Reactive Sputtering Deposition Presentation
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Transcript of Reactive Sputtering Deposition Presentation
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Reactive Sputtering to Increase Sheet Resistance of WSiN Thin FilmsRaymond Chen, Antonio Cruz, Jack Lam, Niteesh Marathe, Camron Noorzad, Yongsheng Sun, Cheng Lun Wu, Disheng Zheng
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Outline1. Problem Identification2. Design Approach3. Evaluation4. Conclusion and
Recommendations
A. Project BackgroundB. Problem ScopeC. Technical ReviewD. Design Requirements
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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 Development5. Sell products and make profit
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Example Product
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MMIC
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Project Goals Develop a fabrication process for WSiN TFRVHs:
1. Produce TFRVHs with desired specifications: 2000 /sq sheet resistance 750 ~1500 thickness 10% Standard Deviation and Uniformity
2. Demonstrate our results were consistent and repeatable
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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
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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
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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
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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
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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
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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]
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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 oC Applications:
Lower power consumption of a capacitive touch screen Mask material for x-ray lithography Hard coating Printer heads
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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.
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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
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Technical Review: WSiN Thin Film Would cause crystallization and loss of nitrogen content around 800oC 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 C1,
The coefficient of thermal expansion of Si is 3.45 106 C1 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
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Technical Review: Sheet Resistance Measure of resistance for thin film materials
instead of a bulk material Sheet resistance is defined as: Rs=(/t)
is materials 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
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Outline1. Problem Identification2. Process Design3. Evaluation4. Conclusion and Recommendations
A. Design Requirementa. Input Parameterb. Output Parameter
B. Design Approacha. Sputtering Systemb. Substrate Biasc. Justification of N2 gas flowd. Deposition Timee. Film Stressf. Thicknessg. Four-Point Probe
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Design RequirementsSputtering 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 < x
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Design ApproachFlowchart of Design:
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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: WSi3N4
Front monitor and chamber of CVC 611 System. Source: [4]
Back of CVC 611 System with the RF Power Supply. Source: [4]
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Design ApproachSubstrate 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]
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Design ApproachJustification 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]
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Design ApproachN2: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.
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Design ApproachDeposition 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 ()
Keep thickness in range Q * T = t Q = deposition rate (/s, assumed constant) T = deposition time (s)
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Design ApproachMachine 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 samples change in curvature between two tests [6]
Measured wafers initial stress before deposition [6] After deposition, wafer is measured again to determine film stress, which is
calculated from wafers change in curvature [6]E = Youngs modulus of substratev = Poissons ratio of substratets = Thickness of substratetf = Thickness of filmr = Radius of curvatureL=Length of traceB=Maximum between chord and trace
Photo of Tencor P2 Long Scan Profiler. Source: [6]
=Ets2/6r(1-)tf
r=L2/8B, L>>B
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Design ApproachMachine 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]
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Design ApproachMachine 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]
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Design ApproachMachine 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]
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Design ApproachMachine for Measuring Film Properties: PANalytical XPert 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 XPert PRO.
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Design ApproachFlowchart of Design:
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Outline1. Problem Identification2. Design Approach
3. Evaluation4. Conclusion and
Recommendations
A. OverviewB. Testing Result
a. Sheet Resistance b. Thicknessc. Stressd. Morphology
C. Assessment + Cost AnalysisD. Future works/ Next steps
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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
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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 (/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
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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]
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Nitrogen Gas Ratio Dependence Ratio test range: 10-20% N2/Ar Agrees with other experiments Exponential curve, just as Kang
et al Reinforces P. Homhuans work:
theory of N interstitials Shorter mean free path for
electrons
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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)
RS1/t Left most point: prone to
statistical error, still within one standard deviation
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Thickness Results
Wafer 8: 915 Wafers 9, 10: 974
Average of 5 measurements All thicknesses within
prescribed range 750 -1500 Some unexpected variation
WSiN Film
Si Substrate
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Thickness
SEM micrographs Wafer 9 Cleaved through
the middle Edge-on view
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Thickness Identical deposition
parameters Wafer 8: 915 Wafer 9: 974 Wafer 8
Wafer 9
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Thickness
Wafer 8 Wafer 9
Variation Q=QAVG Assumed constant
Q Q=IC [11] Ion current I, sputtering
system constant C not expected to change
Sputtering yield must change Age of target
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Thickness Variation
Q=QAVG Assumed constant
Q Q=IC [11] Ion current I, sputtering
system constant C not expected to change
Sputtering yield must change Age of target
A used sputtering target (left) compared to a new target (right). Source: [4]
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StressObserved decrease in film stress with increased N2/Ar
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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
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Stress
SEM micrographs, wafer 9
No delamination or buckling
TOP MIDDLE FLAT
SEM micrograph, patterned wafer 9
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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]
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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
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Composition Confounded by film dimensions
and interaction volume
EDXS data on patterned wafer 9
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Assessment The TFR was successfully
fabricated at Keysight Technologies ~2000 /sq
Margin of Error = 3% 10% Uniformity Thickness>750
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Cost Analysis - Materials WSi3N4 Target - ~$200 Silicon Wafers - $20/wafer
20 Wafers (10 patterned/10 not patterned) = $400 Fabrication/Testing Equipment Provided
Total Material Cost - ~$600
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Cost Analysis - Characterization SEM - $58/hr
6 hrs = $348 XRD - $60/hr
4 hrs = $240 Optical Microscope - Free
Total Characterization Cost = ~$600
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Cost Analysis - Labor Full Time Equivalent
(N2, Ar, CVC operation) Engineer - $15K/month ~ $90/hr
9 engineers x (98 hrs) = $80K Technician - $12K/month ~ $75/hr
1 technician x 18 hrs (2 days) = $1K
Total Labor Costs = ~$81K
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Overall Cost
Investment Type Cost
Materials $600
Characterization $600
Labor $81K
TOTAL $82K
Previous Estimated Cost = ~220K Savings of 220K - 82K = $138K
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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 = Year
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Outline1. Problem Identification2. Design Approach3. Evaluation
4. Conclusion and Recommendations
A. Conclusion
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ConclusionIdentification
Review
Executed Design
Successful Evaluation
Identify the problem
Understand problem scope
Review process details
Research information
Plan design based on information
Controlled experiments
Procured results
All Values were within target requirements
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ConclusionResult
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
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Future Works 1. Possible pre-production for HBT/SFS
2. Use product substratesa. GaAs and InP
3. Further Characterization a. Determine film composition
i. Rutherford Backscattering Spectrometry, XPS (ESCA), Auger spectroscopy for impurities
b. Thermal Coefficient of Resistancei. Variety of carefully controlled experiments.
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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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References 1.) 280C, Four Point Probe Resistivity Mapping System. Digital image. WOTOL, Buy&Sell Industrial Equipment Worldwide. Web.
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3.) A. Lahav, K. A. Grim, I. A. Blech, Measurement of thermal expansion coefficients of W, Si, WN, and WSiN thin film metallizations, Journal of Applied Physics, vol. 67, no. 2, pp. 34-738, January 1990
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9.) M. Powers, Sputter Deposition of Thin Films in HFTC, Santa Rosa, CA: Keysight Technologies, 2015. (slides)
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Thank You