Thin Films Laboratory
Thanks to a generous gift from the VMI Foundation, a Thin Films Laboratory is being created in the Department of Physics and Astronomy. The lab contains the equipment and instruments needed to create and characterize thin films and thin film devices such as interference filters, thin film capacitors, diodes, and transistors. Once the lab is completed, interested students will begin serious undergraduate research in the area of thin films and thin film devices.
The lab contains some of the latest scientific instruments, to which physics majors at other schools may not be exposed at the undergraduate level. Students who choose to do undergraduate research in the department's thin films lab will have the opportunity to work with an atomic force microscope (AFM), scanning tunneling microscope (STM), UV/VIS/NIR spectrophotometer, materials microscope, vacuum evaporation system, and an array of recently purchased electronic instrumentation. Students will have direct, hands-on experience with the type of equipment they will be working with once they graduate from VMI.
The ability to measure the topology (surface characteristics) of thin films is very important in understanding the fundamental nature of a particular material and how that material may play a role in the creation of certain thin film devices. Optical microscopy is one way of seeing details in the surface of materials. Other methods include scanning electron microscopy, atomic force microscopy, and scanning tunneling microscopy. Each method has its advantages and disadvantages, and not all methods are suitable in certain circumstances.
The department's thin films lab contains a Zeiss Axioskop 2 materials microscope. This microscope allows us to visually image films in brightfield, darkfield, and polarized light from 50x to 1000x. With the Total Interference Contrast attachment we are capable of measuring step heights with 20 nm precision.
With the Total Interference Contrast attachment, interference fringes are created where there is a defined step across a material. The height of the step can be determined by measuring the distance between the shifted interference fringes.
The department's thin films lab contains an Atomic Force Microscope (AFM) capable of imaging the surface of materials from 80 (m to less than 1 (m with a resolution of less than 2 nanometers at the lower end. One nanometer (nm) is 10-9 meters. Atomic force microscopy can be used on all types of solid materials and produces a three-dimensional image of the surface. The AFM can operate in either contact or intermittent contact mode (including phase imaging) and can be expanded to include magnetic force microscopy.
In contact mode a cantilever that has a 10 nm semi-circular tip makes contact with and is rastered across the surface of a material with nanometer resolution. The scan head contains a piezoelectric material that contracts and expands in the vertical direction so as to maintain a constant force on the cantilever. This expansion and contraction is then converted into a digitized signal that is used to produce a quantitative, 3-D map of the surface such as Silicon seen in this image.
In wavemode the cantilever is set into oscillation at its natural frequency and is rastered across the surface. Instead of being dragged across the surface as in contact mode, the cantilever "taps" on the surface of the material. The differing heights of the surface structures affect the oscillating frequency of the cantilever. The piezoelectric expands or contracts vertically so as to keep the oscillation frequency unaffected. This produces a quantitative 3-D map of the surface such as TiO2 on a glass slide as seen in this image.
The scanning tunneling microscope (STM) works by the principle of quantum tunneling and can image the surface of conductors or semiconductors. If a conducting probe is held less than 1 nm from the surface of a material and a bias voltage is applied between the probe and sample, electrons can tunnel through the gap (or barrier) between the tip of the probe and the sample. These electrons form a tunneling current that can be measured. If the probe is now rastered across the surface so that the tunneling current is maintained at a constant value, then the probe will have to be raised or lowered by a piezoelectric as greater or weaker tunneling currents are measured due to surface height variations.
A digitized signal is then created that represents the surface topology. The main advantage to using the STM is that we can obtain atomic resolution images such as the image of Highly Oriented Pyrolitic Graphite as shown here. Each bump here represents the location of a carbon atom.
In order to measure the optical properties of thin film materials and devices, we need a means of detecting the amount of light the films will transmit and reflect. In certain instances we need to measure the amount of specular (mirror like) reflection and in other instances the amount of diffuse (rough surface) reflection. These transmission and reflection spectra can be analyzed to reveal such important film parameters as the optical constants, absorbance, and in some cases film thickness. The department's thin film lab has a Perkin-Elmer Lambda 900 UV/VIS/NIR spectrophotometer with a 150 mm integrating sphere, 6( incident specular reflectance attachment, and slide holder for transmission measurements. The instrument is capable of measuring transmitted and specular reflected light from 175 to 3300 nm.
To deposit thin films in a vacuum environment, the thin films lab has a Ladd 30000 vacuum evaporator. The system uses a diffusion pump with liquid nitrogen and can reach 10-7 Torr (1 Torr = 1 mmHg) inside a 12-inch diameter bell jar. Shielded electrodes allow us to deposit solid material either by tungsten boats or filaments.
To characterize the electrical properties of thin films, the lab is equipped with electronic equipment. These include such instruments as a source measure unit, picoammeter, digital multimeters, power supplies, digital phosphor oscilloscope, and RCL meter. These are new instruments from Keithly, Agilent, and Tektronix and are like the ones graduates will find throughout industry or in graduate school.
The lab is outfitted with other equipment typically found in a laboratory setting. A fume hood, nanopure water system, spin coater, computers, and data analysis and programming software are but some of the other equipment that students will find available while pursuing undergraduate research at VMI.