THIN FILM ANALYSIS USING SPECTROMETRY AND ELLIPSOMETRY

 

ABSTRACT

An ellipsometer is currently used for analyzing thin film on top of metal substrates.  However, an ellipsometer can only perform this type of analysis.  On the other hand, spectrophotometers are much more versatile and widely used.  This paper develops techniques for using the spectrophotometers to perform the analyses performed by ellipsometers.  Thus, labs that already possess a spectrophotometer would not need to purchase an ellipsometer nor outside ellipsometry services.  Along with the cost savings, there are added advantages to using spectrophotometer for these analyses, which are discussed along with the disadvantages.     The four main objectives of this project are as follows:

1.   Develop methods and formulas for using a spectrophotometer to accurately measure the thickness (d), the index of refraction (n), and the dispersive formula of a thin dielectric film on a metal substrate.

2.   Compare results and advantages of three methods of measuring the d and n (described in Objective 1).  The three methods include:  spectrophotometry (developed in Objective 1), ellipsometry, and spectrophotometry by use of the NanoSpec.

3.   Use all three methods (of Objective 2) to determine the n and d for a polymer film layered on top of a silicon oxide film, which is, itself, on top of a silicon substrate.  

4.   Develop much less expensive instrument that performs the key analyses of the ellipsometer.    


I.  INTRODUCTION:  MAIN OBJECTIVES:

     This introductory section will expound upon the four Objectives stated in the Abstract:

     1.  Develop methods and formulas for using a spectrophotometer to measure the thickness (d), the index of refraction (n), and the dispersive formula of a thin dielectric film on a metal substrate.

     Determination of the physical properties of oxides (grown or native) and other dielectric thin films on metal surfaces is important to many industries and research fields.  This is especially true of the semiconductor industry, where oxides and nitrides are grown on silicon or gallium arsenide to serve as precise insulators or masks.  The main properties are the thickness (d) and the index of refraction (n) of the dielectric film.  The thickness is useful for determining the insulation and isolation values of the film and the index (n) reveals the chemical and physical quality of the film.

     This first objective seeks to adapt a Cary 5 spectrophotometer to evaluate these properties using interference theory.  The Cary 5 was used, prior to the start of this project exclusively for transmission studies, in which the entire sample is transparent at appropriate wavelengths.  The studies in this work use reflection.  With the previous transmission application the spectrophotometer could only determine the product d*n (thickness times index) of a film.  Thus, requiring prior knowledge of either d or n for the spectrophotometer tests to be of any use.  A method will be developed here for using the reflection method to accurately determine both d and n independently, as well as the dispersive equation for the index of the film.   

     To accomplish this first objective, samples where prepared in the Cleanroom, an adaptive fixture was designed and built for the Cary 5 spectrophotometer, measurements where made, and formulas where derived for use with this set-up.  The principles, errors, and concerns in using this system will be discussed in this report.  Finally, an accurate method for improving the system will be suggested, but not tested.

     2.  Compare results and advantages of three methods of measuring the d and n (described in Objective 1).  The three methods include:  spectrophotometry by use of The Cary 5 (developed in Objective 1), ellipsometry, and spectrophotometry by use of The NanoSpec.

     The three methods mentioned above will be compared.  Their advantages and disadvantages will be discussed.  As well, data from prepared samples will be taken with each and the results compared.

     3.  Use all three methods (of Objective 2) to determine the n and d for a polymer film layered on top of a silicon oxide film, which is on top of a silicon substrate.

     In the semiconductor/integrated circuits industry, multiple layers of films covering a semiconductor substrate is common.  It is therefore, useful to determine d and n for such multi-layers.  However this is a more complicated and less accurate procedure.  For this objective, an vitreous silica layer was grown on a silicon wafer, the thickness of the oxide layer was determined by ellipsometry, then a thin polymer film was laid on top of the silica, and the three methods (of Objective 2) were used to determine the d and n of the polymer film.  This work required extensive formula derivation and computer programming.  A comparison of results from all three methods will be discussed.  

    4.   Develop much less expensive instrument that performs the key analyses of the ellipsometer.  

 

Go to Objective 1                Go to Objective 2                Go to Objective 3                Go to Objective 4


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