1. Reactive Sputtering to Increase
Sheet Resistance of WSiN Thin
Films
Raymond Chen, Antonio Cruz, Jack Lam, Niteesh Marathe, Camron Noorzad, Yongsheng Sun,
Cheng Lun Wu, Disheng Zheng
2. Outline
1. Problem Identification
2. Design Approach
3. Evaluation
4. Conclusion and
Recommendations
A. Project Background
B. Problem Scope
C. Technical Review
D. Design Requirements
3. Project Motivation
• Keysight Technologies has interest of expanding into new markets:
1. Develop new platforms
2. MMIC (High-frequency monolithic microwave integrated circuit)
3. TFRVH (thin film resistor very high)
4. Students Research and Development
5. Sell products and make profit
6. Project Goals
• Develop a fabrication process for WSiN TFRVHs:
1. Produce TFRVHs with desired specifications:
• 2000 Ω/sq sheet resistance
• 750 Å ~1500 Å thickness
• 10% Standard Deviation and Uniformity
2. Demonstrate our results were consistent and repeatable
7. Problem Scope
● Concern of produce TFRVHs on Silicon Wafer
○ Use appropriate deposition method
○ Determine parameter input
○ Achieve priority specification
○ Maintain consistent output
http://project-planners.com/wp-
content/uploads/the_project_triangle1.jpg
8. Technical Review: Reactive Sputtering
• Method of introducing reactive gas into
sputtering to fabricate thin film resistor
• Easy to control deposition properties
• PVD
• Target is bombarded by energetic ions, In
this case Argon ions (Ar⁺)
• Collisions knock and sputter atoms from
the target
• Sputtered atoms flow to be deposited
onto the substrate
magnets
http://ns.kopt.co.jp/English/ca_jou-gi/joutyaku.htm
9. Technical Review: Advantages of Sputtering
• Wide range of possible sputtered materials
• High deposition rates
• High purity thin films (vacuum, low pressure)
• Good adhesion
• Good step coverage and uniformity
• Allow various parameter control
• Available in both DC and RF power
• Magnetron sputtering uses magnets behind
target to attract electrons to facilitate electron-
Argon collision
http://dir.indiamart.com/impcat/sputtering-systems.
html
10. Technical Review: Disadvantages of Sputtering
● Deterioration of equipment and target material
○ High sheet resistance uniformity percentage
■ Bad yield percentage
● Possible sputter gas incorporation into film
11. Technical Review: Why we use RF power
● Power oscillated at radio frequencies sustains the Argon plasma
○ If not. The negative charge applied to target can be neutralized by Ar⁺
○ Ions will not be attracted to target
● Ions are too heavy and slow to follow this frequency
● Electrons can follow this frequency and build up a negative self bias on the
target
12. Technical Review: Why Ar⁺
● Big gas ion
● Inert to WSiN
● Produce high sputtering yield
○ manufacturing process to be
timely and efficient
● Relatively inexpensive and
available in high purity
Source: [9]
13. Technical Review: Tungsten Silicon Nitride
● Ability to reduce the local atomic ordering when sputtered due to argon ion
bombardment
● High melting point of around 3000 o
C
● Applications:
○ Lower power consumption of a capacitive touch screen
○ Mask material for x-ray lithography
○ Hard coating
○ Printer heads
14. Technical Review: Target Processing
● Composite from hot pressing
Tungsten powder and Silicon
Nitride powder
● Because of this, we suspect that
the sputter result will be silicon
nitride and tungsten.
15. Technical Review: Substrate and Chamber
● Silicon substrate (100) orientation with approximately 100 nm silicon dioxide on
top
○ Negative substrate bias: -60 V
■ Better guidance of WSiN movement to substrate and substrate
adhesion, increasing nitrogen content
○ Low cost substrate for experimental purpose
○ One patterned + one non-patterned
● Real substrate will be GaAs and InP
● High vacuum chamber: 10 mTorr
○ lower sputter rate
○ increase the mean free path of sputtered target
16. Technical Review: WSiN Thin Film
● Would cause crystallization and loss of nitrogen content around 800o
C
● Nitrogen atoms bonded to silicon atoms of the Tungsten and Silicon
Amorphous Network increase the resistivity
● Coefficient of thermal expansion of WSiN is 6.37 X 10^-6 °C−1,
○ The coefficient of thermal expansion of Si is 3.45 × 10−6 °C−1
○ This difference can result in significant thermal stresses if the Si
substrate is heated during deposition.
● Amorphously deposited on the substrate
● Very effective at blocking atom diffusion
● Chemical Inertness
17. Technical Review: Sheet Resistance
● Measure of resistance for thin film materials
instead of a bulk material
● Sheet resistance is defined as: Rs
=(⍴/t)
○ ⍴ is material’s resistivity and t is
thickness
● Has unit of ohm but usually use
“ohm/square”
● Only need to specify length and width of the
resistor to define value.
● The ratio L/W represents the number of unit
squares of material in the resistor
18. Outline
1. Problem Identification
2. Process Design
3. Evaluation
4. Conclusion and Recommendations
A. Design Requirement
a. Input Parameter
b. Output Parameter
B. Design Approach
a. Sputtering System
b. Substrate Bias
c. Justification of N2
gas flow
d. Deposition Time
e. Film Stress
f. Thickness
g. Four-Point Probe
19. Design Requirements
Sputtering Input Parameters
Fixed Parameters
RF Power 750 W
Substrate Bias -60 V
Total System Pressure 10 mTorr
Total Flow Rate 40 sccm
Controlled Parameters
Gas Ratio (N2
: Ar) 0.15 : 1
N2
Flow Rate 5.2 sccm
Ar Flow Rate 34.8 sccm
Deposition Time 1027 Seconds
Target Thin Film Parameters
Sheet Resistance 2000 ohm/square
Margin of Error ±3%
Standard Deviation ±10%
Uniformity ±10%
Thickness (x) 750 Å < x <1500 Å
21. Design Approach
● CVC 611 Reactive Sputtering System
○ Older machine in the wafer fab
○ Ion mill chamber to clean wafer before
deposition process [4]
○ Rotating deposition to increase sheet
resistance uniformity [4]
● Sputtering Target: WSi3
N4
Front monitor and chamber
of CVC 611 System.
Source: [4]
Back of CVC 611 System with
the RF Power Supply. Source: [4]
22. Design Approach
Substrate Bias
• Negative bias allows for Ar⁺ ion
bombardment onto substrate, minimizes
long range atomic order
(amorphous thin film) [4]
• Bias repels electrons from depositing onto
the film [4]
• Standard value for the CVC System in the
wafer fab [4]
Diagram of RF Sputtering including Substrate Bias. Source: [4]
23. Design Approach
Justification to Incorporate N2
Gas into Film:
• Increasing sheet resistance = smaller mean
free path of electrons (more defects in film
microstructure) [5]
• Add atoms that bond to the amorphous
network. Saturation point: atoms added as
point defects [5]
• Nitrogen already a part of the target in the
CVC System chamber [5]
Diagram of RF Sputtering including Substrate Bias. Source:
[4]
24. Design Approach
N2
:Ar Gas Ratio
S.M. Kang, et al, showed that
increased presence of N2
gas
in chamber increases sheet
resistance. Sheet resistance of
thin film significantly increases
above ~10%[1].
Gas flow rates calculated
accordingly.
25. Design Approach
Deposition Time
● Keysight suggested deposition time of 20 minutes
○ Confirmed by Kang, et al, in their experiment [1]
● Useful equation: Rs
proportional to 1/t
○ Rs
= sheet resistance (ohm/sq.)
○ t = thickness (Å)
● Keep thickness in range
○ Q * T = t
○ Q = deposition rate (Å/s, assumed constant)
○ T = deposition time (s)
26. Design Approach
Machine for Stress Measurements: Tencor P2 Long Scan
Profiler
• Stressed films bend substrates outward (compressive stress) or inward (tensile
stress) [6]
• Tencor P2 determines film stress by measuring sample’s change in curvature
between two tests [6]
• Measured wafer’s initial stress before deposition [6]
• After deposition, wafer is measured again to determine film stress, which is
calculated from wafer’s change in curvature [6]
E = Young’s modulus of substrate
v = Poisson’s ratio of substrate
ts
= Thickness of substrate
tf
= Thickness of film
r = Radius of curvature
L=Length of trace
B=Maximum between chord and trace
Photo of Tencor P2 Long
Scan Profiler. Source: [6]
σ=Ets
2
/6r(1-ν)tf
r=L2
/8B, L>>B
27. Design Approach
Machine for Thickness Measurement: Tencor P12
Profilometer
• Surface stylus profilometry determines change in height across sample.
• Patterned photoresist was applied onto silicon wafers before deposition.
• WSiN films were deposited onto patterned silicon wafers.
• After deposition, acetone was used to strip away photoresist.
Tencor P12 Profilometer. Source:
[9]
Resist/Deposition/Strip sequence.
Source:[17]
28. Design Approach
Machine for Rs
Measurements: 4P Automatic Four Point Probe, 280C
● Current passes through the outer two probes and film [7]
● Voltage across two inner probes is measured [7]
● Rs
= 4.53 x V/I [7]
● Measures at 25 points for average Rs
value.
Schematic of Four Point Probe
machine. Source: [7]
280C Four Point Probe, Model 4D. Source: [8]
29. Design Approach
Machine for Surface Topography and Chemical
Composition: FEI SCIOS Dual Beam FIB/SEM
● Scanning Electron Microscope (SEM)
○ Surface topography and composition at high
resolution
○ Electron beam shoots at sample and interacts
● Energy-Dispersive X-ray Spectroscopy (EDXS)
○ Separates characteristic X-rays into elements
○ Relative amounts of elements in sample
● Electron Backscatter Diffraction (EBSD)
○ Measures electrons diffracted from atomic planes
○ If crystalline, gives crystal orientation and grain
size.
Photograph of a SEM. Source: [16]
Interaction of Electron Beam with sample.
Source: [16]
30. Design Approach
Machine for Measuring Film Properties:
PANalytical X’Pert PRO
• X-Ray Reflectivity (XRR)
• Shoots X-Rays at film sample from a
range of small, grazing angles.
• X-rays reflect toward detector.
• Gives information about film
thickness, density, surface
roughness, and degree of
crystallinity.
Basic concept of XRR.
PANalytical X’Pert PRO.
32. Outline
1. Problem Identification
2. Design Approach
3. Evaluation
4. Conclusion and
Recommendations
A. Overview
B. Testing Result
a. Sheet Resistance
b. Thickness
c. Stress
d. Morphology
C. Assessment + Cost Analysis
D. Future works/ Next steps
33. Overview of Results
● Graph compares our last 3
wafers using all of the
same final parameters:
○ Dep time= 1027 s
○ 15% N to Ar ratio
● Rs
close to 2000
● Good consistency
● Wafers 9 and 10:
sputtered simultaneously
34. Sheet Resistance
• 8: batch-to-batch comparison
• 9 and 10: wafer-to-wafer
comparison
• Standard engineering margin of
error = 3%
• Note: 9 and 10 only have same
RS
, not std. dev. or uniformity.
Wafer
Number
#8 #9 #10
Rs (Ω/sq) 1975 2060 2060
Margin of
Error (%)
1.25 3.00 3.00
Std.
Deviation
(%)
5.64 5.86 6.05
Uniformity
(%)
10.13 10.61 11.12
35. Causes of Variation
● Target condition affects
sputtering:
○ Wear pattern directs
sputtered atoms
○ Batch-to-batch variation
● Old sputtering system
A used sputtering target (left) compared to a
new target (right). Source: [4]
36. Nitrogen Gas Ratio Dependence
• Ratio test range: 10-20% N2
/Ar
• Agrees with other experiments
• Exponential curve, just as Kang
et al
• Reinforces P. Homhuan’s work:
theory of N “interstitials”
• Shorter mean free path for
electrons
37. Film Thickness Dependence
• Dep time was altered after
viewing results of 15% N to fine
tune RS
• Thinner films yield higher RS
(less is more)
• RS
∝1/t
• Left most point: prone to
statistical error, still within one
standard deviation
38. Thickness
● Results
○ Wafer 8: 915 Å
○ Wafers 9, 10: 974 Å
● Average of 5 measurements
● All thicknesses within
prescribed range 750 Å-1500 Å
● Some unexpected variation
WSiN Film
Si Substrate
41. Thickness
Wafer 8
Wafer 9
● Variation
○ Q=QAVG
○ Assumed constant
● Q∝γ →Q=ICγ [11]
○ Ion current I, sputtering
system constant C not
expected to change
○ Sputtering yield γ must
change
■ Age of target
42. Thickness
● Variation
○ Q=QAVG
○ Assumed constant
● Q∝γ →Q=ICγ [11]
○ Ion current I, sputtering
system constant C not
expected to change
○ Sputtering yield γ must
change
■ Age of target
A used sputtering target (left) compared to a
new target (right). Source: [4]
44. Stress
● Residual vs. thermally induced stresses
○ Thermal stress not significant [8]
● Residual stress due to
○ Ar+
contamination
○ Densification effects
● Stress reduction due to
○ Change in microstructural regime [15]
■ Further characterization to
confirm
46. Stress
● No delamination or buckling was observed
● Stress greater on GaAs substrates than on Si
substrates
○ Lattice constants, CTE
● Stress on GaAs can be reduced by annealing
[3]
○ Possible increase in resistivity [4]
47. Morphology
• Previous studies of
WSiN thin films
suggested our film
would be amorphous
[3,4]
• EBSD showed no
crystallinity
• XRD indicates degree
of roughness
X-ray reflectivity curve
53. Overall Cost
Investment Type Cost
Materials $600
Characterization $600
Labor $81K
TOTAL $82K
● Previous Estimated Cost = ~220K
○ Savings of 220K - 82K = $138K
54. Return on Investment
● Estimated Leverage Sales (Keysight Technologies) - $13M/year
○ $10M HBTs, $3M SFSs
● Estimated Cost of Production is Half the Estimated Sales
○ $13/2 = $6.5M Cost of Investment
○ Total Cost = Production + Labor = $6.5M + $82K = $6.582M
● Estimated Time of Return on Investment Based on Information Provided
○ $6.582M/$13M/yr = ½ Year
57. Conclusion
Result
● 2000 Ohm/sq.
● 3% difference in
range
● 10% standard dev.
Risks and Concern
● There is a run to
run variation which
will affect the data
● Must watch out for
the life cycle of the
target.
Recommendation
● 750W power
● -60V constant biasing
● 10 mTorr Total
Pressure
● 40 sccm flow rate
● A 15% nitrogen to
argon flow
● 1027 sec. deposition
time
58. Future Works
1. Possible pre-production for HBT/SFS
2. Use product substrates
a. GaAs and InP
3. Further Characterization
a. Determine film composition
i. Rutherford Backscattering Spectrometry, XPS (ESCA),
Auger spectroscopy for impurities
b. Thermal Coefficient of Resistance
i. Variety of carefully controlled experiments.
59. Acknowledgements
The authors would like to thank:
• Nick Kiriaze
• Rijuta Ravichandran
• Steven Zhang
• Ricardo Castro
• Michael Powers
• Vache Harotoonian
• Erkin Seker
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