Dear Margi, quartz crystal (piezoelectric) is commonly used for thickness measurement during the thin films deposition. It measures a mass change per crystal area by measuring the change of frequency. This frequency changes due to growing mass of material on the surface of the quartz crystal during the deposition. http://www.tectra.de/qmb.htm

In case your deposition system is not equipped by such device, you can calculate your deposition rate from thickness of the deposited film divided by deposition time. Such deposition rate is valid only for the exact same deposition conditions of the films. Be careful in case of reactive deposition, the deposition rate changes rapidly with amount of oxygen incorporated to the sputtering target.

Dear Margi, quartz crystal (piezoelectric) is commonly used for thickness measurement during the thin films deposition. It measures a mass change per crystal area by measuring the change of frequency. This frequency changes due to growing mass of material on the surface of the quartz crystal during the deposition. http://www.tectra.de/qmb.htm

In case your deposition system is not equipped by such device, you can calculate your deposition rate from thickness of the deposited film divided by deposition time. Such deposition rate is valid only for the exact same deposition conditions of the films. Be careful in case of reactive deposition, the deposition rate changes rapidly with amount of oxygen incorporated to the sputtering target.

The answer of this question is depending on your growthing system. For example; if you coating the wafer by using a spin coater, the thickness of the films depends on the viscosity and the spin speed of the system (rpm). 

If you are using a sputter, it depends on the temp, pressure, target and the plasma gas. But as far as I know, you can make a guess after calibration the system, On the other hand, the angle of the sample and the target effects the thickness!! For example, you can distinguish the color difference of the film on sample.

 If you growth SiO2, you can use the link http://www.cleanroom.byu.edu/color_chart.parts/sio2_chart.jpg.

Regards.

Dear Margi,

The best method is QCM for sputttering. QCM needs some parameters to measure thickness such as acoustic empedance, tooling factor etc. Especially the tooling factor is so critical. In our lab, we have QCM and XPS. We use XPS to determine the deposition rate after we find it. We set tooling factor according to deposition rate to measure thickness with QCM.

You can use also profilometer of course you can not do it during sputtering.

Dear Margi, Sputtering is a thin film deposition technique. To control the thickness during sputter deposition of thin film, QCM is the most widely used tool for in-situ thickness measurement as it is cheaper as well as easy to install. An expenssive option is RHEED. But if QCM or RHEED are not available, you can optimize thin film thickness ex-situ using a SURFACE PROFILOMETER. In that case, you should deposite films for varying deposition time, measure their thickness ex-situ,  determine the deposition rate and use it as standard for further depositions. Same way you can also optimize your deposition rate (i.e. thickness) using XRR.

Dear Margi,

Having agreed with the previous responses, I would re-calibrate the measurement for after ten to twenty depositions assuming that the growth rate is changing. Suppose you have no tools like QCM, RHEED, XPS, RBS etc. you can deposit one layer in a certain period and measure the thickness ex-situ (XRR, profilometry or ellipsometer). But one has to take care about oxidation.

Best.

Dear Margi,

The acoustic method is best, which I know. But question is that is method of deposition of film do you use. If you use magnetron sputtering system QCM is best, if you use ion ablation method, you can use calculation of beam power and time ( energy of beam) with calibration of thickness before your real measurements. With pulsed ion ablation method to easy to have  control after knowing of rate of evaporation of target and beam power ( that is kinetic energy and current). With DC ion ablation is a little complicated. You can look DC ion ablation systems from Kurt J. Lesker ( it is in my memory, maybe I am not right). I used parameters of power supply for magnetron sputtering system and vacuum. I made calibration of thickness using surface profilometer. Right now laser microscopy allows to have 3D picture and it also can be used for calibration of thickens of film and calculate the rate of deposition. If you know rate of deposition you can transfer to thickness. Of course you can use method of measurements of conductivity, but it is for conducting films. What type of films do you have? Good Luck!!

I believe the question is `how to control?'. This is entirely dependent on the method used. Every method has some parameters for deposition experiment. Vary the most sensitive one and standardize by measuring the thickness. If impearmeable to ions, you can try impedence. Elipsometry has already been given by others.  

Dear margi, kindly provide details about the thin film....

Dear Margi, controlling the thickness of a thin film depends on the deposition technique that you are working with. Can the sputtering process do it? Yes, it can. However, it depends on the equipment. It is pretty commun that sputtering devices have a panel where you can know the thickness as well as other parameters. If your sputtering does not have this features, I would recommend you to make some thickness measurements at different deposition periods. In that way, you can estimate more o less how much deposition time is what you will need in order to get an specific thinckness

Is your question about 'How to Control the Thin Film Thickness' or "How to Measure the Thin Film Thickness"?   Also, are you referring to liquid film or solid film?  

Actually for calibrating thickness measurements with quartz crystal, I used to weight samples before and after depositing. I worked with 50 to 100 nm Al films deposited by evaporation on super smooth quartz discs. Thickness variation between this films should less than 5 nm. A good scale and clean handling and it worked fine.

The previous responses are right when mentioning that QCM is often used for estimating the thickness of growing films.

But I would like to say more about optical monitoring systems:  If your film is transparent and your sputtering systems has windows, you can shine a laser beam on your substrate, collect the light with a photodetector, and observe the oscillation of the signal while your film is growing, which will give you the thickness of the film.  If your substrate is transparent as well, you can measure the transmitted signal in the same way.

In addition, a bright white light beam and a ccd detector will give you a reflectance or transmittance spectrum of your films during the growth, a spectrum that you can model to extract the thickness (convenient for multilayer systems).  With a more complex setup, you can also use ellipsometry for the in-situ measurement.

These kind of thickness monitoring systems are very common in the optical coating industry.

I agree with all the above answers of thickness variation in the sputtering process. Now that what is available at your facility of sputtering?. In case you do not have the insitu Thickness monitor while sputtering, in case your tool do not have any automatic controllers. Then try with other methods suggested above. Best way is to measure the thickness using the profilometer or cross section SEM, then estimate the rate of deposition for one set of conditions for instance the constant Ar pressure, constant target power, constant substrate to target distance. Try to vary the time of deposition and continue for at least five depositions to be more accurate. Then you get the straight line, thickness versus the time of deposition. From the extrapolation of this line you can estimate the desired thickness. But in case of reactive sputtering again follow the same trend putting all parameters constant. Additionally you may refer how I have optimized the thickness of ZnO films: Supporting information of: http://onlinelibrary.wiley.com/doi/10.1002/smll.201500924/full

Thanks for all the answers, it all made me more understand how the growth of thin films. so there are many ways to control the thickness of thin film, actually i use DC Magnetron Sputtering. The parameter that i can control in the sputtering process is time of growth, temperature, plasma power, current, and Voltage. The are some theory or growth rate to predict the thickness of thin film from the parameter, i've been looking for some journal but have not yet found it. i want to make hard magnet thin film Strontium Ferrite (SrFe12O19), thank you very much my friends

CMIIW 

I have been reading all the previous answers, and there is still one very important parameter that you need to take into consideration when using magnetron sputtering. As some people suggested in other comments, you could make a very nice calibration studying how the deposition rate is affected by gas sputtering pressure, gas mixture, gas purity, substrate temperature, voltage in the target , current in the target, power in the target, and substrate bias. Still, one day you return to your system to make a new experiment and you discover that the deposition rate is not correct, even if you control all the previous parameters. What happened? There is one parameter than very few number of people checks: strength of the magnetic field in the cathode! The magnetic field in the magnets of the cathode do not have constant power, they decrease with the time. Specially you can have a big change if one day you forget to refrigerate correctly the magnetron! A small variation of the magnetic field can cause a huge change on the plasma conditions at the cathode and your previous calibrated deposition rate is not more valid! So, when you are studying the parameters affecting your deposition rate, you need to check from time to time the strength of the magnets with a magnetometer.

Good luck in your experiments! - Esteban.-

Thanks so much Prof. Broitman for completing the previous answer. Is there ways to control exactly the thickness. For example, I want to make thin films with a thickness of 20-30 nm, how to control the parameter of sputtering (magnitude of temperature, strength of magnetic field, voltage, Subtracts, grain size of target, etc), whether there is a calculation to determine the thickness with these parameters? I have been look for it but have not yet found it, or we are required to try and error ( with DC magnetron sputtering method) to get exactly thickness of thin film. Thanks so much to you all friends

CMIIW

In thermal vacuum evaporation, you have an approximated " a priori" way to calculate the film thickness if you make some reasonable approximations. The theory is very simple and you can find it in many books. However, there is no theoretical formula "a priori" to calculate the deposition rate of a DC magnetron sputtering. The reason is that you need to know the sputtering rate of the material from the target at the plasma conditions you are doing the experiment (impossible to calculate even with theoretical simulations), diffusion of the material through the substrate, chemical reactions if it is reactive sputtering, adsorption and mobility of adatoms on the surface, atc, etc etc..

In simple metal deposition with inert plasma (Ar), the deposition rate vs time should be constant when all parameters are constant. There is an exception at the beginning, when you are forming the first layer because the adsorption of adatoms on the substrate are different than on a layer of the same material you have already deposited. But after the substrate has been covered with a layer of your material, the deposition rate of the single metal should be constant if everything is constant.  However, as soon as you have reactive magnetron sputtering of more complex materials, the problem is more complicated because you have chemical reactions on the surface of the forming film, chemical reactions in the plasma, and chemical reactions on the target which are not linear with time.

As many people said before, you need to fix parameters, and make few depositions to get a thickness vs time "calibration curve". From the calibration  (which in principle could be not linear) you can infer the thickness for other intermediary deposition times.