#### 1.    What is spectroscopic ellipsometry (without mathematics)?

• Spectroscopic ellipsometry is a non-destructive, non-contact, and non-invasive optical technique which is based on the change in the polarization state of light as it is reflected obliquely from a thin film sample.  Ellipsometry uses a model-based approach to determine thin film, interface, and surface roughness thicknesses, as well as optical properties (and much more!) for thin films ranging in thickness from a few Å to several tens of microns.  Also, spectroscopic ellipsometry can be performed either ex-situ or in-situ, in static or kinetic mode, for various application needs.  For more in-depth information on ellipsometry, please view the “Introduction to Spectroscopic Ellipsometry” webinar on our Webinars & Videos page.

#### 2.    What is spectroscopic ellipsometry (with mathematics)?

• Spectroscopic ellipsometry measures ψ and Δ, both of which describe the output elliptical polarization state after linearly polarized light is reflected obliquely off of a thin film sample.  The parameters ψ and Δ are related to the complex Fresnel reflection coefficients according to: ρ = tan ψ e = rp/rs.  After collecting ψ and Δ, a model representing the thin film structure must be built in order to determine thickness and/or optical constants.  For more information about ellipsometry, please view the “Introduction to Spectroscopic Ellipsometry” webinar on our Webinars & Videos page.

#### 3.    What are the advantages of spectroscopic ellipsometry?

• Spectroscopic ellipsometry is non-invasive, non-destructive, non-contact, and can allow a user to determine several film properties simultaneously.  The technique is fast and requires no sample preparation.  It is also precise, reproducible, very sensitive to thin films below 10 nm, and it covers a wide spectral range from 190-2100 nm.  Spectroscopic ellipsometry is applicable to almost any thin film material and it is ideal for in-situ applications.

#### 4.    What types of materials can be measured with spectroscopic ellipsometry?

• Materials suitable for spectroscopic ellipsometry include semiconductors, dielectrics, polymers, organics, and metals.  Ellipsometry can also be used to study solid-liquid or liquid-liquid interfaces.  For some examples of the use of ellipsometry over a wide range of applications, please view our Application Notes.

#### 5.    What are Is, Ic, and Ic’?

• Typically, ellipsometers do not measure ψ and Δ directly.  Instead, they measure functions of ψ and Δ.  In the case of Phase Modulated ellipsometers, such as the UVISEL and UVISEL 2, the three measureables are: Is, Ic, and Ic’, which are functions of ψ and Δ according to Is = sin2ψ sin Δ, Ic = sin 2ψ cos Δ, and Ic’ = cos 2ψ.  When combined, Is and Ic provide an accurate measurement of Δ over the full range from 0° to 360° and Is and Ic’ provide an accurate measurement of ψ over the full range from 0° to 90°.

#### 6.    What can ex-situ spectroscopic ellipsometry measure?

• It is important to note that spectroscopic ellipsometry is an indirect technique which does not measure thin film thickness and/or optical properties directly.  In order to determine thin film thickness and/or optical properties, a model-based approach must be used.  ex-situ spectroscopic ellipsometry allows for the characterization of a range of thin film properties including layer thickness, surface roughness thickness, interface thickness, optical constants, composition, band gap, composition, crystallinity, grading, anisotropy, and uniformity by depth and area.  It can also be used to calculate the depolarization factor and the Mueller Matrix coefficients.  For some examples of the use of ellipsometry over a wide span of applications, please view our Application Notes.

#### 7.    What can in-situ real time spectroscopic ellipsometry measure?

• It is important to note that spectroscopic ellipsometry is an in-direct technique which does not measure thin film thickness and/or optical properties directly.  In order to determine thin film thickness and/or optical properties, a model-based approach must be used.  Besides thin film thickness and optical properties, in-situ spectroscopic ellipsometry can also be used to determine nucleation and growth parameters, precise optical properties without significant surface roughness or oxide, and film growth profiles.  For some examples of the use of ellipsometry over a wide span of applications, please view our Application Notes.

#### 8.    Can spectroscopic ellipsometry measure unknown samples?

• Since ellipsometry is a model model-based approach, it is helpful to know something about your sample (number of layers, materials, etc.).  However, if little to nothing is known about the sample, it can still be studied with ellipsometry as long as it is a simple single layer on a known substrate.  Ellipsometry can also be used to determine the optical properties of the substrate if its material is unknown.

#### 9.    What is polarization, and what types of polarization exist?

• Polarization is defined by the orientation and phase of the electric field vector.  We can describe polarization as a superposition of two orthogonal waves.  The most general state of polarization, known as elliptical, allows for an arbitrary phase difference and arbitrary relative amplitudes of the two orthogonal waves.

Two special cases of elliptical polarization are known as circular and linear polarization.  To obtain circular polarization, the two orthogonal waves must be 90° out of phase and have equal amplitudes.

To obtain linear polarization, the two orthogonal waves must be in phase but they may have arbitrary amplitudes.

Images taken from Fujiwara, H. “Spectroscopic Ellipsometry Principles and Applications,” John Wiley and Sons, 2007.

#### 10.    What are optical properties?

• Optical properties are comprised of two components:  the refractive index, and the extinction coefficient.  The refractive index, denoted by n, is the ratio of the speed of light in a vacuum to the speed of light in the material.  The extinction coefficient, denoted by k, is related to the absorption loss in the material.  Together, these two components make up the complex refractive index, given by N = n-ik, which describes electromagnetic radiation’s interaction with materials (speed change and absorption loss).

#### 11.    What thickness range can spectroscopic ellipsometry measure?

• Depending on the type of material, ellipsometry can measure thickness from a few Å to tens of microns.  How far into the material the light can travel depends on the material’s absorption coefficient.  It is important to keep in mind that for optically opaque samples, such as metal films greater than about 60 nm, ellipsometry can only determine optical properties and NOT thickness.  This is because of the strong absorbing nature of metals and thus a very shallow light penetration depth.  For transparent or semi-transparent samples, on the other hand, the maximum thickness that’s measurable by an ellipsometer depends on the spectral resolution of the instrument.  If you have questions about specific samples, please contact us at ellipsometry.us@horiba.com.

#### 12.    What spectral range do HORIBA Scientific’s ellipsometers  cover?

• Depending on the instrument, the spectral range can be anywhere from 145-2100 nm.  For example, the Auto-SE and Smart-SE instruments have a fixed spectral range from 450 nm to 1000 nm, the UVISEL system offers four different spectral ranges: 190-880 nm, 210-880 nm, 245-2100 nm, and 190-2100 nm, and the UVISEL 2 spectral range is from 190-1000 nm with an optional NIR extension up to 2100 nm.  We also offer the UVISEL 2-VUV instrument which has a spectral range from 145-880 nm, also with an optional NIR extension up to 2100 nm.  To decide which spectral range is best suitable for your applications, you can refer to the next two FAQ’s or contact us at ellipsometry.us@horiba.com for more information.

#### 13.    What can we learn from the NIR region?

• The NIR region is useful for measuring the thickness of materials absorbing in the visible range, as well as thick samples.  The NIR region can also be helpful if you need to know optical properties in the NIR region.

#### 14.    What can we learn from the FUV region?

• The FUV region is useful for determining the band gap, crystallinity, composition, and absorption of dielectrics and semiconductors.  It also helps measure ultra-thin films and films with a low index contrast.  And, measurements over the FUV range are, of course, necessary for applications where you need to know the material’s optical properties over that region; e.g., photolithography.

#### 15.    What is the difference between spectroscopic ellipsometry and reflectometry?

• Both spectroscopic ellipsometry and reflectometry are non-contact optical techniques, and both require modeling to obtain a result.  A reflectometer, however, measures an intensity ratio of light, whereas spectroscopic ellipsometry measures the change in the polarization state of light (i.e. electric field vector).
Reflectometry
Ellipsometry

Reflectometry is not sensitive to small changes in thin film thickness, so it is generally used on thicker (> 100 nm) samples, whereas ellipsometry is very sensitive to ultrathin films.  The figures below show reflectometry (top) and the parameter Δ, measured by ellipsometry (bottom) for a native oxide layer on c-Si.  Clearly, reflectometry is not sensitive to the ultrathin layer, while ellipsometry is.  Also, since reflectometry is intensity-based, things like changes in the lamp intensity can alter the results.  Ellipsometry is polarization-based, so lamp intensity is only important to obtain good signal to noise.  An ellipsometer can measure reflectance and transmittance, as well.

Reflectometry Results:

Ellipsometry Results:

#### 16.    Should I choose reflectometry or ellipsometry for thickness measurements?

• Reflectometry is capable of measuring single and thick films (>100 nm), but it does not carry as much sensitivity as ellipsometry does for handling thin films (<100nm).  Also, reflectometry is generally used for single layered transparent films; it is not useful for certain samples, such as those that are multilayered, anisotropic, absorbing and/or graded.

#### 17.    What are the application fields of spectroscopic ellipsometry?

• Basically, anywhere there are thin films; , including: photovoltaics, microelectronics and semiconductors, flat panel displays, optoelectronics, metallurgy, optical coatings, biochemistry, nanotechnology, polymers, and organic materials.  Some of the materials measured with ellipsometry and used in the above applications include c-Si, a-Si, p-Si, mc-Si, CdTe, CIGS, CdS, SiN, SiC, GaAs, AlGaAs, AlN, InGaN, SnO2, SiO2, PET, PEN, ZnO, PbS, PbSe, TiO2, Al, Ag, Au, and carbon nanotubes, just to name a few.  If you are unsure whether your application is suitable for spectroscopic ellipsometry, please contact us at ellipsometry.us@horiba.com and an applications scientist will be glad to help you determine whether ellipsometry is suitable for your applications.  You can also find examples of the use of ellipsometry over a wide range of applications by viewing our Application Notes