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Inductively coupled plasma chemical vapour deposition (ICP-CVD)
Dr. Owain Thomas
Introduction
A wide range of insulating thin films are used in modern VLSI circuits providing electrical isolation
between conducting regions within a device, and as a final capping passivation layer. Silicon dioxide,
silicon nitride and oxynitrides are widely used. Various deposition methods are available dependant on
deposition temperature. Atmospheric pressure chemical vapour deposition and low pressure chemical
vapour deposition methods typically require high temperatures in the region of >400 °C whereas the use
of plasma enhanced chemical vapour deposition PECVD) typically requires deposition temperatures of
<400 °C.
Considerable interest has been directed towards the ability to deposit high density dielectric films at even
lower temperatures (<150 °C), especially in temperature-sensitive devices such as organic LEDs. By using
the ICP-CVD technique, Oxford Instruments have developed a deposition process in which high quality
films can be deposited with high density plasma, low deposition pressures and temperatures.
Experimental
Low temperature depositions are typically achieved by using plasma in which the gases react in a glow
discharge. This discharge ionizes the gases, creating active species that react at the wafer surface. The
most common method is a parallel plate reactor in which the sample sits on a grounded bottom electrode
and radio frequency voltage is applied to the top electrode. This creates a glow discharge between the
two plates and the gases flow radially through the discharge. Typically the bottom electrode is heated to
100-400oC and this method is usually referred to Plasma enhanced chemical vapour deposition (PECVD).
However in order to deposit high density films dielectric films at even lower temperatures (<100oC) OIPT
have developed a high-density-plasma (HDP) source in which the plasma electrons are excited in a
direction parallel to the chamber boundaries.
The HDP source used is the inductively coupled plasma (ICP) chamber, in which the plasma is driven by a
magnetic potential set up by a coil wound outside dielectric walls (typical design see figure 1). The
direction of the electron current is opposite to that of the coil currents which are, by design, parallel to
the chamber surfaces. When the plasma is excited in this manner the operating pressure can subsequently
be lowered. The lower limit of the pressure is typically dictated by the efficiency of the particular source.
In most materials processing plasmas the electron heating is primarily resistive, and the impedance of the
plasma scales with the density of neutrals available for inelastic collisions. As the impedance (pressure) is
lowered so is the ability of the source to drive the plasma.
This publication is the copyright of Oxford Instruments Plasma Technology Limited and which (unless agreed by the
company in writing) may not be used, applied or reproduced for any purpose, or form part of any order or contract or be
regarded as a representation relating to the products or services concerned.
© Oxford Instruments Plasma Technology Ltd, 2010. All rights reserved.
ICP SOURCE
WATER COOLING
BLANKED / VIEW
PORT
EXTRACTION PRESSURE
RELIEF PORT
PORT
GAS POD CONTAINING
UP TO 12 MFC
CONTROLLED GAS
LINES
RF MATCHING
UNIT
GAS INLET
ICP TOP
RF COIL
ENCLOSURE
PLASMA
RF
INDUCTION
COIL
RF
GENERATOR
O RING
SEALS
SEAL
ELECTROSTATIC
SCREEN
DIELECTRIC
TUBE
WAFER
SUPPORT
SAMPLE
CHAMBER LID
LOWER
ELECTRODE
PROCESS
CHAMBER
SiH4 Gas ring
Vacuum
TURBO
measurement
TURBO
BACKING
VALVE
ROTARY
VANE
PUMP
PNEUMATIC
GATE VALVE
N
2
APC
Chamber
vent
Vacuum
measurement
GATE
VALVE
TURBO
ROTARY
TURBO BACKING VANE
VALVE
PUMP
Vacuum
Switch
APC
Controller
CRYO COOLING
LOWER ELECTRODE HEATING
THERMOCOUPLE
Automatch
Turbo
Controller
RF GENERATOR
(LOWER ELECTRODE
BIAS)
Figure 1: OIPT ICP-CVD system
For plasma depositions there are additional system features:•
The inductively coupled coil is connected to a 13.56 MHz, 3.0kW RF generator via a matching unit.
•
The ICP coil power controls the dissociation of the plasma and the density of the incident ions in
the chamber.
•
The lower electrode is separately powered by another 13.56 MHz 300W generator, which allows
independent control of the bias voltage, i.e. the energy of the ions on the sample.
•
In order to reduce the plasma-induced damage during deposition processes and the stress level in
deposited films, the ICP-CVD system has been operated in a purely "ICP" mode by applying RF
power (100 to 2000W) to only the ICP coil, but no RF power on the lower electrode.
•
Helium pressure was applied to the back of the wafers to provide good thermal contact between
chuck and wafer.
•
o
o
The system has precise control of substrate temperature from -150 C to +400 C by using electrical
heater and liquid nitrogen. This wide temperature range is important for the advanced plasma
deposition processes of different substrate materials.
•
Pure silane (100% SiH4) is introduced into the deposition chamber through a gas distribution ring.
Other gases such as N2 and N2O are introduced into the ICP source chamber
•
Automatic pressure controller (APC) is used to control the pressure (2 to 20mTorr).
© Oxford Instruments Plasma Technology Ltd, 2010. All rights reserved.
A summary of the ICP-CVD system configurations are shown in table 1 below
Feature
System
80Plus
System100
System133
ICP-CVD180
ICP-CVD380
ICP-CVD380
240mm
240mm
Up to 330mm
Load locked
Load locked
Load locked
150mm with carriers 200mm with carriers Up to 300mm with
options available for options available for
carriers options
multi-wafers or small multi-wafers or small available for multipieces
pieces
wafers or small
pieces
No
Various dopants
Various dopants
Various dopants
Dopants
available which
available which
available which
include PH3, B2H6,
include PH3, B2H6, include PH3, B2H6,
GeH4
GeH4
GeH4
No
No
No
No
Liquid Precursors
MFC controlled 8 or 12 line gas 8 or 12 line gas box 8 or 12 line gas box 8 or 12 line gas box
box available
available
available
available
gaslines
20°C
to
400°C
0°C
to
400°C
0°C
to
400°C
0°C
to 400°C
Typical Wafer
stage temperature
range
Yes
Yes
Yes
Yes
Insitu plasma
clean
ICP
Electrode size
Loading
Substrates
ICP65
240mm
Open Load
50mm wafers
System100
Table 1: ICP-CVD Tools from Oxford Instruments
© Oxford Instruments Plasma Technology Ltd, 2010. All rights reserved.
Process Results
ICPCVD can be used to deposit several materials e.g. SiO2, SiNx, SiOxNy, a-Si and SiC. In this paper we will
concentrate mainly on the ability to deposit high quality SiO2 and SiN films at substrate temperature as
low as 20 °C. In an ICP-CVD chamber the silicon dioxide films are deposited by reacting silane which is
introduced through the gas distribution ring and nitrous oxide which is introduced through the ICP source.
Additionally silicon nitride films are deposited using silane which is introduced through the gas
distribution ring and nitrogen which is introduced through the source. Alternatively ammonia can also be
used to deposit silicon nitride but the use of nitrogen results in a higher quality film which will be
explained in more detail later.
Typical process parameters which are discussed here include deposition rate, film thickness uniformity,
refractive index, film stress, wet etch rates, and breakdown voltage.
Deposition Rates
Wet Etch Rates
ICP-CVD Film
Refractive index
Breakdown voltage
Film Stress
Deposition Rate
Traditionally ICP-CVD processes results in lower deposition rates than PECVD films. Typical deposition
rates for silicon oxide and silicon nitride are >8nm/min but higher deposition rates are now possible in
which results can be seen in the next section. In a similar way to conventional parallel plate deposition
methods many process parameters can be adjusted in order to control the process. Figure 2 and 3 below
show typical deposition rate trends with different process parameters.
ICP CVD SiNx at <100oC
ICP CVD SiO2 at <100oC
220
180
170
Dep Rate (A/min)
Dep Rate (A/min)
200
180
160
140
120
160
150
140
130
120
110
100
100
100
200
300
ICP Power (W)
3
5
7
Pressure (mT)
5.9
6.9
200
300
ICP Power (W)
Silane flow (sccm)
Figure 2: Effect of ICP power, pressure and silane
flow on ICP-CVD SiNx deposition rate
Refractive Index
100
7.9
3
5
7
Pressure (mT)
5.9
6.9
Silane flow (sccm)
Figure 3: Effect of ICP power, pressure and
silane flow on ICP-CVD SiO2 deposition rate
© Oxford Instruments Plasma Technology Ltd, 2010. All rights reserved.
7.9
The refractive index can be controlled by varying the ratio of the Si:N for silicon nitride deposition or Si:O
for the silicon oxide deposition. Silicon nitride films have typical refractive index of 2.00 (at 633nm)
although this value can be adjusted by varying the silane and nitrogen flows. Silicon dioxide films have
typical refractive index of 1.46. The RI value can be adjusted by varying the silane and nitrous oxide flows.
In both films a higher refractive index value usually indicates a silicon rich film. Figure 4 and 5 below show
the relationships of refractive index with different gas flow ratios.
o
ICPCVD SiNx at <100 C
o
ICPCVD SiO2 at <100 C
2.06
1.465
Refractive Index
Refractive Index
2.04
2.02
2
1.98
1.96
1.46
1.455
1.45
1.445
1.94
1.44
1.92
1.16
1.18
1.2
1.22
1.24
1.26
0.3
Figure 4: Variation of refractive Index with
SiH4:N2 gas ratio
0.32
0.34
0.36
0.38
0.4
Gas Flow Ratio (SiH4:N2O)
Gas Flow ratio (SiH4:N2)
Figure 5: Variation of refractive index with
SiH4:N2O gas ratio
Film Stress
In some applications such as MEMS the ability to control film stress is very important. Film stress is usually
calculated by measuring the curvature change pre- and post-deposition of the film. This difference in
curvature as a result of film deposition is used to calculate stress by way of Stoney’s equation, which
relates the biaxial modulus of the substrate, thickness of the film and substrate, and the radius of
curvatures of pre- and post-process.
In ICP-CVD silicon nitride and silicon oxide depositions the film stress can be controlled by changing
various parameters. Process pressure has the biggest influence on the silicon nitride film stress and is
shown in figure 6a below. By increasing the process pressure the film stress can be controlled from
compressive to tensile. Figure 6a also shows that very low stress can be obtained by fine tuning the
process pressure.
ICP-CVD silicon oxide films typically show compressive stress. The film stress can be adjusted by changing a
combination of parameters including SiH4:N2O ratio, temperature and RF power. Figures 6b and 6c below
shows the effect of SiH4:N2O gas ratio and temperature with film stress. Low compressive film stress can be
obtained by increasing the SiH4:N2O gas ratio and decreasing the deposition temperature.
© Oxford Instruments Plasma Technology Ltd, 2010. All rights reserved.
ICPCVD SiNx at <100oC
400
200
Stress (MPa)
0
-200
0
2
4
6
8
10
12
-400
-600
-800
-1000
-1200
Pressure (mTorr)
Figure 6a: Variation of SiNx film stress with
process pressure
ICPCVD SiO2
0
Film Stress (MPa)
-50
0
50
100
150
200
250
300
350
-100
-150
-200
-250
-300
-350
Temperature oC
Figure 6b: Variation of SiO2 film stress with
temperature
ICPCVD SiO2 <100oC
0
0.1
0.15
0.2
0.25
Film Stress (MPa)
-50
-100
-150
-200
-250
-300
SiH4:N2O ratio
Figure 6c: Variation of SiO2 film stress with
SiH4:N2O gas ratio
© Oxford Instruments Plasma Technology Ltd, 2010. All rights reserved.
0.3
Wet Etch rates
Quality of the film is most readily shown by wet etching, normally carried out with buffered oxide
etchants (BOE) which are typically blends of 49% hydrofluoric acid (HF) and 40% ammonium fluoride
(NH4F) in various predetermined ratios. Typically BOE buffered oxide etchants are used to etch window
openings in silicon dioxide layers. The primary application is the etching of thermal oxide layers in IC
production. The etch rate of the film by aqueous NH4F/HF solutions, with or without surfactant additives,
depends on three primary factors: NH4F range, etching temperature, and specific HF content. Standard
BOE etchants (40% NH4F/ 49% HF blends) contain over 30% NH4F, a range where the HF content has
primary influence on etch rate.
When testing wet etch rates of the film its usually good practice to measure the etching rate based on a
thermal oxide layer as a reference. A low etching rate film usually indicates a high density film. Figures 7
and 8 shows wet etch rate data of SiNx and SiO2 deposited using both ICP-CVD and conventional PECVD.
The data shows that films deposited at low temperature using ICP-CVD gives comparable film process
performance with films deposited using high temperature conventional parallel plate PECVD at 300 °C.
ICP-CVD v PECVD
Wet Etch Rates (10:1 BHF at 200C)
ICP-CVD v PECVD
Wet Etch rates (10:1 BHF at 200C)
450
5000
400
ICP-CVD
nm/min
300
20
< 90
-
250
100
<20
< 210
200
<3
350
ICP-CVD
PECVD nm/min
Temp
ºC
ICP -CVD
4500
3500
WER / nm/min
WER (nm/min)
200
< 100
330
< 39.4
150
100
ICP-CVD
nm/min
PECVD
nm/min
70
<1300
-
150
<160
PECVD
4000
PECVD
Temp
ºC
-
200
3000
< 920
300
<70
<310
2500
400
-
< 170
2000
500
-
< 90
1500
600
-
< 56
1000
50
500
0
0
50
100
150
200
250
300
350
0
0
0
Figure 7: Variation of SiNx wet Etch rate with
electrode temperature
50
100
150
200
250
300
350
400
450
500
550
600
650
Temperature ºC
Temperature / C
Figure 8: Variation of SiO2 wet etch rate with
electrode temperature
Breakdown Voltage
The breakdown voltage is usually measured by applying a ramped voltage across the dielectric film. The
film is normally deposited on a conductive bottom layer (either a doped Si wafer, or a metal layer)
together with a metal layer deposited on top of the deposited film. The metal layer is usually patterned
either through a shadow mask or by lift-off to form small test pads (typically <<1x1mm). To contact to
such small pads a wafer probe station is usually required. Al/Si metal layers are common but other metals
could be used. It is important that the interfaces are flat and smooth, i.e. no hillocks or bumps on the
underlying metal, and no particles on the surface or in the film, otherwise the breakdown voltage will be
significantly reduced (the metal deposition process may need some optimisation if the customer does not
have this set-up as a standard test already). This is one reason for having as small a test pad diameter since
it is possible to minimise the chances of having a particle within your measurement area. The voltage is
then ramped up until a high current peak is observed (i.e. breakdown of the film). The voltage required
depends on the film thickness (e.g. 6MV/cm = 120Volts across a 2000Å thick film).
In ICP-CVD film depositions the electrical characteristics of SiNx deposited at low temperatures (~RT) have
shown breakdown electrical fields of more than 3x106 Vcm-1 with low leakage currents [1,2]. Table 2
below shows the effect of temperature on the breakdown voltage of ICP-CVD SiNx deposited films.
© Oxford Instruments Plasma Technology Ltd, 2010. All rights reserved.
In addition ICP-CVD SiO2 also shows high breakdown voltage when deposited at low temperatures. Figure
9 shows the breakdown electrical fields of >8MV/cm when the SiO2 film was deposited at 150oC. In
comparison a typical SiO2 film deposited by PECVD at 300oC results in an electrical breakdown electrical
fields in the range of >5-6MV/cm.
Temperature
ºC
20
150
200
300
Breakdown Voltage Breakdown Voltage
ICP-CVD
PECVD
MV/cm
MV/cm
>3
>7
>3
>4
>5
Table 2: ICP-CVD SiNx typical breakdown voltage values
Current density A/cm
2
-1
10
-2
10
-3
10
-4
10
-5
10
-6
10
-7
10
-8
10
-9
10
-12
-10
-8
-6
-4
-2
0
Electric field (MV/cm)
Figure 9: Variation of current density with electric field for ICP-CVD
SiO2 film deposited 120oC. The results show breakdown voltage
~>8MV/cm.
Step Coverage
The step coverage is the ratio of film thickness along the walls of a step to the thickness of film at the
bottom of the step. This is referred to S/T and/or S/B in the figure (*) below. For conformal coverage the
ratio of S/T and/or S/B is 1. Typically good step coverage is achieved by using high temperatures (>300oC)
however it is possible to achieve excellent step coverage at low temperature using ICP-CVD. Figure (*)
below shows ICP-CVD SiNx film coverage when deposited at 20oC. In addition the step coverage also
depends on the step height and width.
© Oxford Instruments Plasma Technology Ltd, 2010. All rights reserved.
T
Substrate
S
B
Figure 10a: (Above) Definition of step
coverage
Figure 10b: (Right) SEM images of cross
section of 50 nm ICP–CVD SiN deposited
at 22oC on 150 nm metal with good step
coverage.
Courtesy of University of Glasgow [2]
OIPT New Process Improvements
Film Thickness Uniformity
Process improvements have also been made in which improved film thickness uniformity has been
achieved based on our new patented hardware design [3]. The new hardware design also allows the user
the ability to deposit layers over larger areas with excellent film thickness uniformity. The patented
hardware design is based on a new style showerhead design which we call a transmission plate. The
transmission plate is then placed in the chamber and sits between the high density plasma source and the
substrate.
The transmission plate has been optimized by adjusting the hole sizes and distribution in order to achieve
maximum film thickness improvement. The transmission plate is made of the aluminium alloy 6082 with
sufficient thickness to maintain the plate close to the chamber temperature by lateral conduction, even
when running with high ICP powers. It was found that in order to achieve “best” film thickness
uniformity for silicon nitride and silicon oxide depositions two different variants of the plates were
required.
© Oxford Instruments Plasma Technology Ltd, 2010. All rights reserved.
Figures 11 and 12 (below) show two different transmission plates for an ICP180 source.
Figure 11: Image of the silane gas ring and gas transmission
plate inside the process chamber during a plasma process
a
b
Figure 12: Two gas transmission plates. (a) Transmission plate 1 is optimised to
deposit SiO2. (b) Transmission plate 2 is optimised to deposit SiNx
Figure 13 shows a larger transmission plate which is required for the ICP380 source in order to deposit ICPCVD films with substrates up to 300mm with excellent film thickness uniformity.
Figure 13: Transmission plate used with the ICP380 source
© Oxford Instruments Plasma Technology Ltd, 2010. All rights reserved.
Figure 14 and 15 shows an example of SiNx film thickness distribution over 100 mm and 200mm silicon
wafer, using an ICP180 and an ICP380 source respectively. Oxford Instruments’ ICP-CVD systems now offer
these improved process improvements, and users will also be able to easily upgrade their existing ICP-CVD
system in order to able to achieve even better film performance.
Silicon Nitride Film thickness distribution over 4 inch wafer
N-S
W-E
1100
Thickness (Å)
1000
900
800
Uniformity across 100mm wafer
Uniformity = ((max-min)/2*average)*100%
700
±1.50%
600
500
400
0
10
20
30
40
50
60
70
80
90
100
Distance across wafer (mm)
Figure 14: ICP-CVD SiNx film thickness uniformity over 100mm using a
Plasmalab System100 with an ICP180 source
Silicon nitride film thickness distribution over 8inch wafer
3000
Thickness (A)
2500
2000
1500
N-S
W-E
1000
500
0
0
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200
Distance across wafer (mm)
Figure 15: ICP-CVD SiNx film thickness uniformity over 200mm using a
system100 with an ICP380 source
© Oxford Instruments Plasma Technology Ltd, 2010. All rights reserved.
Typical film thickness uniformity performance for low temperature depositions also depends on the ICP
source used. Table 3 shows the different film thickness uniformity depending on the ICP source.
ICP Source
Wafer Size
50mm
<±6%
<±2%
<±1%
ICP65
ICP180
ICP380
100mm
<±3%
<±2%
150mm
<±5%
<±3%
200mm
<±5%
Table 3: Typical ICP-CVD film thickness uniformities
ICP-CVD for production
High deposition Rate depositions
Deposited films such as Silicon nitride and silicon oxide are used in HBLEDS to passivate the final devices.
Current methods include batch PECVD processing which has a typical load of up to 8 x 4” substrates (and a
much larger load of 2” substrates) with a growth rate of 14-15 nm/min. Considerable amount of interest
recently have been directed towards single wafer LED processing which requires higher deposition rates to
maintain throughput requirements. It is also known that the deposition temperature must also be kept as
low as possible. These requirements restricts the ability of conventional PECVD which require high
temperatures and low deposition rates in order to allow high quality material to be deposited, probably
through allowing sufficient time for excess hydrogen to outgas from the growing film.
We have already discussed that high density films can be deposited at low temperatures (<150oC) using the
ICP-CVD technique but with typical deposition rates of 8nm/min. However recent development work at
OIPT has achieved much higher deposition rates of > 140nm/min at the same low temperatures, whilst
maintaining good film quality, film thickness uniformity and film stress control. These recent advances
have shown the capability of ICP-CVD in achieving high quality films at low temperatures with high
throughput. The higher deposition rate processes were achieved by increasing the ICP power and gas flow
mixture as shown in figure 16 below. The gas flow ratio for SiN and SiO2 deposition were then adjusted in
order to tune the refractive index (figure 17).
ICP380 ICPCVD SiNx High rate at 150C
160
Dep rate / nm/min
140
120
100
80
60
40
20
0
0
20
40
60
80
100
120
140
160
180
Total gas Flows (Sih4+n2) sccm
Figure 16: Variation of deposition rate with total gas flows
for ICP-CVD SiNx deposited at 150oC
© Oxford Instruments Plasma Technology Ltd, 2010. All rights reserved.
o
High Rate SiO2 at 150 C
D ep. rate [nm/min]
200.0
180.0
160.0
140.0
120.0
100.0
80.0
60.0
40.0
20.0
0.0
150
170
190
210
230
250
270
Total flow [sccm] (SiH4:N2O ratio constant)
Figure 17: Variation of deposition rate versus total gas
flows for ICP-CVD SiO2 deposited at 150oC
Process Repeatability
One of the most important factors of a deposition system is the ability to deposit the same film over and
over again. The repeatability and stability of the ICP-CVD process in which tests have been carried out by
depositing high deposition rate SiO2 (>140nm/min) at low temperatures (<150oC) on 75 x 100mm wafers.
Results are shown in figure 18, 19, and 20 below.
ICPCVD SiO2 at <150oC
Deposition rate [nm/min]
200
180
160
140
120
100
80
Dep. Rate
60
40
20
0
1
5 10 15 20 25
26 30 35 40 45 50
51 55 60 65 70 75
Wafer number
Figure 18: Wafer to wafer deposition rate repeatability of
<+/-2% with film thickness uniformity of <+/-3% over
100mm wafer
o
ICPCVD SiO2 at <150 C
1.51
Refractive Index
1.49
1.47
1.45
1.43
1.41
Refractive Index
1.39
1.37
1.35
0
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
Wafer number
Figure 19: Wafer to wafer refractive index
repeatability of <+/-0.3%
© Oxford Instruments Plasma Technology Ltd, 2010. All rights reserved.
o
ICPCVD SiO2 at <150 C
0
-20
1
5 10 15 20 25
26 30 35 40 45 50
51 55 60 65 70 75
Stress [MPa]
-40
Stress
-60
-80
-100
-120
-140
-160
-180
-200
Wafer number
Figure 20: ICPCVD SiO2 film stress repeatability over 75 wafers
ICP-CVD amorphous silicon and ICP-CVD silicon carbide
In addition to SiO2, SiOxNy and SiNx layers ICP-CVD can also be used to deposit other materials such as
amorphous silicon (undoped and doped) and silicon carbide.
Amorphous silicon is usually deposited using pure silane with small flows of argon in order to help strike
the plasma. Additional hydrogen is also used in order to improve the film quality. Dopants can be added
in the form of phosphorus and boron in order to change the conductivity of the layer which is particular
important in photovoltaics applications. Figure 21 below the effect of Phosphorous flow on deposition
rate for ICP-CVD amorphous si layers.
Deposition Rate (nm/min)
ICPCVD a-Si
7
6
5
4
3
2
1
0
10%PH3/Ar (sccm)
Figure 21: Effect of phosphorous gas flow on ICP-CVD a-Si
deposition rate
ICP-CVD can also be used to deposit silicon carbide. Silane is normally mixed with methane and argon is
also used to help with plasma striking. The refractive index of the SiC can be tuned by adjusting the gas
flow ratio of silane to methane. Figure 22 and 23 shows the relationship between refractive index, film
stress and methane/silane gas flow ratio.
© Oxford Instruments Plasma Technology Ltd, 2010. All rights reserved.
ICPCVD SiC
ICPCVD SiC
2.7
0
-50
0
2
4
6
8
10
12
14
16
2.5
Film Stress (MPa)
Refractive Ind ex
2.6
2.4
2.3
2.2
2.1
2
1.9
-100
-150
-200
-250
-300
-350
1.8
0
2
4
6
8
10
12
14
-400
CH 4 : SiH4 gas flow ratio
Figure 22: Variation of refractive index with
methane/silane gas flow ratio
CH4:SiH4 gas flow ratio
Figure 23: Variation of film stress with
methane/silane gas flow ratio
ICP-CVD Plasma cleaning
Chamber Plasma cleaning
In ICP-CVD processing, a significant proportion of the tool time is devoted to plasma cleaning using
etching gases to clean the process chamber. There are a number of clean gases available such CF4, C3F8,
C2F6 and NF3. However in our ICP chambers we nominally use SF6 due to ability to achieve higher etching
rates, cleaner by products and experienced etching processes which we have modified in order to
successfully clean inside the chamber. Alternative gases which we have also used are CF4 and C3F8.
The clean gases whether its SF6 or CF4 is usually used with either O2 or N2O in order to reduce the by
products formed after the clean. The clean consists of using the ICP power and also power to the
electrode. This is used to promote the fluorine in order to achieve faster etching rates. A wafer is also
suggested to be placed on the table in order to protect the surface i.e. reduce over cleaning in this area.
The plasma cleaning time and the cleaning intervals depends on the nature of the deposition. For
example if a high stress film is deposited in the chamber then the maximum deposition before cleaning is
required is reduced due to the potential of the film flaking from the chamber walls onto the sample.
Typical thickness and cleaning guidelines are shown below.
•
•
•
Cleaning should be carried out after >5microns of film deposition.
Cleaning time is dependent on type and thickness of film deposited.
Typical cleaning time is 2hours for 6-8 microns of film deposition.
Following a plasma chamber clean it is important to run a pump purge recipe in order to minimise
particulates. A typical sequence is shown below:Repeat 30 times/1min pump/1min N2 purge, 100sccm, 50mT/Loop
Conditioning of the chamber is an important step in order to achieve a repeatable process. We have
observed that ~0.5microns of deposition is required for conditioning. Figure 24 shows how the deposition
rate and refractive of the process stabilises after a chamber plasma clean and chamber conditioning.
© Oxford Instruments Plasma Technology Ltd, 2010. All rights reserved.
Repeatability Tests
ICPCVD SiNx at 20oC
Conditioning
200
2.14
2.12
180
2.1
Dep Rate
RI
2.08
2.06
140
2.04
120
2.02
2
100
1.98
80
R.I.
Dep Rate (A/min)
160
1.96
1.94
60
1.92
40
1.9
1.88
20
1.86
0
1.84
0
1000
2000
3000
4000
5000
6000
7000
8000
9000 10000 11000 12000 13000 14000 15000 16000 17000 18000
Total Thickness (A)
Figure 24: Effect of chamber conditioning on process repeatability
Surface precleaning
A plasma pre-treatment process can be applied to a particular surface in order to avoid delamination of
the deposited films especially when the film comes under some thermal or mechanical stress. Good
adhesion of the deposited films onto the underlying material depends on the type of surface and also the
type of residues on the surface. An oxygen based plasma pre-clean has the greater effect in removing
organic residues wheras a hydrogen based plasma pre-clean has the greater effect removing inorganic
residues.
If a substrate material other than Silicon is used such as Gallium Arsenide or Gallium nitride a plasma pre
treatment process is essential to achieve good film properties. For example, adhesion and quality of the
deposited film can be improved by applying a hydrogen based pre clean process prior film deposition. This
has been carried out by using an ammonia/nitrogen plasma pre clean where the ammonia dissociates into
nitrogen and hydrogen and the resulting hydrogen attacks the underlying surface giving a hydrogenated
surface which provides a good interlayer between film and substrate. The subsequent deposited film then
shows good film properties such as good adhesion, low pinholes and good electrical characteristics.
Summary
In this paper we have shown that ICP-CVD can be used to deposit various materials including SiO2, SiNx, aSi and SiC. By using the ICP-CVD technique high quality films are deposited with high density plasma, low
deposition pressures and temperatures which results in minimizing film contamination, promoting film
stoichiometry, reducing radiation damage by direct ion-surface interaction, and eliminating device
degradation at high temperatures.
© Oxford Instruments Plasma Technology Ltd, 2010. All rights reserved.
References
[1] K. Elgaid, H. P. Zhou, C. D. W. Wilkinson and I. G. Thayne : Microelectronic Engineering 73–74 (2004)
pp. 452–455.
[2] H. P. Zhou, K. Elgaid, C. D. W. Wilkinson and I. G. Thayne : Japanese Journal of Applied Physics
Vol. 45, No. 10B (2006) pp. 8388–8392
[3] EP1889946A2 ‘Surface processing apparatus’ O Thomas, AJV Griffiths, MJ Cooke (2007)
© Oxford Instruments Plasma Technology Ltd, 2010. All rights reserved.