ARTICLE Hyperspectral mid-infrared ellipsometric measurements in the twinkling of an eye
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VOL. 32 NO. 2 (2020) ARTICLE Hyperspectral mid-infrared ellipsometric measurements in the twinkling of an eye Karsten Hinrichs, Christoph Kratz and Andreas Furchner Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V., Department Berlin, Schwarzschildstr. 8, 12489 Berlin, Germany Contacts karsten.hinrichs@isas.de ( 0000-0002-6580-7791) andreas.furchner@isas.de ( 0000-0002-5448-7956) Introduction also interpretation with respect to film FT-IR spectroscopy both in far-field and Fast, contact-less and destruction-free thickness, molecular structure, composi- near-field IR-related techniques. Typical hyperspectral infrared (IR) techniques tion and homogeneity of these param- applications are time-resolved 13 and that enable large-area mapping within eters. hyperspectral IR ellipsometry8 (Sentech short measurement times are highly Instruments), photothermal AFM-IR 2 relevant for research and industry in Laser-based IR imaging (e.g., from Bruker/Anasys), visible- environmental, biomedical, material and Mid-infrared (MIR) imaging beyond clas- diffraction limited spectroscopy11 [e.g., space applications. Laser-based meth- sical Fourier-transform infrared (FT-IR) optical photothermal infrared (O-PTIR) ods provide high optical throughput as techniques recently showed promising spec troscopy from Photothermal well as high spectral, spatial and tempo- developments for material science,1–8 Spectroscopy Corp.], and scattering ral resolution, and are thus of particular medical diagnostics and the study of near-field IR spectroscopy1,5 (e.g., from interest for analytical, process, laboratory biological samples.9–11 IR spectroscopic Neaspec and Bruker). and field applications. applications are significantly broadened In this article, we focus on MIR laser- In this article, we focus on a recently and strengthened by modern technical based polarimetric imaging (i.e., hyper- introduced, rapid, laser-based hyper- advances such as upconversion-based spectral ellipsometry) as a new method spectral method for thin-film analysis in hyperspectral imaging,4,9 hyperspectral to simultaneously reveal vibrational, the mid-IR fingerprint range. Featuring nanospectroscopy,1–3,5–7,10 frequency- structural and thickness information.8,13 a polarimetric single-shot design, both comb techniques,12 visible-diffraction phase and amplitude information limited optical photothermal IR multi- Method of hyperspectral related to refractive and absorption indi- spectral imaging,11 and QCL-based single-shot ellipsometry ces are simultaneously recorded. A tune- single-shot polarisation dependent The developed hyperspectral IR laser able, pulsed quantum cascade laser hyperspectral concepts.7,8,13 polarimeter incorporates a four-chan- (QCL) enables spectrally highly resolved IR imaging as a linear optical technique nel division-of-amplitude concept14 that (
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The polarimeter (Sentech j = 0, 1, 2,… number of layers, with rp and showing exemplary images and spec-
Instruments) consists of a tilt- and rs being the p- and s-polarised complex tra obtained from the multidimensional
height-adjustable sample mapping reflection coefficients. The optical data cube.
stage (50 mm × 50 mm) and an auto- constants N = n + ik (N: complex refrac- Figure 1a shows complementary
collimation unit for defined sample tive index; n: refractive index; k: absorp- phase and amplitude contrast images
alignment. A pulsed external-cavity tion index) are related to the complex at different IR wavelengths. These
QCL (MIRcat 2100, Daylight Solutions) dielectric function e = e1 + ie2 via N = √e. images readily identify areas with vary-
is employed as a brilliant, broadband- Measuring both Y and D yields ing thicknesses (mainly in cos D) and/
tuneable radiation source. A custom- complementary information regard- or a chemical structure (mainly in tan Y).
built beamsplitter optics divides and ing the sample’s optical and structural The chemical contrast image in Figure 1b
directs the laser beam into four paral- properties, thus providing a means to shows the difference in band amplitudes
lel detection channels, each equipped in-depth sample analysis. Various routes of two vibrational markers at 1706 cm−1
with a polarising unit (KRS-5 wire- are possible for evaluating the structure- (associated with C=O carboxyl vibra-
grid polariser, Specac) and a photo- related baselines and material-specific tions of pure MyA) and at 1379 cm−1
voltaic InAsSb detector (P13894-211, vibrational bands of hyperspectral IR (associated with complexed MyA), thus
Hamamatsu). A custom-built gated- ellipsometric data, including direct spec- enabling the direct visual domain separa-
integrator electronics synchronises the tral interpretation, optical modelling and tion of pure and complexed MyA.
polarisation-state analysers of the four multivariate analysis. A spectral tan Y linescan along the red
channels, thus enabling single-shot line in Figure 1b is presented in Figure
ellipsometric measurements. Hyperspectral IR 1c, highlighting the distinct vibrational
By selecting specific polariser settings ellipsometry characteristics of the different sample
and additional optical elements like Single-shot hyperspectral IR polarimetric areas. Comparing the variations in spec-
retarders, it is possible to tailor the imaging of a sample provides ellipsomet- tral intensities reveals that regions of pure
four detection channels. This flexible ric parameters dependent on wavelength MyA are rather homogeneous, whereas
approach allows one to measure various and up to three spacetime coordinates. those with complexed MyA exhibit
polarimetric parameters, for example, Regarding data interpretation, character- pronounced heterogeneity in surface
ellipsometric amplitudes (tan Y), phases istic vibrational bands such as carboxyl, coverage. Pure and complexed domains
(D) or specific Mueller-Matrix elements CHx and amide bands16 of lipids, proteins are separated by an area with low cover-
related to the sample’s polarimetric prop- and polymers yield detailed insights into age.
erties (e.g., circular/linear dichroism and the film’s properties.13 Such data, there- A detailed vibrational analysis of, for
birefringence).15,7 fore, enable unprecedentedly compre- example, the n(C=O) band can eluci-
In this contribution, we focus on hensive, laterally resolved analysis with date molecular interactions and orien-
single-shot measurements of the ellip- respect to molecular structure, chemistry, tations in such complex heterogeneous
sometric angles Y and D. Using a differ- interactions, optical anisotropy, composi- samples. Beyond such a spectral inter-
ent polariser setting a in each of the four tion, morphology, film thickness and vari- pretation, a polarimetric approach can
detection channels, the single-shot ellip- ations thereof. (as stated in Reference 15) “provide a
someter measures Y and D from the In the following, we present two exam- number of contrast mechanisms besides
intensities Ia according to: ples: one on the simultaneous phase traditional unpolarized radiation intensity,
and amplitude mapping of a heterogene- including linear depolarization, circular
I90 I0
cos 2Y , ous surface and one on a time-depend- depolarization, cross-polarization, direc-
I90 I0
ent investigation of individual sample tional birefringence and dichroism.” The
I45 I135
sin2Y cosD spots under external temperature stimuli. high contrast resulting from the ellipso-
I45 I135 (1)
metric measurement could have tremen-
Further technical details regarding the Hyperspectral imaging of dous application potential for the imaging
device and measurement protocol can heterogeneous thin films and characterisation of complex surfaces
be found in References 8 and 13. The first sample is a drop-cast (e.g., tissue classification for cancer iden-
The ellipsometric angles Y and D are thin film of myristic acid (MyA) tification, quality and morphology control
functions of the incidence angle j0, the [ C H 3 ( C H 2 ) 12 C O O H ] o n a g o l d of materials). In this regard, hyperspec-
wavelength l, the optical constants of s u b s t r a te .13 T h e h o m o g e n e o u s , tral data analysis based upon neural
substrate (NS), ambient medium (N0) 150 nm thick film was partially chem- networks, multivariate analysis and/or
and layers (Nj), as well as of the indi- ically modified using droplets of optical modelling could be a promising
vidual layer thicknesses dj, NaOH dissolved in ethanol, resulting tool for gaining a detailed understanding
in areas of pure MyA, intermediate of the structural and chemical specifici-
rp regions and areas of complexed-like ties of the sample.
tanY e i D F 0, , NS , N0 , N j , d j
rs MyA. Hyperspectral measurements Together with the high stabilit y
(2) of the film are presented in Figure 1, concerning disturbing environmental
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Figure 1. Hyperspectral IR ellipsometry images of a partially chemically modified, drop-cast MyA film. a) Phase and amplitude contrast at different
wavelengths. b) Chemical contrast between pure and complexed MyA. c) tan Y linescan across the heterogeneous sample surface and exemplary
ellipsometric spectra. Reprinted with permission from Reference 8 (©2019, Optical Society of America).
absorptions,7 the capabilities of the µs to hour range. This point is illustrated While both mode 1 and 2 are present
IR laser ellipsometer make accessible in Figure 2, which displays the temper- below the phase transition, only mode
multiple new IR spectroscopic imaging ature-resolved investigation of the ther- 2 is found above it. These observed
applications regarding the analysis of mal phase transition of a 150 nm thick changes reveal a tight interrelationship
structured, anisotropic films and biolog- MyA film around 55 °C.13 The amplitude between molecular interactions and
ical samples. Interesting systems to and phase images in Figure 2b show the thermo-induced phase transition.
be investigated with the device range n(C=O) band progression (cf. spectra in As these presented measurements
from functionalised surfaces and coat- Figure 1c). Characteristic band compo- demonstrate, the new IR polarimeter
ings, to polymer and protein materials, nents due to differently interacting C=O covers multiple time scales ranging from
minerals, as well as solar cells, OLEDs groups can be identified. A strongly inter- µs to minutes. The results highlight the
and other optoelectronic devices and acting component (mode 1) is found in applicability and sensitivity of the tech-
sensors. the solid phase at 50 °C, and a weakly nique for time-dependent analyses of
interacting one (mode 2) occurs at non-cyclic, irreversible processes and
Time-resolved studies of thin- higher wavenumbers mainly in the liquid reactions. We see powerful applications
film phase transitions phase at 60 °C. in process and quality control, but also in
Spectra of single spots can be monitored Figure 2c shows the time-dependent rheology, relaxation and related studies.
within about 100 ms, and single wave- development of the two n(C=O) band Because of the ellipsometer’s small spot
lengths even as rapidly as 10 μs. The IR components obtained from Figure 2b size (0.03 mm2), low sample volumes
laser polarimeter, therefore, pushes new during slow heating. Figure 2d displays and amounts can also be studied.
possibilities for time-resolved measure- the corresponding single-wavelength Currently, we are working on coupling
ments of non-cyclic processes via flex- monitoring of mode 1 during rapid cool- microfluidic flow cells17 to the instrument
ible measurement durations from the ing with a time resolution of 200 µs. in order to image and analyse processes
12 SPECTROSCOPYEUROPE www.spectroscopyeurope.comVOL. 32 NO. 2 (2020)
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Figure 2. Time-resolved IR laser ellipsometry of the solid-to-liquid phase transition of a fatty-acid thin film upon heating/cooling. a) Chemical struc-
ture of MyA. b) n(C=O) band amplitude and phase during slow heating. c) Oscillator amplitudes of mode 1 and 2 fitted from the spectral images.
Reprinted with permission from Reference 13 (©2019, Optical Society of America). d) Sub-ms single-wavelength monitoring of mode 1 during rapid
cooling.
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