCl4 ↑
Si + 4Cl 
SiO2
oxide
Fig. 6. Chlorine ion-enhanced etching of
photoresist patterned polysilicon. The major
reactive species in plasma include energetic
chlorine ions (Cl+) and reactive neutrals (Cl,
Cl2, SiCl2).
O2 + e − → 2O + e −
e-
+ e
+
+
e-
+
e-
ee-
+
+
+
E
+
}
sheath
Resist + O → CO2 + H 2 O
Si
Silicon
Fig. 7. An oxygen plasma ashing photoresist
process.
increases, shadowing effect of the neutral species
due to their non-zero reaction probabilities on the
sidewall of the feature can cause concentration
gradients within the feature and significantly alter
the etching profiles and the etching uniformity. The
etching products or by-products with high sticking
probabilities can deposit on the surfaces within the
feature and alter the profile evolution. Specifically,
etching of photoresists or electron impact
dissociation of the etching products (SiCl4) lead to
the formation of carbonaceous contamination and
SiCl2 that form passivation layers in the feature and
prevent the sidewall from being etched. The balance
between simultaneous etching and deposition
controls the overall profile topography change
during plasma etching processes.
Moreover,
shadowing of the isotropic electrons and positive
charging on the silicon dioxide surface in etching
high aspect ratio (width/depth) features can build up
an electrical field on the oxide surface to distort the
ion trajectory. These etching phenomena are highly
convoluted and a thorough understanding of the
fundamental mechanisms by which the etching
anisotropy is achieved is required to develop rapid,
directional, high resolution and damage-free etching
processes.
Photoresist ashing is typically done with an
oxygen plasma, as shown in Fig. 7. Resist trimming
allows finer feature definition and will be detailed in
the Epilogue.
Other processes including ion
implantation, surface modification, and surface
preparation will be discussed in the later part of this
course.
Major References for Part B
1. H. H. Sawin, Plasma processing for microelectronic
fabrication, Lecture notes, MIT (1996).
2. B. Chapman, Glow discharge processes, Wiley
(1980).
3. M. A. Lieberman and A. J. Lichtenberg, Principles of
plasma discharges and materials processing, Wiley
(1994).
4. J. R. Hollahan and A. T. Bell, Fundamentals of
plasma chemistry: Techniques and applications of
plasma chemistry, Wiley (1974).
5. J. F. O’Hanlon, A User’s guide to vacuum
technology, Wiley (1989).