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Science of Thin Films
(Formerly the Rainbow Wafer Kit)
This kit supports the Deposition Overview for Microsystems Learning Module. It can also be used as an activity in the Etch Overview for Microsystems Learning Module.
This kit and supporting learning modules are a study of thin films used to fabricate MEMS or microelectromechanical devices. The learning modules discuss the various types of thin films and how these films are deposited, grown and etched. The kit allows one to further study the characteristics of thin films, specifically silicon dioxide.
The Science of Thin Films Kit uses a rainbow wafer to study the characteristics of silicon dioxide, etch rates and light interference. A rainbow wafer is an oxidized silicon wafer that has been etched using a manual process, resulting in several layers of silicon dioxide (oxide) of different thicknesses. The various thicknesses create a novel rainbow pattern on the wafer. The Rainbow Wafer Kit allows the participants to explore some of the basic concepts of MEMS fabrication such as thermal oxidation, thin film interference, and oxide etch. Participants interpret oxidation graphs, estimate oxide thicknesses, and calculate etch rates. They also observe how light affects the physical appearance of transparent thin films such as silicon dioxide.
The kit contains two rainbow wafers and the Etch Overview for Microsystems Learning Module. See More
Kit refills are available which consist of additional Rainbow Wafers
Where can I use this kit?
SCME's Deposition Overview for Microsystems Learning Module supports several STEM concepts which are needed to understand microsystems fabrication and design. In this module, the deposition of silicon dioxide (a type of glass) is described. The description includes the basic concept of silicon oxidation (similar to the formation of iron oxide, or rust), and the basic chemistry involved in two different oxidation processes – direct oxidation of silicon with oxygen gas or an interaction of the silicon crystal with water vapor. The effect on growth rate (micrometer/hour) as a function of time, temperature, and oxide thickness are explored leveraging concepts such as diffusion, mathematics, and graphing.
To create the striped “rainbow wafer,” the oxide is step-wise etched by lowering the wafer in an HF (hydrofluoric acid) solution for a fixed distance (the height of a stripe) and for a fixed time interval (e.g. 1 minute or 20 seconds). At the end of each time interval, the wafer is lowered one more step. Therefore, moving from the top of the wafer to the bottom, the top stripe was not etched, the next stripe was only etched for one time interval, each stripe below that was etched an increasing amount of time, while the bottom stripe was etched the total time.
The resulting thin oxide film stripes each appear as a different color due to the thin film interference effects of light reflecting off of the top surface of the oxide and the silicon substrate. Students learn how to view the colors for the most accurate estimate of oxide thickness. The colors seen are cross referenced with a color chart to determine the oxide thickness. Using these data, the students plot oxide thickness versus etch time and determine the etch rate by applying a straight line fit to determine the slope (or etch rate). This information is then used to determine the time required to remove a given amount of oxide.
In summary, the students use STEM concepts in the application of thin film deposition, etch rate and thin film measurements as used in microsystems fabrication to solve a typical problem encountered by a MEMS technician. When taught in secondary schools, this learning module ties together STEM to a future job function, giving students an answer to the age old question “When will I ever use that?” and “Why I should care?”
- Chemistry - These wafers are made by first depositing a layer of Silicon Dioxide (SiO2) on a silicon wafer substrate. This is done by exposing the bare crystalline silicon substrate to oxygen. The growth rate is dependent on the crystal orientation, temperature, oxygen concentration and type of oxidation process (i.e. "wet" or "dry" oxidation). Using water vapor to deliver the oxygen ("wet oxidation"), yields a faster deposition rate than using oxygen gas ("dry oxidation"). This comparison is analogous to the formation of rust (FeO2) in humid vs. dry climates. The oxide layer was subsequently removed using a wet etchant containing hydrofluoric acid. The color of the oxide is used to determine the oxide thickness and then analyzed to determine the etch rate.
- Physics - One can use this kit to investigate several optical properties. Thin film interference is responsible for the varying colors. Some of the incident light is reflected off the top surface and some off of the glass/silicon interface resulting in constructive/destructive interference depending on the wavelength of light, index of refraction of the glass and its thickness. This information can also be used in the physics class to plot and determine the etch rate of the process.
- Mathematics - The acquired thickness data can be plotted as a function of etch time. Then a straight line curve fit can be applied, or approximated (best fit through the graphed points). The slope of the fitted line represents the etch rate (oxide removed per minute). One can further delve into the goodness of the fit (is it really linear?). This would be a great capstone for students to apply graphing, and a real-world application of a line function.
- MEMS (Microsystems) Fabrication - The fabrication of microsystems devices include the deposition of optically transparent materials (such as oxides) and the subsequent thinning of these oxides (wet isotropic etching) to change their electrical or optical properties, or the removal of the oxides to free components that need to move (release). Measuring the thickness of these materials is done in the fabrication facility using measurement systems which rely on the thin film interference effect (eye, thin film measurement tools).