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NASA Aeronautics Research Institute
Concept Demonstration of Dopant Selective Reactive Etching (DSRIE) in
Silicon Carbide
NASA Aeronautics Research Mission Directorate (ARMD)
2014 Seedling Technical Seminar
February 19–27, 2014
Robert S. Okojie, PhD
NASA Glenn Research Center, Cleveland OH
Email: [email protected]; phone: (216)433-6522
NASA Aeronautics Research Institute
Outline
Reactive Ion Etching: The Basics
Observed Phenomenon
Technical Approach
Results
Innovation and Impact on Mission
Next Steps
February 19–27, 2014
NASA Aeronautics Research Mission Directorate 2014 Seedling Technical Seminar 2
NASA Aeronautics Research Institute
Reactive Ion Etching (RIE): The Basics
February 19–27, 2014
NASA Aeronautics Research Mission Directorate 2014 Seedling Technical Seminar 3
Halogen based gas (i.e., SF6) is ionized to create F
ions and
neutrals. Neutrals react chemically with target. F ions accelerate toward the target substrate under E-field and are adsorbed. F ions assist neutrals chemical reaction, leading to enhanced chemical etching of target. With argon added and ionized, Ar
+ physical bombardment of
target causes erosion. Results in increased chemical reaction and higher etch rate.
E-Field
F- F
- F- F
- F
-
P.H. Yih et al.: A Review of SiC Reactive Ion Etching in Fluorinated Plasmas, phys. stat. sol. (b)202, 605 (1997)
Neutrals
NASA Aeronautics Research Institute
Observed Phenomenon
February 19–27, 2014
NASA Aeronautics Research Mission Directorate 2014 Seedling Technical Seminar 4
Hole
Etched diaphragm in semi-insulating SiC
Boss
Free standing, 2 m highly doped
n-type SiC resistor
Semi-Insulating
SiC substrate
Hole due to over-etch
Free standing, 2 m highly doped n-type
SiC resistor
Unintentional rupture of micro diaphragms was observed during routine RIE
(with SF6 + Ar) of semi-insulated (SI) 4H-SiC substrate.
Observed free standing 2µm thick “cantilevers” of the patterned highly doped
n-type homoepitaxially grown 4H-SiC piezoresistor layer on the SI SiC.
Etch selectivity between SI and highly doped n-type SiC is proposed
RIE
NASA Aeronautics Research Institute
Technical Approach
February 19–27, 2014
NASA Aeronautics Research Mission Directorate 2014 Seedling Technical Seminar 5
SF6= Flow Rate (sccm) SI N-Type(Nd=3.8 x 1019 cm-3) 45 3 8 60 4 9 75 5 10
Calibration patterns etched in n-type and semi-
insulating SiC samples.
Fixed process conditions: RF Power=400 W; Base Pressure = 25 mT; Etch Time=2 hours
Photolithography and Al pattern definition with
calibration mask
RIE
Al mask stripped off and performed depth profile
measurement
Depth Profile
NASA Aeronautics Research Institute
February 19–27, 2014
NASA Aeronautics Research Mission Directorate 2014 Seedling Technical Seminar 6
Etching selectivity between SI and highly doped n-type
substrates in SF6 gas only.
Selectivity Between SI and N-Type 4H-SiC
480
500
520
540
560
580
30 40 50 60 70 80
Etc
h R
ate
(Å/m
in)
SF6 Flow Rate (sccm)
SI-SiC N-Type SiC (3.8 x 10^19 cm^-3)
RF Power = 400 WBase Pressure = 25 mT
Higher doping increases
selectivity
NASA Aeronautics Research Institute
February 19–27, 2014
NASA Aeronautics Research Mission Directorate 2014 Seedling Technical Seminar 7
Etching selectivity between highly doped p- and n-type 4H-SiC
substrates in SF6 gas only.
600
650
700
750
800
850
900
950
1000
30 40 50 60 70 80 90
Etc
h R
ate
(Å/m
in)
SF6 Flow Rate (sccm)
DH0852-14 (p-2e20), 0.5 µm epi BQ0339-10 (n>2e19), 2 µm epi
RF Power = 400 WBase Pressure = 25 mT
Selectivity Between N- and P-Type 4HSiC
Higher doping increases
selectivity
NASA Aeronautics Research Institute
500
600
700
800
900
30 40 50 60 70 80 90
Etc
h R
ate
(Å/m
in)
SF6 Flow Rate (sccm)
2 x 10^19 cm^-3 3.8 x 10^19 cm^-3
February 19–27, 2014
NASA Aeronautics Research Mission Directorate 2014 Seedling Technical Seminar 8
Selectivity in N-Type Concentrations
Selectivity increasing with
increased dopant
concentration
NASA Aeronautics Research Institute
DSRIE Release of N-Type Cantilevers
February 19–27, 2014
NASA Aeronautics Research Mission Directorate 2014 Seedling Technical Seminar 9
Suspended 2 µm
thick single crystal
SiC piezoresistor
Proof mass
suspended by 2 µm
thick single crystal
SiC piezoresistor
SEM paste!
Suspended 2 µm
thick single crystal
SiC piezoresistor
NASA Aeronautics Research Institute
~220 µm
~3 µm
DSRIE Release of N-Type Cantilevers
NASA Aeronautics Research Institute
Innovation and Impact on Mission
February 19–27, 2014
NASA Aeronautics Research Mission Directorate 2014 Seedling Technical Seminar 11
Mission Challenges: Thermoacoustic instabilities in combustors are known to be precursor to flame-out or damage to engine components. Existing instability prediction models have high uncertainty margins. Need environmentally robust and reliable pressure sensors for model validation and improvement.
Existing Technology Gap: SoA sensors placed feet from test article-limits frequency bandwidth; Water cooling adds “vortex noise” to corrupt signal. Robust and reliable sensors needed for direct (no water cooling) measurement of sub-psi dynamics at >500 ºC. 600 ºC SiC pressure sensor technology currently exists, but SiC fabrication technology cannot produce the ultra-thin diaphragms to achieve the high sensitivity needed to resolve sub-psi pressure dynamics.
Innovation: DSRIE will result in ultra-thin (< 10 µm) SiC diaphragms to accurately resolve sub-psi dynamics at temperatures in excess of 500 ºC (current capability).
Flush mounted
SiC dynamic sensor
Benefit of SiC Etch Stop Mechanism
~5 µm etch stop diaphragm
Semi Insulating SiC
NASA Aeronautics Research Institute
Next Steps
February 19–27, 2014
NASA Aeronautics Research Mission Directorate 2014 Seedling Technical Seminar 12
Improve further understanding of the DSRIE mechanism -Investigate various doping levels and other halides to optimize etch selectivity
Demonstrate 5-10 µm thick diaphragm SiC pressure sensors -Based on circular plate theory, maximum deflection (for deflections << thickness) of a clamped circular plate is expressed as:
Where: =maximum deflection (m); P=applied pressure (Pa); r=diaphragm radius (m); E=Young’s Modulus (Pa); h=diaphragm thickness (m); =Poisson ratio
For E =475 GPa for SiC; h =5 µm; r =1 mm; = 0.212
The predicted ~ 1 µm and applied pressure is ~345 Pa (0.05 psi)
~5 µm etch stop diaphragm
NASA Aeronautics Research Institute
Summary/Conclusion
February 19–27, 2014
NASA Aeronautics Research Mission Directorate 2014 Seedling Technical Seminar 13
Confirmed existence of dopant and conductivity selectivity in SiC during reactive ion etching with SF6 gas;
While the etch selectivity ratio between the SI and n-type SiC using SF6 gas is < 2, free standing n-type cantilever structures were realized;
Validation of this proof of concept allows the investigation of other halides in which the etch selectivity ratio may be higher.
More experimental data is needed to determine the effective role of doping concentration and conductivity.
NASA Aeronautics Research Institute
Acknowledgement
February 19–27, 2014
NASA Aeronautics Research Mission Directorate 2014 Seedling Technical Seminar 14
Kimala Laster/Ariana Miller - sample fabrication Beth Osborn-Sample preparation Laura Evans and Pete Bonacuse - SEM images Kelley Moses and David Spry- RIE system health maintenance Sponsor: ARMD Seedling Fund