Infrared Blocking of Ultrathin Aluminum Films on Freestanding Polyimide Substrates
Bruce Lairson, Jacob Betcher, and Travis Ayers
Luxel Corporation, Friday Harbor, WA
M. Wieland
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Aluminum /Polyimide Advantages Polyimide (<200nm)/Aluminum (<100nm): -Good Soft X-ray transparency -Good mechanical durability -Atomic oxygen resistance -Vis-IR-LWIR blocking For these ultrathin films, it was noticed that UV-Vis-IR transmission could vary greatly depending on filter constraints -Filter film tension -Filter Size -Coating method (E-beam, thermal, sputtering)
Microcalorimetry/Astronomy
Contamination Blocking Filters
Future NIF Windows?
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Tran
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sion
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Wavelength (nm)
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Transmission of Two 25nm Al/200nm Polyimide Filters
Visible Transmitted Image
• Infrared transmission depends on coating parameters • IR transmission varies greatly, visible transmission largely unaffected • Different IR transmission not detectable from visible spectra or images
Path A
Path B
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Aluminum Optical Density Depends on Processing Parameters
Case B
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T%
Wavelength (nm)
25nm Al/50nm freestanding polyimide
25nm Al/500nm freestanding polyimide (LEH Window)
Parameters: -Sputter, e-beam, or thermal -Rate -Vacuum impurities -etc. etc. Luxel has a well-developed LEH window process with 25nm Al and 500nm polyimide Perform DOE Screening to isolate variables and responses
Experimental Runs on 200nm thick polyimide -Various coating methods, cleaning methods
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-Visible Transmission (450-650nm) varies little. -Infrared transmission (e.g. 1800nm) varies greatly
Cleaning No clean Air Plasma IB Clean ArH Plasma Air Plasma No clean No Clean No CleanRate A/sec) 10 10 10 10 10 10 10 10
On/Off Substrate
On Substrate
OffSubstrate
TopOn
SubstrateOn
SubstrateOn
SubstrateOn
Substrate
OffSubstrateBottom
OffSubstrate
TopCrucible None None None None None None None None
Clean None 1 min air IB 5 min ArH 20min air 20min None None NoneBase
Pressure 3.00E-06 5.00E-07 5.00E-07 5.00E-07 5.00E-07 3.00E-06 1.50E-06 1.50E-06
MethodSystem 5 Ebeam
System 6 Ebeam
System 5 Ebeam
System 6 Ebeam
System 6 Ebeam
System 6 Ebeam
System 5 Ebeam
System 5 Ebeam
Post Treatin situ None None None None None None None None
T1800 % 1.57 20.75 1.85 1.26 1.81 1.4 13.4 16.65T450-650 % 3.79 6.48 4.41 3.98 5.78 3.36 6.7 6.7
Change in IR Optical Density for Selected Process Changes 25nm Al/200nm polyimide
-Coating on freestanding films results in lower infrared optical density -Plasma cleaning prior to aluminization might be mildly beneficial
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Condition:Optical DensityChange (1800nm)
Off vs On Substrate System 6 Ebeam Evap -1.17Off vs On Substrate Thermal Evap -0.92Off vs On Substrate System 5 Ebeam Evap -0.89High Dose Air Plasma vs No Plasma Clean -0.08Low Dose Air Plasma vs No Plasma Clean -0.03Low Dose ArH plasma vs No Plasma Clean 0.02Ion Beam Clean vs. No Clean 0.03System 6 vs System 5 0.04High Dose ArH plasma vs. No Plasma Clean 0.05Front vs back side coating 0.05
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Alu
min
um T
hick
ness
(nm
)
Polyimide Thickness (nm)
Existing Capability
Desired Capability
Small Size Capability
Desired thicknesses for IR-dense aluminum on polyimide
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Goal: Produce IR-blocking aluminum on <200nm thick polyimide in large (~100mm) areas
-Aluminum is Optically Coupled to the Polyimide Film How to separate Intrinsic IR density of Aluminum from Substrate Interference? Does the microstructure change for the different aluminum processes? 8
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Wavelength (nm)
Transmission of 25nm Al on 100-200nm Thick Polyimide
Incident Light
Transmitted Light
Top: Al deposited onto 200 nm freestanding polyimide. Bottom: Al deposited onto 200 nm polyimide while still on the imidization substrate.
-Dark dendrites appear along the ground path.
SEM Images of Aluminum on Polyimide
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1 mm
ON SUBSTRATE ON SUBSTRATE
Optical Model for Discontinuous Aluminum Transmission
Islands
Holes (=continuous film+ “-islands”)
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-Complication: εb changes with island geometry (ignored) -OK to use effective dielectric constant if scattering vector is small
Optical Model for Discontinuous Aluminum Transmission
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Wavenumber (1/micron)
Die
lecc
tric
con
stan
t
Continuous
400nm
1600nm
800nm
Island size: D~λLorentz*geometry factor
Assume 5 optical layers: -Polyimide -Continuous Al layer (Palik) -Al with λLorentz=1600nm =800nm =400nm
Interband absorption
-Lorentz layers defined as Mie scattering function convolved with Bulk Aluminum* -Define Parameter “%Continuous” = Thickness bulk Al/Total modeled Al thickness
*E. Palik, “Handbook of Optical Constants of Solids”, Academic Press, 1998 -Interband absorption at 800nm shifts and disappears for islanded Al (neglected)
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Optical Model Fits for 40nm Aluminum Transmission
-Model describes infrared transmission increases well -Loss of interband absorption peak for islanded Al is not captured by the model -IR transmission increase is due to film discontinuity
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Wavelength (nm)
200nm Freestanding polyimide 50 % Continuous Al
100 nm Polyimide 90% Continuous Al
Fused Silica 96% Continuous Al
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(%)
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Tem
pe
ratu
re (
°C)
Deposited Al Thickness (nm)
100nm freestanding polyimide
500nm freestanding polyimide
100nm polyimide on substrate
-Heating:adiabatic condensation of Al, IR from source -Radiative cooling
Substrate Temperature Increase During Al Deposition
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dg(n
m)
Temperature (°C)Activation Energy for Grain Growth in Aluminum Coatings A. Jankowski, J. L. Ferreira, J. P. Hayes, Thin Solid Films 491 (2005) 61-65
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Transmission of 25nm Al (Same coating run) on Various Thickness Polyimide Substrates
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Wavelength (nm)
765nm Polyimide
101nm Polyimide
214nm Polyimide on wafer
Tran
smis
sion
(%)
Freestanding100nm 10% Continuous
Freestanding 765nm 20% Continuous
On Substrate 200nm 80% Continuous
Thicker freestanding polyimide yields more continuous Al films Thermal contact with a substrate provides enough thermal reservoir to keep coating cool
Standard 25nm Al/50nm Polyimide 0% continuous
Infrared-Dense 25nm Al/50nm Polyimide 80% continuous
Transmission of 25nm Aluminum /50nm Polyimide
15 Luxel Confidential
Coated Freestanding
Polyimide film placed in thermal contact with a thick substrate using a heat transfer layer
Plasma Clean of Polyimide Prior to Al Coating
-On-substrate reflected color changed from blue to silver -Improvement of 10% in %Continuous
16 Luxel Confidential
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Wavelength (nm)
No Ion Clean
Ar+ Ion Clean
100mm 20nm Al/100nm Polyimide Transmitted Light Image
A New Process for Al/ <200nm Polyimide
-Need uniform thermal contact with heat sink -Must avoid generating defects in ultrathin film
17 Luxel Confidential
Coated film removal
ion cleaning
Film attachment to substrate using heat transfer medium
Fabricate freestanding ltrathin polyimide film
Thin aluminum deposition
Large area dense aluminum-on-polyimide
freestanding film
Standard Method New Method
aluminum-on-polyimide freestanding film
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sion
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Wavelength (nm)
30nm freestanding30nm low temperature20nm, freestanding20nm low temperature
Far-Infrared Transmission of Low Temperature Aluminum/Polyimide Process
Thermal Wavelengths
18 Luxel Confidential
1.E-06
1.E-04
1.E-02
1.E+00
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Tran
smis
sion
at 1
0 µm
Thickness (nm)
Standard AlDense AlBulk Al
Infrared Density of Low Temperature Al/Polyimide Process Various aluminum thicknesses
19 Luxel Confidential
30nm Standard Al
20nm Dense Al
Atomic Oxygen Resistance of Low-Temperature Al Process
-Low Temperature Aluminum process improves atomic oxygen durability
-Photograph of 200nm thick aluminized polyimide filters after 1.5E10 atomic oxygen atom exposure
20 Luxel Confidential
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inum
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ckne
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m)
Polyimide Thickness (nm)
Existing Capability
Desired Capability
Demonstrations
Small Filter Capability
>75mm diameter low temperature Al demonstration films
21 Luxel Confidential
Conclusions: -Poor infrared blocking of aluminum films on <200nm polyimide is due to discontinuity in the films -Discontinuity is primarily caused by elevated temperature during coating of a freestanding film -An optical model based on Lorentz oscillators (“islands and holes”) describes relative transmission changes in the visible to far infrared -Reducing the coating temperature and implementing an ion clean improves Al to near-bulk optical properties
22 Luxel Confidential
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