Optical Characterization of Organic Semiconductors by Spectroscopic Ellipsometry
SE02 Optical Characterization of Organic Semiconductors by Spectroscopic Ellipsometry
Spectroscopic Ellipsometry Solution Thin Film Measurement Capabilities Spectroscopic Ellipsometers are optical thin film measure- ment tools for determining film thickness and optical con- Interfacial Behaviour stants (n,k) of thin film structures. They are widely used in the microelectronics, display, • Interface thickness photonics, photovoltaics, lighting, optical and functional • Composition of mixed materials forming interface coatings, biotechnology industries. • Monitor interface thickness in real-time: film growth, adsorption • Monitor real-time changes at interfaces Advantages of Ellipsometry Surface Measurement - Non destructive technique • Roughness thickness - No sample preparation • Native oxide thickness • Any surface film thickness - Rapid measurement and simple to operate • Depolarization coefficient - No reference measurement needed - Very sensitive for ultra-thin film measurement down to 1Å Surface - Single and multi layer thin film measurement Film - Information rich for layer stack description (interface, roughness, film gradient, film anisotropy etc…) Interface Substrate - Direct and accurate determination of optical constants (n,k) When compared with other optical metrology instruments Thickness Measurement Optical Properties the unique strengths of spectroscopic ellipsometers are • From a few Å to ~30 μm • Optical constants (n,k) and α based on their highly precise and simple experimental • Single and multi layers • Optical bandgap Eg measurements plus physical and material information de- • Transmittance, Reflectance rived from optical constants characterization. Material Properties • Gradient, anisotropy, composition, crystallinity, microstructure infor- mation provided by the optical constants characterization • Film porosity
Thin Films & Materials Range - Substrate materials: Silicon, GaAs, glass, sapphire, plastic, metals… - Film materials • Dielectrics: Al2O3, SiO2, TiO2,… • Semiconductors: Silicon, SiGe, compound alloys ~ out ~ in ~ • Thin metal films: Ag, Al, Cr, Cu E p E p rp i ()δ −δ ρ = tan ()Ψ e iΔ = = e p s ~ out ~ in ~ • Transparent conductors: ITO, ZnO, SnO2 E E rs s s • Polymers, organic materials Measured data: Ψ and Δ SE02
At the Heart of Optical Constants •Absorption coefficient α Capabilities The absorption coef- Spectroscopic Ellipsometers are the tool of choice for ficient (α) is simply measuring optical constants (n,k) (also called complex di- given by the relation: electric constants) of materials. The precision of optical constants are in the range of 10-3. 4πk α = in cm-1 λ
It provides an insight into material properties such as: - Composition, - Degree of ordering or crystallinity, - Hydrogen content, - Conductivity, Through measurement of optical constants, SE can pro- - Porosity, vide detailed knowledge of : -Aging, And any other variables that affect the optical constants of •Electronic properties of material such as band gap Eg materials. Three methods are implemented in the software to calcu- late the optical bandgap value, including: n - Tauc model based on the relation: αhν = A(hν − Eg ) where hν is the photon energy, α the absorption coeffi- 100% polymer cient, Eg the band gap and A is a constant. The extrapolation of straight line to()αhν n = 0 axis gives the value of the band gap. For a direct transition, n=0.5, 70% polymer + 30% void and an indirect one n=2. 4 -1 -E04 value defined as the energy for which α=10 cm . - A directly calculated Eg parameter from a dispersion Refractive index decreases with Optical constants change due to formula (Tauc Lorentz, Adachi-New Forouhi, Afromovitz, porous films effect of aging on a-NPD layers New amorphous, Tanguy)
•Optical gradient and anisotropy of materials A graded film exhibits a change to its optical constants through the layer. It specifies one value of (n,k) at the bot- tom and another one for the layer top.
Graded layer
Glass SE02
A uniaxial anisotropic film exhibits two different sets of Large Range of Spectroscopic optical constants, commonly called ne (for extraordinary Ellipsometers ray) and no (for ordinary ray). HORIBA Jobin Yvon spectroscopic ellipsometers are available for research and manufacturing metrology cov- Uniaxial anisotropic ne ering spectral ranges from vacuum UV to near-IR. Models layer include: Glass • Table-top for R&D and QC with UVISEL and Auto SE n o UVISEL
Optical constants of PEDOT- PSS film exhibiting uniaxial anisotropy – N oriented
Auto SE For biaxially anisotropic films 3 different sets of optical constants are measured, nx,ny,nz and kx,ky,kz. Fully automatic for fabs with FF-1000* and UT-300*
FF-1000 UT-300 n PET z
ny
nx
*FF-1000 and UT-300 integrates UVISEL or Auto SE ellipsometers
•In situ for thin film process monitoring and In line for roll to roll processing with adapted configurations of UVISEL or Auto SE
Main Features
UVISEL Auto SE
Liquid crystal Technology Phase modulation modulation
Spectral range 142-2100 nm* 440-850 nm
Monochromator with PMT + InGaAs detectors Detection system CCD Multiwavelengths with PMT detectors
Microspot optics Down to 60x180µm Down to 25x60µm At 70°
Vision External CCD camera Integrated *5 models with different spectral ranges are available SE02
Unique Integrated Vision System Ellipsometry Power for Organic The Auto SE ellipsometer integrates the unique Semiconductor Applications MyAutoView vision system. It allows the user to view the measurement spot on the sample and to choose the opti- HORIBA Jobin Yvon Expertise mum position and spot size accordingly. Materials (n,k) @ 633 nm* Advantages of Sample Type Examples P3HT n=2.374 – k=0.171 MyAutoView PEDOT:PSS n =1.492 – k =0.029 Direct visualization of o o Patterned the measurement spot ne=1.484 – ke=0 sample on the pattern area PPV no =1.81 – ko=0.011 Sample with Optimization of the ne=1.55 – ke=0 inhomogeneous position of the
surface (stripe, measurement spot Alq3 1.715 Versatile Optical Measurement Capabilities PET nx=1.634 – ny=1.658 – nz=1.483 HORIBA Jobin Yvon ellipsometers provide a large range PEN nx=1.881 – ny=1.729 – nz=1.677 of optical measurement capabilities. TAC n=1.462 – k=0 Optical PTFE n=1.308 – k=0 For what ? measurement *depending on manufacturing process and conditions Film thickness, optical constants, and Ellipsometric data many other parameters for layer Organic Semiconductor Applications stacks To control the growth process of thin Kinetic ellipsometric • Organic Photovoltaic Device films – in terms of film thickness, data optical constants, composition, … ZnO 167 Å Mueller matrix Complete description of anisotropic (polarimetric data) films or substrates P3HT 143 Å PEDOT: PSS 543 Å Additional optical information Polarized/unpolarized measured by ellipsometer, that can Glass reflectance/transmittan also be used in addition to ce ellipsometric data for better accuracy Optical dimension of structure Scatterometry (patterned periodic sample) SE02 • Organic Thin Film Transistor of OLED device encapsulated either by a thin dielectric layer or a glass cap. Pentacene 1707 Å SiO2 92 Å Encapsulation by dielectric layer c-Si Measured from the encapsulation layer side Sample structure LiF Note the accurate character- ization of optical constants of BCP: Cs pentacene exhibiting 4 ab- Glass sorption peaks at 654, 630, 580 and 540 nm. Thickness (nm) n@633nm BCP:Cs 63.6 1.661 LiF 74.4 1.327 • Small Molecule OLED Device Total thickness by 138 ell ipsometry ETL Alq3 338 Å EML Doped Alq3 386 Å Excellent correlation was obtained with a stylus profilome- HTL NPD 487 Å ter where a total thickness of 135 nm was measured. HIL CuPC 141 Å ITO 1021 Å Encapsulation by glass cap Glass Desiccant agent roughness Glass α-NPD Sample structure Impossible to measure from the layer side due to the desiccant which fully absorbs ellipsometric incoming light ⇒ Measured from the glass side Thickness (nm) n@633nm α-NPD 54.1 1.793 • Encapsulated OLED Devices Usually, organic materials are encapsulated to avoid en- vironmental aging due to oxygen and water absorption. Ellipsometric analysis can be performed on such samples. The two examples below show accurate characterization SE02 • Roll to Roll Thin Film Process Electrical Conductivity* Mapping SiOx PET In line thickness control of SiOx barrier layer on PET film *Derived from Drude dispersion fitting • Transparent Conducting Oxides • Films with Low Index Contrast Doped SnO Phase Modulated Spectroscopic Ellipsometry makes pos- 2 sible the characterization of films with low index contrast BCP: Cs 482 Å such as a SiOx layer deposited on glass substrate. Glass Thickness and optical constants of the layer were deter- mined. Thickness Mapping of layer 2 SiOx 4468 Å Glass Optical Transmission Mapping* s - Printed in France - 12/2008 lly binding under any circumstance *T at 633 nm is not This document contractua