concept demonstration of dopant selective reactive etching … · 2016-07-22 · nasa aeronautics...
TRANSCRIPT
NASA Aeronautics Research Mission Directorate (ARMD)
2015 Seedling Phase II Technical Seminar
November 17 & 19, 2015
Concept Demonstration of Dopant Selective Reactive Etching (DSRIE) in
Silicon Carbide Robert S. Okojie, PhD
NASA Glenn Research Center, Cleveland OH
Email: [email protected]; phone: (216)433-6522
https://ntrs.nasa.gov/search.jsp?R=20160009223 2020-03-23T14:26:17+00:00Z
Outline
Motivation
Technical Challenge
Observed Phenomenon
Technical Approach
Results
Innovation Impact on Mission if implemented
Conclusion
Problem Statement-Motivation
Lower emissions (LE) jet-engines are critically dependent on lean-burning (LB) operation (low fuel/air ratio).
LB combustion is susceptible to thermo-acoustic instabilities, producing pressure oscillations that can reduce component life and potential engine failure.
Existing CFD instability prediction models have high uncertainties in high temperature regime.
SoA dynamic pressure sensors are placed distance away from test article-limits frequency bandwidth; Water-cooled sensors introduce “vortex noise” that corrupt signal.
Need high temperature (>500 C) pressure sensors to validate CFD models.
HeatRelease
Oscillations
AcousticOscillations
Motivation (SoA dynamic pressure sensor versus SiC Static Sensor)
Flush mounted SiC dynamic pressure transducer
PCB dynamic pressure transducers mounted on
semi-infinite line
0
0.1
0.2
0.3
0.4
0.5
0 200 400 600 800 1000 1200 1400
p
(p
si, p
-p)
Freq. (Hz)
26.06.2013 15-11-19 (703 F) 26.06.2013 15-11-50 (703 F) 26.06.2013 15-12-29 (703 F)
26.06.2013 15-46-49 (703 F) 26.06.2013 18-33-07 (1085 F) 26.06.2013 18-34-11 (1086 F)
PCB Dynamic pressure transducer at P3
0
0.1
0.2
0.3
0.4
0.5
0 200 400 600 800 1000 1200 1400
p
(p
si, p
-p)
Freq. (Hz)
26.06.2013 15-11-19 (703 F) 26.06.2013 15-11-50 (703 F) 26.06.2013 15-12-29 (703 F)
26.06.2013 15-46-49 (703 F) 26.06.2013 18-33-07 (1085 F) 26.06.2013 18-34-11 (1086 F)
SiC 235 static pressure transducer at P3
161 Hz (1085 F, 1086 F)
0
0.2
0.4
0
0.2
0.4
p
si, p
-p
psi
, p-p
Uncooled SiC static pressure sensor at P3
Water cooled PCB dynamic pressure sensor at P3
Results:
• Reduced SiC data compared favorably well with the reduced PCB data.
• Low frequency noise (<500 Hz) suppressed the static SiC sensor output.
Value PropositionA real SiC dynamic pressure sensor will operate uncooled
at high temperature, increase sensitivity, increase S/N ratio at lower frequencies
Note: The thick diaphragm SiC sensor not designed for dynamic operation
Technical Approach
Based on circular plate theory, maximum deflection (for deflections << thickness) of a clamped circular diaphragm is expressed as:
Where: d=maximum deflection (m); P=applied pressure (Pa); r=diaphragm radius (m); E=Young’s Modulus (Pa); h=diaphragm thickness (m); =Poisson ratio
E = 475 GPa for SiC; h (research goal) = 5 µm; r =1 mm; = 0.212Calculated maximum stresses on diaphragm: Center=6.27 MPa; Clamped
edge=-10 MPa. Both well below the fracture strength of SiC
The predicted applied pressure is ~345 Pa (0.05 psi)
d=𝟑𝑷𝒓𝟐
𝟏𝟔𝑬𝒉𝟑(𝟏 − 𝟑)
Goal: Obtain SiC diaphragm thickness 5 µm or less!
Goal: Realize ultrathin diaphragms in 4H-SiC
Reactive Ion Etching (RIE) for Diaphragm Fabrication: The Basics
Halogen-based gas (i.e., SF6) is ionized to create F ions and neutrals. Neutrals react chemically with target low etch rates.
With argon added and ionized, Ar+
physical bombardment of target increases surface area increased chemical reaction and higher etch rates.
E-Field
F-F
-F-F
-F
-Neutrals
Conventional SiC RIE
Conventional RIE (with SF6 + Ar) of semi-
insulated (SI) 4H-SiC substrate to create
thinner diaphragms resulted partial
etch-through of the micro diaphragms.
Observed free standing, 2-µm 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.
If controlled and reproducible, could
lead to the realization of ultrathin
diaphragms by RIE.
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
RIE
Experiment: RIE Etching of SI and N-type 4H-SiC with SF6
SF6= Flow Rate (sccm) SI N-Type(Nd=3.8 x 1019 cm-3)45 3 860 4 975 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
DepthProfile
Etching selectivity between SI and 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
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 4H-SiC
DSRIE Release of N-Type Cantilevers from SI SubstrateFirst proof of concept
11
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
Investigation of Selected HalidesBCl3, HBr, and Cl2
• Wafer Clean10 minutes in Acetone and 10 minutes in IPA
• Nickel Deposition230 nm thick nickel in Cooke E-beam Evaporator
• Optical lithographyAZ 5214 E 4000 RPM about 1.3 um thick; Bake at 110 degrees for 1 minute; Expose with NASA mask for 40 seconds at Karl Suss MJB Aligner; Develop in AZ327 for 4 minutes.
• Wet Etch4 minutes in Nickel Etchant (Type TFB from Transene Company); 10 minutes in Acetone to remove photoresists
• ICP-RIE Etch100 W for RIE and ICP power settings; Gas flow rates from 5 sccm, 10 sccm, 15 sccm, to the maximum of 20 sccm; First depth measurement with Alpha Step
• Nickel Removal4 minutes in Nickel Etchant; Second depth measurement
Etching with HBr
80
90
100
110
120
0 5 10 15 20 25
Etc
h R
ate
(Å/m
in)
Flow Rate (sccm)
Semi-Insulating p-type (2x10^19 cm^-3) n-type (3x10^19 cm^-3)
HBr Only ICP Power=100 W
Etching with HBr + Ar
0
50
100
150
200
250
0 5 10 15 20 25
Etc
h R
ate
(Å/m
in)
HBr Flow Rate (sccm)
Semi-Insulating p-type (2x10^19 cm^-3) n-type (3x10^19 cm^-3)
HBr + ArAr fixed at 20 sccm
Etching with BCl3
60
85
110
135
0 5 10 15 20 25
Etc
h R
ate
(Å/m
in)
Flow Rate (sccm)Semi-Insulating p-type (2x10^19 cm^-3) n-type (3x10^19 cm^-3)
BCl3 Only
Etching with BCl3 + Cl2 + Ar
100
150
200
250
300
0 5 10 15 20 25
Etc
h R
ate
(Å/m
in)
BCl3 Flow Rate (sccm)
Semi-Insulating p-type (2x10^19 cm^-3) n-type (3x10^19 cm^-3)
BCl3+Cl2 + ArCl2 fixed at 5 sccmAr fixed at 20 sccm
Summary of ER Selectivity Between SI and N-Type 4H-SiC
Gases Power (W) Flow rate (sccm)
ER
SF6 400 60 1.03
HBr 100 10-20 ~1
BCl3 100 15-20 1.33
BCl3 + Cl2+ Ar 100 BCl3 (15), Cl2
(5), Ar (20)1.37
Facility limitation prevented investigation of higher power and flow rates
Ultrathin Diaphragm with BCl3 + Cl2 + Ar etching
Full diaphragms
Cut out diaphragms
Wafer edge
3 µm 4HSiC n-type epilayer
150 µm 4HSiC SI substrate
GoalStart with fast ER recipe through most of SI substrate, then switch to DSRIE:BCl3 (15 sccm)Cl2 (5 sccm), Ar (20 sccm)Power 100 W
Results
Microtrenching
Ultrathin Diaphragm with BCl3 + Cl2 + Ar etching
Diaphragm thickness between 2 and 3.5 µm
Ultrathin Diaphragm with BCl3 + Cl2 + Ar etching
Observed microtrenching and etch non-uniformity across wafer causes reduced
yield. Etch conditions and chamber optimization are required to increase yield
Semi-insulated 4H-SiC substrate (300 µm)
Highly doped N-type 4H-SiC epitaxial layer (2 µm) Piezoresistor fabrication
e) Diaphragm formation: fast reactive ion etchingf) Nickel strip and switch to DSRIE to form
diaphragm
Contact metallization
a) Starting wafer b) Define, pattern, and dry etch piezoresistors
Highly resistivity polycrystalline CVD SiC (> 2 µm)
d) Nickel electroplated maskc) Chemical vapor deposited SiC
g) Piezoresistor oxide passivation and contact
via etch,
h) Metal deposition, pattern and etch
Fabrication Process Steps of Ultrathin Diaphragm Sensors Using DSRIE
Passivation and Contact via etch
~7 µm
Fabrication Process Steps of Ultrathin Diaphragm Sensors Using DSRIE
Diaphragm microtrench eliminated
Diaphragm thickness between 4 and 7 µm
Before
Fabrication Process Steps of Ultrathin Diaphragms-Sensor Side
Piezoresistor
Contact via
Outline of diaphragm
Next Step: Complete fabrication, package and test SiC dynamic pressure
sensor with ultrathin diaphragms
Innovation and Impact on Mission
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 (< 5 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
Summary/Conclusion
We investigated the possible existence of dopant and conductivityselectivity in SiC during reactive ion etching with selected halidegases;
Within the limits of experimental space, the maximum etchselectivity between the SI and n-type SiC was 1.37, enough torelease free standing n-type cantilevers and diaphragms of less than10 µm thick.
Applied process to fabricate array of ultrathin SiC diaphragms forpressure sensors.
Acknowledgement
Sponsor: ARMD Seedling FundUniversity of Illinois Urbana Champagne-Matrix Experiments
Dr. Robert Shinavski (Roll-Royce)- SiC deposition Kimala Laster/Ariana Miller - Sample fabrication
Beth Osborn/Michelle Mrdenovich-Sample preparationLaura Evans and Pete Bonacuse - SEM images
Kelley Moses and David Spry- RIE system health maintenance