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664
Fig.
24.
Chapter 5.2: Plasma Etching
Microwave Window
Electromagnets
Microwave
Wafer
RF Biased Chuck
A schematic diagram of a diffusion ECR source reactor.
lems associated with these sources are the inherent high cost and noncompatibil-
ity with existing parallel-plate production etchers.
MATCHING NETWORK FOR ECR SOURCES
The microwave systems usually employ one to four cylindrical tuning stubs in-
serted into the waveguide between the power source and the load [85]. These tun-
ing stubs can be adjusted either automatically or manually. For automatic tuning
to minimize reflected power, the sensors inserted in the waveguide measure the
standing waves in the waveguide and feed back the information to the signal pro-
cessing unit for calculation of the impedance in the chamber. The tuning stubs are
adjusted accordingly to minimize reflected power.
5.2.7 New Trends
665
5.2.6.6 Summary of Various Reactor Configurations
Various reactor configurations are summarized
in
Table
2.
5.2.7
NEW TRENDS
Some
of the
recent trends influencing plasma etch technology
are
briefly
de-
scribed
in
this section.
Table
2
Summary
of
Various Reactor Configurations
Category
Barrel
Diode
RIE
MERIE
Hollow-anode-
enhanced triode
Magnetically
enhanced triode
ECR
ICP
Excitation
Scheme
Inductive or Capacitive
13.56 mHz
Capacitive 13.56 mHz
Capacitive 13.56 mHz
Capacitive 13.56 mHz
Capacitive 13.56 mHz
Capacitive single or
dual freq., 100 kHz,
13.56 mHz
Microwave 2.45
gHz
Inductive 13.56 mHz
Plasma
Density
(ions/cm^)
<10'o
<10'o
<10'o
lO'O-lO"
10»0-10'2
lO'O-lO"
10'0-10'2
10'°-10'2
Pressure
Range
(mtorr)
>1000
300-10,000
30-300
10-50
10-100
10-50
0.1-10
1-100
Comments
Batch etcher
Isotropic, mainly
used for ashing
Batch etcher
Isotropic
or
directional
Batch etcher or SWE
Anisotropic
Excellent uniformity
Easy
to
scale up
SWE,
anisotropic,
acceptable unifor-
mity, difficult
to
scale up
SWE,
high localized
plasma density,
excellent uniformity,
easy to scale up
SWE,
anisotropic.
acceptable unifor-
mity, difficult
to
scale up
SWE,
very-low-
pressure operation,
high density
at
source, difficult
to
scale up
SWE,
high plasma
density, relatively
easy
to
scale up
666 Chapter 5.2: Plasma Etching
END POINT DETECTION
End point detection of
a
given etch process has become very critical because very
thin layers (<100 A) used for modern VLSI and ULSI circuits cannot tolerate
overetching. Laser reflection interferometry [86] was used in early days for end
point detection. At present, optical emission spectroscopy (OES) technique is be-
ing widely used in production for end point control. In OES, the end point is de-
termined by following time dependence of a selected wavelength of the optical
spectmm emitted by the plasma representing either a reactant or a by-product. By
monitoring the change in optical signal associated with completion of an etch re-
action, the end point can be called and the etch process can be terminated.
Presently available OES end point detection equipment are reaching a limit for
the etch applications involving etching of very little exposed areas (<2%) on the
wafer (such as via etch), due to very low signal-to-noise ratio. A major emphasis
is being given to optimization of the detection capability of the OES equipment
[87,88].
In parallel, several other techniques such as ellipsometry [89], plasma
impedance monitoring [90], infrared absorption spectroscopy [91], and neural
networks [92] are also being investigated.
CLUSTER TOOLS
In the past few years, many etch chambers have been integrated on the same plat-
form in a form of a cluster tool. The concept of a cluster tool was developed to
improve yield by reducing particles through automated, in-vacuum transfer be-
tween etch modules. The throughput is also increased because many operations
can be done in parallel. Clustering, however, places stringent requirements on
equipment reliability due to interdependence of the equipment set and the inher-
ent lack of flexibility [93,94].
ELECTROSTATIC CHUCKS
Electrostaic chucks are being integrated on proven etch platforms by almost all
equipment vendors as a replacement for conventional Helium backside or cryo-
genically cooled clamped electrodes [95]. The electrostatic chuck holds the wafer
with no front side contact, offering significant improvement in total number of
dies that can be processed. In addition, the elimination of moving clamp parts help
reduce particle generation and simplifies chamber design.
ORIGINAL EQUIPMENT MANUFACTURE [OEM] SUPPORT
In past few years, the up-time and reliability of equipment has become very criti-
cal,
because of financial constraints in high-volume manufacturing of semicon-
References 667
ductor circuits. The software and robotic systems used on an etch tool are con-
stantly being upgraded. Also, in many cases the etch equipment is linked to the
OEM through a telephone line so that the performance of the machine can be
monitored on-line and potential problems can be diagnosed and resolved without
sending out a field service engineer.
GUARANTEED PROCESSES
In the initial days of the semiconductor industry, the majority of the etch
processes were developed by engineers in the fabrication facility, on an in-house-
built or a vendor-supplied reactor. In the past few years, the trend has been shifted
toward process development by the OEM. At present, the majority of the produc-
tion machines are being sold with a guaranteed process performance for a specific
application.
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CHAPTER
5.3
Ion Beam Technology
Michael
J.
Powers
Commonwealth Scientific
Corporation
5.3.1
INTRODUCTION
Ion beam processes, broadly defined, include a wide range of unique applications.
The ability to control independently the beam's energy, size, and mass over many
orders of magnitude gives these processes a large amount of flexibility. Beam en-
ergies can range from up into the MeV, as routinely used in Rutherford back-
scattering spectroscopy (RBS) for compositional analysis of materials [1], and
down to just a few eV for plasma processing [2]. Beam sizes can range from mi-
crometers (/im), in focused ion beam systems [3], up to tens of centimeters in ion
implanters [4] and broad ion beam technology as discussed in this chapter. Ion
masses can also range from that of a proton beam (ionized hydrogen) up to, in
theory, that of the heaviest element on the periodic table. The scope of this chap-
ter is limited to broad, low-energy ion beams (50 eV-2 keV) used for etching and
deposition of thin films in research and production environments.
The broad beam ion source was originally conceived in NASA's electric space
propulsion program [5], and active research continues for this application [6].
The thin-fihn industry has since adapted the technology to meet needs in a wide
range of
applications.
Industrial broad beam ion sources consist of two main com-
ponents: a discharge chamber where a plasma is formed, and some form of ex-
traction optics that accelerates and directs positive ions out of the plasma toward
a substrate or target. A major distinction between ion beam processing and other
ISBN 0-12-325065-7 Copyright (0 1998 by Academic Press
j25.()0 All righiN of reproduction in any form reserved.
672
5.3.1 Introduction
673
plasma processes is that the plasma is maintained remotely from the process
chamber. Diameters of the optics (and the beams) in commercial sources range
from 1cm up to about 40cm [7] in diameter. The two most widely used methods
of plasma generation define two distinct types of ion soures: the Kaufman direct
current (DC) ion source [8] and the radio frequency (RF) ion source. The optics
for each are nearly identical and in many cases interchangeable.
In a Kaufman ion source, as shown in Figure 1, thermionic cathodes (heated
tungsten filaments) emit electrons that are accelerated within the discharge cham-
ber toward the anode by DC voltage. A plasma is formed by electron impact ion-
ization of gas (typically argon) atoms or molecules in the discharge chamber. Gas
flow of
1
-50 seem (depending on the source size and system pumping) maintains
the discharge chamber at a pressure of about 1 mtorr. The extraction optics limit
the flow conductance and allow the system pressure to be in the low 10""^ torr
range, several orders of magnitude less than typically found in plasma processing
systems [2]. Electromagnets or permanent magnets are often used to form a weak
magnetic field (10-100 G) within the source to enhance the plasma density and
uniformity by altering the electron trajectories as they accelerate toward the anode.
Flg.l.
L
Discharge-chamber wall
Screen
grid -,
Gas —• Discharge
chamber
1
' ' Accelerator
I y^ grid
Ion
:S-'
Cathode
Anode
beam
Neutralizer>,
(a) Kaufman DC ion source, cut-away view, and (b) schematic diagram of power supply
configuration.