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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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

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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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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)

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Continuous

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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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200nm Freestanding polyimide 50 % Continuous Al

100 nm Polyimide 90% Continuous Al

Fused Silica 96% Continuous Al

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Tem

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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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765nm Polyimide

101nm Polyimide

214nm Polyimide on wafer

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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

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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

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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

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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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30nm freestanding30nm low temperature20nm, freestanding20nm low temperature

Far-Infrared Transmission of Low Temperature Aluminum/Polyimide Process

Thermal Wavelengths

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1.E-06

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Thickness (nm)

Standard AlDense AlBulk Al

Infrared Density of Low Temperature Al/Polyimide Process Various aluminum thicknesses

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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

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Demonstrations

Small Filter Capability

>75mm diameter low temperature Al demonstration films

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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

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