PECVD NEEDs Deposition Rate Control in Production
Transcript of PECVD NEEDs Deposition Rate Control in Production
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PECVD NEEDs Deposition Rate Control in Production
Lutz Eichhorn1
Michael Weinmann2, Klaus Zieger2, Michael Klick1
1Plasmetrex, 2UMS
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Overview
Motivation
Problem description
Innovation Plasma model NEED for process analysis Combined sensor head development
Plasma modes
Correlations & Results
Summary
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Motivation
Plasma enhanced chemical vapor deposition processes are among of these plasma processes which are to control difficultly. The process result indeed depends on the plasma parameters and strongly on the chamber state like wall coating and temperature. Depending on RF power the plasma changes often between - and to γ- mode and influences the deposition and process stability An in-situ control of the deposition process with the layer thickness and the refraction index as well is a requirement for reproducible manufacturing. Because d/ε is included in the capacitance of the device, the deposition rate and permittivty or refrection index are a very important parameters.
The subject matter expertise (SME) of interacting processes in a CVD chamber enables a knowledge-driven analysis and production control.
Model based plasma parameter which describe the plasma condition within the process chamber are used to control the deposition process.
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Problem description
Process instabilities, drifts, jumps of plasma parameters between different levels while processing as well as a global drop down of deposition rate of Si3N4 was
observed at CVD tools Delta_201 from SPTS. No tool parameter was suitable to indicate these instabilities.
There are known dependencies from the daily work: High deposition rate and high RF power lead to better Uniformity of film thickness Process pressure influences the uniformity of refractive index nSi3N4 = 2,
Processing cycle: clean → pre-coat → deposition
Can plasma parameters be used to characterize the plasma and indicate the quality of the deposited layers?
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Overview
Motivation
Problem description
Innovation Plasma model NEED for process analysis Combined sensor head development
Plasma modes
Correlations & Results
Summary
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Innovation - Plasma Model NEED
NEED (Non-linear Extended Electron Dynamics) is a plasma-physical model developed in particular for CVD processes to determine plasma parameters like: Plasma Resistivity Resonance Frequency Uniformity edge RF Fundamental_A
Pressure range: 60 Pa ... 1 kPa (0.5 ... 7.5 Torr)Plasma excitation frequency: > 5 MHz
The smart Plasma Metrology system uses a comprehensive, full 2d fluid plasma and a 1d nonlinear sheath model called NEED (Nonlinear Extended Electron Dynamics). This model allow the real-time analysis of the RF current and the characterization of all classical PECVD tools.
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NEED Process Parameters
The parameters provided by the new plasma physical model have a real physical background and technological meaning.
Plasma Resistivity of plasma bulk Normalized, proportional to electron collision rate / plasma density Reflects chemistry due to dependence of collision rate Gas and chamber wall temperature, gas flows First wafer effect Process: Deposition rate (DR)
Resonance Frequency of chamber plasma & chamber Plasma density Substrate holder position and impedance
(susceptor, heater block) Process: Hardware faults of heater and RF supply
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The plasma resistivity is the normalized, specific, Ohmic resistance of the plasma bulk. The higher this ratio the higher the RF bulk power → Chemistry.
Plasma Resistivity Fundamentals
No influence to the plasma resistivity
Plasma resistivity =νeffωgen
ωpe (ne)2 =
ϵ0ωgen
μe ne e0=
ϵ0ωgen
κe= ϵ0ωgenρbulk
Plasma resistivity reflects:
- Deposition rate- Chamber state- Temperature
(first wafer effect)
Le - Inductive reactance
density of electrons
LGL
- Inductive reactanceof grounded line
Equivalent electrical circuit
Plasma sheath
Plasma sheath
Plasma bulk
Capacitive reactance
Capacitive reactance
Ohmic resistance
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NEED Process Parameters cont’d
The parameters provided by the new plasma physical model have a real physical background and technological meaning.
UniformityEdge parameter at edge of large substrates usingskin depth (≥ 450 m),
Approximately proportional to plasma resistivity -2
Process: Uniformity of Deposition rate (DR)
RF Fundamental (RF current without harmonics) Generator and Matchbox faults Process: Deposition rate changes driven by variation of real plasma power
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NEED for plasma CVD
Due to the different design and pressure range of etch and PECVD chambers, the delivered parameters are different. RF driven PECVD chambers have usually a movable and heatable wafer chuck so that the wafer is not really on ground potential.
This must be incorporated in a combined plasma and chamber model. Finally that special setup of PECVD chambers hinders the algorithm to determine the electron density separately and thus we are not able to determine the collision rate from the plasma resistance.
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Innovative combined electrical and optical Sensor Development
A special sensor, which combines electrical (NEED) and optical (OES) methods was developed for process control at AVIZA/SPTS® Delta 201. OES was used to characterize and optimize the cleaning progress. Further the plasma parameters in particular the plasma resistivity show process instabilities which were also verified by OES.The correlation analysis showed which plasma and tool parameters are suitable for a reliable process characterization in production.
OES NEED
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Overview
Motivation
Problem description
Innovation Plasma model NEED for process analysis Combined sensor head development
Plasma modes
Correlations & Results
Summary
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- and g-mode visible in Plasma Resistivity
A higher resistivity at high RF power indicates a change of plasma from the - to the g -mode. In this case RF current, electron energy distribution (EEDF), and plasma density are locally different. The change of these plasma modes is dependent on chamber state.
Why does the plasma resistivity increases with RF power ?
RgLe - Inductive reactance
density of electrons
Equivalent electrical circuit
Plasma sheath
Plasma sheath
Plasma bulk
Capacitive reactance
Capacitive reactance
Ohmic resistance
90 140 190 2403,5E-03
4,0E-03
4,5E-03
5,0E-03
5,5E-03
6,0E-03
6,5E-03R² = 0,9908
Plasma Resistivity vs. RF Power
RF Power [W]
Pla
sm
a R
es
isti
vit
y
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Electron Heating in g and Mode – Driving the Chemistry
At low RF power, the electrons are mainly heated in the plasma bulk by Ohmic heating, by collisions. This is the so-called -mode. At higher RF power, the high energetic ions provide secondary electrons from the chamber and substrate surface. They are accelerated in the sheath, so they have a large energy.
The yield of electrons (secondary electrons coefficient g) depends on ion energy (RF power) and unfortunately strongly on the surface conditions (chamber conditioning).
The transition is characterized by the formation of glow seams near the space charge layer (in the pictures N2O).
Alpha to gamma mode transition in hydrogen capacitive radio-frequency discharge, Essam Abdel-Fattah, Can. J. Phys. 91, 1062 (2013).
See also: Modes and the alpha-gamma transition in rf capacitive discharges in N2O at different rf frequencies, V. Lisovskiya et al. Phys. Plas. 13, 103505 (2006).
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Literature to g and Mode
In the g-mode, there appears an additional resistance Rg in the equivalent circuit, parallel to the plasma. The higher the voltage drop at this resistance, the higher the ion energy, the higher the emission rate of secondary electrons.
The increase in the resistivity can be approximated by a series expansion with the layer capacitance CshTot, with the bulk resistance R and the equivalent resistance Rg for accelerated secondary electron emission:
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Overview
Motivation
Problem description
Innovation Plasma model NEED for process analysis Combined sensor head development
Plasma modes
Correlations & Results
Summary
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RF Power Variation
Secondary (g) electrons increase the Plasma Resistivity and Deposition Rate with higher RF power.
The RF power was varied at chamber 1 from 100 W to 250 W to find out the transition point from to g mode.
d/ε controls the capacitance of the device, therefore the Deposition Rate is a cucial parameter.
There is always a dependency between Deposition Rate and Refractive Index: The higher the Deposition Rate, the less are compactness and Refractive Index of the deposited film.
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Correlation between Plasma-Resistivity and Deposition Rate Tool 1
The different slope in the dependency of Deposition Rate and RF power (right diagram) indicate the different (,g) plasma mode at tool 1.The Plasma Resistivity is much more meaningful and robust for the process analysis. It shows despite the change of plasma mode a very stable relation of Deposition Rate and plasma resistivity (left diagram).
0,0035 0,0040 0,0045 0,0050 0,0055 0,006015
20
25
30
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40
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R² = 0,9438
Depo-Rate vs. Plasmareistivity
Resistivity
Dep
o-R
ate
[n
m/m
in]
second measurement
100 125 150 175 200 225 25015
20
25
30
35
40
45Depo-Rate vs. RF Power
RF Power [W]
Dep
o-R
ate
[nm
/min
]
-mode
g-mode
second measurement
First measurement
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Correlation between Plasma-Resistivity and Deposition Rate Tool 2
Further investigations at another tool (2) show the same behavior and indicate also the transition from - to g-Mode.The transition into the g-Mode is shown at an RF power of approximately 180 W.The correlation of Deposition Rate and Plasma Resistivity is also very much comparable to tool 1.
100 125 150 175 200 225 25015
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25
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Depo-Rate vs. RF power
RF Power [W]
De
po
-Ra
te [
nm
/min
]
0,0035 0,0040 0,0045 0,0050 0,0055 0,006015
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30
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40
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50R² = 0,987
Depo-Rate vs. Resistivity
Reistivity
De
po
-Ra
te [n
m/m
in]
-mode
g-mode
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Comparison of Tool 1 and 2
In the -mode, there is no significant deviation from chamber to chamber. In the surface dependent g-mode (> 180 W or 180 mA) the chambers differ.The deviation at higher power and parallel displacement in the right diagram seems to be caused by different surface conditions (g) and reaction rate of species at chamber wall.
30 80 130 180 23015
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45
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Depo-Rate vs. RF_Fundamental Tool 1 and 2
tool1_1. measurement
tool1_2. measurement
tool2
RF_Fundamental [mA]
De
po-R
ate
[nm
/min
]
0,0035 0,0040 0,0045 0,0050 0,0055 0,0060 0,006515
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35
45
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R² = 0,98728
R² = 0,94377
Depo-Rate vs. Reistivity
tool1
Linear (tool1)
tool2
Linear (tool2)
Resistivity
De
po-
Rat
e [n
m/m
in]g-mode:
different surface
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Correlation between deposition rate and refractive index
As already indicated by the power variation, an increased deposition rate leads to layers of lower compactness and therefore also to a smaller refractive index.
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1,98
2,00
2,02
2,04R² = 0,8995
Refractive Index vs. Deposition Rate
Dep Rate [nm/min]
Re
fra
ctiv
e In
de
x
Monitor wafers
80 100 120 140 160 180 200 220 240 26015
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30
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40
45
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1,9
2
2,1
2,2
2,3Depr Rate/ Index vs. RF Power
DR (nm/min) index
RF Power [W]
De
po R
ate
[nm
/min
]
Re
fract
ive
Ind
ex
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Plasma Parameter Variation
The changes of the RF current distribution in the chamber is likely attributed to plasma mode jumps from the α to γ and vice versa.
The plasma jumps spontaneously within a process or the next time the process starts, as shown in the diagram.
Because large impact of the RF current to the power, P = I 2 R , small changes lead to a big impact.
Deposition Rate Pre-coatDeposition
Time [s]R
F-F
unda
men
tal
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Parameters with no visible correlation have been removed. Data sets without Hercules N data were not taken into account.Target variables: deposition rate DR, thickness d, refractive index n, and film uniformity. The table shows the (linear) correlation coefficient.Film parameters and plasma parameters show a very good correlation, tool parameters do not.
Correlation Analysis – First Classification of Parameters
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Verification of SEERS by OES
The complementary of parameters N2
+ emission line (471 nm) and plasma resistivity show a similar curve shape at pre-coat and deposition process
Jumps of the OES intensity and plasma resistivity are probably caused by a switch between - and g-mode of the plasma. The running-in effect of the plasma resistance is maybe temperature triggered.
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Correlation between Plasma Parameters and Layer Quality
Plasma parameters show correlations with the layer quality (Deposition Rate and Refractive Index).In particular, the plasma parameters Plasma Resistivity and RF_Fundamentals_A (RF current) very clearly reflect the relationship to the layer quality.Jumps in these parameters during the deposition indicate some times large change in the plasma conditions.An increased deposition rate usually leads to lower Refractive Index (compactness of the layers)Correlations between layer quality and tool and NEED parameters (plasma parameters) good correlation between Deposition Rate and Plasma Resistivity moderate correlation of deposition rate to the RF peak-to-peak voltage VPP.
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Excellent Correlation of Deposition Rate and Plasma Resistivity
Due to the good correlation to the Deposition Rate, the Plasma Resistivity provides the opportunity of online process monitoring
New matchbox type and hardware changes in chamber
DR drop in test run
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Summary
NEED is a model-based plasma characterization method for the classical PECVD production plasma process.
NEED shows the presence of two plasma modes with the recipe close to the crossing point. Sometimes the plasma switches also spontaneously, likely triggered by surfaces conditions and so by mandatory plasma clean.
Excellent correlations between plasma parameters and deposition rate and refraction index were obtained – also over two chambers. These correlations can be easily used for production and quality control. Tool parameters show usually only hardware defects.