Notes of a Filmmaker
by Katherine Develos-Bagarinao
A typical day ensues. I strut to the lab to make my ‘films.’ Instead of a labgown, I put on my white cleansuit and enter the filmmaking quarters. Yes, I’m a filmmaker, but I do not belong to the entertainment sector by any means! I’m talking about “thin films” – or layered materials, which are typically in the scale of nanometers (1,000,000 times smaller than a millimeter). And in lieu of the big white screen, I use a different method of “viewing” my films. Advanced thin film technology is one of the main reasons why we have integrated circuits in our computers and practically anything electronic, enabling us to process enormous data using smaller and smaller computer chips. Thin film coatings on metals, glasses and plastics have yielded superior qualities and properties. Even the anti-reflective coated eyeglasses now perched on top of my nose have benefited from this technology. As thrilling as these applications may be, however, the making of the films themselves is an entirely different matter, but just as exciting nonetheless. Let me now give you a behind-the-scenes sneak preview of how the films are made.
Part I. The Making of the Film
Inspecting the PLD system prior to a growth run
There are various ways of making thin films – the most common of which are evaporative processes, that is, the materials are “grown” onto another material through processes of evaporation and condensation. The growth of the film is epitaxial, which means that the crystalline structure of the growing film is similar to that of the substrate, the material on which it is grown. I use single crystal substrates as substrates for the films. The method of deposition I use is called pulsed laser deposition or PLD. Simply, I use a high-powered laser, for example, a KrF (krypton fluoride) excimer laser, to evaporate the materials I want to grow as thin films. However, it is not just a matter of simply evaporating a material and letting it condense on the substrate (how do you make them stick to the surface, for instance?). In an atmospheric pressure of 760 Torr, the collision of the evaporated species with the atoms of the gases present in the air would practically prevent these species from ever arriving at the substrate. So the deposition must be done in a highly controlled vacuum. Actually the deposition chamber is an ultra-high vacuum (UHV) environment, with typical background pressures within the range of 10-8 to 10-9 Torr. Below is a picture of the deposition chamber I use.
Main deposition chamber and accessories of the PLD system
Aside from the main chamber where the deposition is carried out, there are other paraphernalia like the subchamber where the substrate is first mounted, port windows for the entrance of the laser beam, valves leading to other components such as the turbo molecular pumps and rotary pumps. These pumps are kept constantly running to maintain the pressure to within the UHV range.
Ok, so far I’ve already mounted the sample in the deposition chamber, now what are the other preparations I need to do? One important aspect, especially in the growth of high-critical-temperature (High Tc) superconductors,* is the deposition temperature. By sufficiently increasing the temperature, adequate energy will be supplied to the species at the surface of the substrate and lead to crystalline growth. In case the temperature is too low, the resulting film is not crystalline, but amorphous (disordered structure).
Typical growth temperatures, in the case of Y-Ba-Cu-O** (or simply, YBCO) films, lie within the range of 700-800 °C. Ever had the experience of being scalded by boiling water? Imagine heat seven or eight times as intense! You wouldn’t want to get near anything that hot. Fortunately, the deposition chamber has adequate cooling systems that prevent overheating of the whole chamber. The heating process is radiative, where a tungsten lamp is used to heat the substrate from behind. A temperature controller is used to program and control the temperature. I use Inconel mounts to hold the substrate which are attached to its surface by silver paste. The picture below shows the sample mount which has been heated up to ~760 °C. Imagine steel that glowing red-hot!
Another aspect is reactive gas. For a multi-component material like Y-Ba-Cu-O, the oxygen has proven to be a very troublesome element. The resulting films are usually oxygen-deficient, and thus the superconducting quality of the film is poor. In order to produce films with excellent superconducting properties, it is not only necessary to provide enough oxygen gas during the deposition (typically in the range of 200-300 mTorr), but the films must also be exposed to very high quantities of oxygen (typically a few hundred Torr) after the deposition, particularly during cooldown. This is to compensate for the deficiency in the oxygen of the films. In this way, oxygen atoms can be reabsorbed back into the film.
The deposition process itself is simple. PLD takes pride in being one of the easiest ways of depositing films. After the routine checks of the temperature and background pressure, I switch on the laser, then input the values of the pulse energy and rate I desire. I run the laser, and the beam is reflected via mirrors and focused by appropriate lenses into the deposition chamber, which houses the target material. The target material for the growth of YBCO is ceramic powder which has been pellitized using high pressures to achieve a very high volume density.
View of the YBCO target (left side) and glowing red-hot substrate holder (temperature at ~800 degrees Celsius)
The laser beam hits the YBCO target and creates a plume which expands towards the substrate.
The ensuing interaction of the laser beam with the target material is quite a sight to look at! The resulting color of the plume depends on the target material. In the case of YBCO, the resulting color is pinkish white. For the growth of YBCO on sapphire substrates, I also use a buffer layer called CeO2 (cerium oxide) to prevent chemical diffusion between the two materials and to provide a good lattice matching (which is important in the case of heteroepitaxial*** growth). The plume resulting from a CeO2 target is bluish white. Of course, for safety, I use goggles which are especially made to protect the eyes against ultraviolet (UV) radiation which emanates from excimer lasers.
* High Tc superconductors – Superconductors are materials which exhibit extremely low or near zero resistance below a certain temperature, called the critical temperature (Tc). Early superconductors are metals, with Tcs of more or less 20 K. High Tc superconductors are cupper-oxide based materials, and have Tcs greater than the boiling point of liquid nitrogen of 77 K.
** Correctly, the YBa2Cu3Oy is the superconducting phase (123 phase), with a Tc of ~90 K.
*** Homoepitaxy involves the growth of film on the same material; heteroepitaxy involves the growth of films on dissimilar substrates.
Part II: Viewing the Film
Operating the SPM apparatus to view a deposited YBCO film
There are various aspects as to the evaluation of the film quality. These include the crystalline properties, composition, surface morphology, and superconducting properties (in the case of superconducting thin films). For me, the study of the surface morphology is certainly the most exciting, because I get to “view” the films and see how they look. Of course, in the nanometer scale, it is impossible to see how the films really look like with the naked eye. On a macroscopic scale, the resulting YBCO films are shiny and black, and unfortunately there’s not much else you can see. Quite a boring sight… that is, of course, until you view them using state-of-the-art equipments. My favorites are the scanning electron microscope (SEM) and the scanning probe microscope (SPM).
Using these apparatuses, it is possible to view the morphology of the films even up to the nanometer scale. You’ll be pretty much surprised at how so much different the structure of the films are compared to just a plain, shiny surface. However, using the SEM and SPM are not exactly ways of “seeing” the film in that sense of the word. In SEM, electrons (called primary electrons) bombard the surface and collide with the atoms in the material. The electrons which are scattered from these collisions, called the secondary electrons, are collected and appropriate electronic equipment “interpret” the energies of these electrons as representing the surface of the material. The secondary electrons then become one’s “seeing eyes.” On the other hand, in SPM, a cantilever oscillating in free air is used to “scan” the surface of the sample. Specifically, in the case of tapping-mode atomic force microscopy**** (AFM), a piezo stack excites the cantilever’s substrate vertically, causing the tip to bounce up and down. A laser beam is focused onto the cantilever’s head.
Sample stage, cantilever, and accessories of the SPM
As the cantilever bounces vertically, the reflected laser beam is deflected in a regular pattern over a photodiode array, generating a sinusoidal, electronic signal. Appropriate electronic equipment translates these signals into surface features of the sample. Hence the cantilever becomes some sort of a “seeing rod.” The pictures here show some of the typical surface morphologies of the YBCO films which were viewed using the SEM and SPM. Clearly, from the SEM results, the film surface is not smooth, and a large amount of outgrowths protrude from the surface which is otherwise homogeneous. In the case of AFM, scanning over much smaller scales shows that the film surface has mini-islands stacked on top of each other, forming a terraced structure!
Of course, ultimately it is not just a matter of making films and watching how they look like under the scrutiny of the microscopes. My goal is to investigate how the film forms under different conditions and parameters, and how the conditions affect the resulting quality of the film. In particular, our lab would like to use these films for device applications, hence, there are electrical properties which need to be studied and optimized as well.
And oh yes, we’ve got awards for the best films as well. For superconductivity research, the lure of being able to make the best film remains an endeavor that many a researcher has ventured to. Filmmaking is business, and we’re on a race to get the best film. We’ve got enough action and adventure, horrors, frustrations and fulfillments of sorts – and hey, we just might get the chance to change the world with these films.
|SEM image of a YBCO film||
Surface morphology of the YBCO film as viewed through AFM
****AFM belongs to the family of SPM methods which includes STM (scanning tunneling microscopy)