Honestly it is not so easy to eliminate the effect of substrate in an optical measurement of thin films. You should measure transmittance of your thin film-substrate structure regarding the pure substrate. However it does not mean that you will entirely take into account the effect of the substrate because you will have different reflection from air-substrate and thin film-substrate interfaces. For the reflectance measurement the best will be a black substrate to prevent reflectance from the thin film-substrate interface. If you have your thin film only on a transparent substrate just put black paper behind the substrate. It will prevent the reflectance from the substrate-air interface. However you will still have the parasitic reflectance from the thin film-substrate interface.

It should be mentioned that there is the envelope method, which is applicable in the case of a transparent thin film on a thick transparent substrate. You just need to measure transmittance with interference effect and all optical parameters can be calculated as well as the thickness of the thin film (you do not need to measure reflectance and take into account the effect of your substrate). This article is for additional information. Good luck!

http://www.sciencedirect.com/science/article/pii/S0925346712002716

It is very simple...the reflectance (R) and transmittance (T) of the film measured by spectro-photmeter gives in percentage. First you need to convert both R and T into absolute values by dividing 100. Secondly you need to get the T of a blank substrate and then divide this T from your data T. The resultant will be the T of your film.

Honestly it is not so easy to eliminate the effect of substrate in an optical measurement of thin films. You should measure transmittance of your thin film-substrate structure regarding the pure substrate. However it does not mean that you will entirely take into account the effect of the substrate because you will have different reflection from air-substrate and thin film-substrate interfaces. For the reflectance measurement the best will be a black substrate to prevent reflectance from the thin film-substrate interface. If you have your thin film only on a transparent substrate just put black paper behind the substrate. It will prevent the reflectance from the substrate-air interface. However you will still have the parasitic reflectance from the thin film-substrate interface.

It should be mentioned that there is the envelope method, which is applicable in the case of a transparent thin film on a thick transparent substrate. You just need to measure transmittance with interference effect and all optical parameters can be calculated as well as the thickness of the thin film (you do not need to measure reflectance and take into account the effect of your substrate). This article is for additional information. Good luck!

http://www.sciencedirect.com/science/article/pii/S0925346712002716

what kind of optical characterization as you hear (optical transmission infrared absorption spectroscopy)?

For example, in the case of semiconductor thin films deposited on glass corning, the software for processing the data collected during the measurement of the transmission factor (T%) consider the effect of substrate basis through the index set equal to 1.55.

An effective method for studying the electronic properties of semiconductor thin films is a measurement of the photoluminescence spectra. In this case, you can get information about the value of band gap, the presence of different types of defects, the position of energy levels as well as the optical quality of thin films. Certainly the substrate can to some extent will affect on the optical spectra structure of thin films. This is due to the different thermal expansion coefficient of thin film and substrate material. Therefore, it is important to choose the substrate material where this value is close to that for the value of the film material. By changing technological conditions of growing films can get such materials of high optical quality. An example of such research is the work which attached.

If you have access to a spectroscopic ellipsometer, you can in principle isolate the optical properties of the substrate and the deposited film or films stack. The main limitation of this device is that your film must be of optical quality.

If you could tell us what sort of information you are looking for, It would be much helpful to find the better solution to your problem.

I guess that the main question that you should answer is what kind of optical measurements you want to perform. If it is an transmission/absorption measurement it is fairly easy, as you must subtract the substrate absorbtion from the samples abs. spectrum.With photoluminescence it is more problematic, as you have to check the emission spectrum of the substrate you are using with the same excitation wavelength you use in your experiment. Feel free to contact me if you would need any further advices.

Nicodem @ What is a problem to study the photoluminescence spectra of the investigated thin films and substrate materials ? Why such experiment is more problematic than the measurement of the absorption spectra of thin films and substrate materials ? In the case of thin films with thickness about several mkm the photoluminescence spectrum observes only from thin films, since the excited light is absorbed by such films fully. Thus, in this case there is no need to conduct additional studies of the photoluminescence spectra of substrate material.

@Yuriy - of course, you are right - in most cases study of substrates photoluminescence spectra is not necessary, although in some cases, especially with extremally thin or transparent layers while using some specific wavelengths (usually from UV to blue) you can encounter some serious problems. I'm working with proteins embedded in thin polymer films deposited on glass, and while exciting with a 405 nm layers one can observe also glasses emission (centered around 640 nm and extremally broad). Usually, when the protein concentration is relatively high this is not a problem, although when you go lower, for example in order to do single molecule spectroscopy, you have to switch to quartz to avoid high background coming from the substrate.

This discussion is so interesting. I had high refractive index in thin film of polymer prepared in Si substrate. The value of refractive index is about 2.5 which it is affected by Si substrate. So I wounder if there is any methoid to elimnate the substrate effect espically in very thin film within 10 nm?

How did you measure the refractive index of the thin layer? Have you tried elipsometry? How your measurement setup looks like? is it possible to use an immersion oil objective?

Thanks for reply. Yes I tried ellipsometry. I got the thickness almost the same as obtained from x-ray reflectivity. My set up is acceptable but as I said before the refractive index is so large in very thin film which I want to calculate density from it. The density is about 5 g/cm3 which is unreliable. The high refractive index of Si substrate affected the value of the film. So how can I eliminate the substrate effect?

Dear Hamdy Mohamed,

Maybe it will be better for refraction measurements if you change the material of the substrate. If you deposit your polymer thin film onto a substrate with the refractive index close to that of the polymer, the reflection from the polymer-substrate interface will be reduced and thus you will have less effect of the substrate. Many polymers possess the value of dielectric constant close to 3. Therefore, glass or quartz substrate may be useful in your case. Maybe it can help.

Good luck.

Well, spectroscopic ellipsometry is in principle able to do the job well if you use several angles. Mono wavelength ellipsometre is lacking precision for such job.

Ideally, use three to four angles around the Brewster angle of the substrate (for Silicon, it is about 72°) and as many wavelength as you can in UV-Vis-NIR range. Take care, for getting a good precision, you need to use a model which describes perfectly the optical properties of your substrate ( for Si substrate, you can find some informations in the following web page)

http://refractiveindex.info/?group=CRYSTALS&material=Si

Take a reference substrate of the same kind and normalize to your measurement.

Dear Hamdy Mohamed, optical parameter measurements of thin film on any substrate it'd usful to do for three different thickness of films.It must help you.

Dear Elena. Frolova, Thanks for you comment. The refractive index is almost normal for thicker film within 20 nm but so high for thinner film with 10 nm.

The thinner samples may give reliable results at lower angles relatively glancing angles. Try two close angular measurements in ellipsometry.

Chandramohan

Does anyone know the method similar to envelope method for absorbing thin film - polymeric film which contain luminescent nano/microcrystals? I am interested to measue absorption, then also luminescence properties of these films because of luminescent ions in nano/microcrystals.