Description

Non-destructive technique used to determine parameters of thin layers such as polymer films by measuring the ratio of reflectivities of beams polarized parallel and perpendicular to a surface as a function of wavelength from 275 to 850 nm ( 4.5 - 1.5 eV). Thickness and refractive index for each layer in a multilayer structure may be determined. Software for multilayer modeling is available.

Equipment

Sopra ES4G Spectrometer

• Light Source: Xenon lamp. Intensity drift is < 0.2 % per hour.
• Polarizer: A quartz ROCHON birefringent beam splitter, usable for a wavelength range of 200 - 2500 nm, rotates at a rate of 40 cycles per second, and its position is tracked with an accuracy of 5.5 x 103 degrees.
• Sample Stage: Stage height as well as biaxial tilt can be adjusted to give maximum signal, and a pneumatic sample holder holds the sample on the stage
• Goniometer: The source-polarizer arm and the analyzer arm are mounted on a goniometric support. The angle of incidence may be varied continuously from 45 to 90 degrees with a setting accuracy of 0.01 degrees.
• Analyzer: A GLAN TAYLOR calcite prism, usable for a spectral range of 200 - 3000 nm, can be rotated by a stepper motor, and its position tracked with an accuracy of 5 x 103 degrees.
• Spectrometer: A prism/grating system double monochrometer for separating wavelengths with the following features:
• Two variable slits, adjustable in width from 20 m to 2 mm; the least count of the vernier adjuster is 2 m; an intermediate slit of width 400 m.
• Spectral range: 230 - 930 nm
• Resolution at 313 nm = 0.05 nm
• Dispersion: 31.4 nm/mm, related to grating order
• Scanning speed maximum of 930 nm/m. Dual monochromators driven by two stepper motors
• Numeric aperture: f/10; focal length of entrance and exit mirrors = 500 mm
• Prism made of SiO₂, with an angle of 30 °
• Ruled grating features:
• Number of grooves: 600/mm
• Ruled area: 52 x 52 mm
• Blaze wavelength: 2.7 m
• Blaze angle: 54 ° 6'
• Monochromatic efficiency: 64 % at 640 nm in the fourth order

Accessories

Computer support: IBM PS/2 system. Calibration, measurement, analysis, and data storage procedures are computerized with software supplied by Sopra. A database provides the refractive indices of a large number of materials versus wavelength in the visible range.

Applications

Present data analysis capabilities using measured ellipsometric data are:

• Quartz cells at angles 65 and 75.5 for in situ (adsorption ) measurements in solution.
• In situ measurement software to take readings at one specified wavelength as a function of time.
• Calculation of refractive index and thickness for a single layer dielectric film on a substrate over the spectral range of measurement, provided substrate optical constants are known.
• Calculation of real and imaginary parts of the refractive index of an absorbing single layer film on a substrate over the spectral range of measurement, provided substrate optical properties and film thickness are known.
• Calculation of real and imaginary parts of the dielectric response of a bare reflecting material, possibly for use as a substrate.
• Multilayer modeling capabilities, using a regression algorithm. Parameters that can be varied to fit measured and experimental curves are:


• (i) thickness of each layer;
• (ii) angle of incidence; and
• (iii) composition of each layer for a maximum of two components per layer (including void fraction, so changes in density from bulk conditions are also measurable).


• CIE studies include polymer adsorption on functionalized surfaces, some in situ and some timed, kinetics of block copolymer adsorption, and polymer and non-polymer multilayers:


• Modification of biomedical device surfaces with a photosensitive derivative of polyacrylamide
• Nucleation and growth of thermally evaporated thin gold films
• Optical constants of nitride and oxide thin films
• Surface segregation in polyolefin block copolymer thin films
• Adsorption of water soluble block copolymers on silica
• Commonly used materials include crystalline Si, amorphous Si, silicon oxide, Ag, Au, Cu, Pt, Ge, Al, aluminum oxide, ZnS, etc. (Ask specialist for more details.)

Specifications

Important working characteristics of the ES4G machine are:

1. maximum sample size is 8 inches diameter, 0.5 inch thick.
2. Working spectral range of 275850 nm ( 4.51.5 eV).
3. Dispersion = 0.31.4 nm/mm depending on grating order.
4. Slit width limited resolution (Dl).
   • Completely open slits (W = 2 mm), Dl = 5.6 nm (for dispersion = 1.4nm/mm).
   • Least slit width (W = 20 µm), Dl = 0.56 nm (for dispersion = 1.4nm/mm).
5. The photo multiplier tube has been corrected for non-linear effects up to a level of 106 photons reaching the photo cathode in 50 turns of the polarizer.
6. Signal to noise (S/N) ratios for shot noise can be improved by programming the computer to perform an experiment as many as 11 times at a single wavelength and taking the average value of the ellipsometric parameters, S/N ~ n^1/2, where n is the number of times the experiment is performed.
7. Measurement time is auto-normalized for each measurement step to a certain number of photons (106) or to a fixed number of rotations of the polarizer (1000), whichever happens first. Resulting variations in the time required to make a full spectral scan may range from < 60 s up to 2.5 h, depending on the number of readings taken at each wavelength, sample characteristics, and the desired accuracy.
8. Angle of incidence can be set with an accuracy of 0.01 °.