Metal surfaces are known for their very high surface energies and interesting chemistry. In modem integrated circuit microfabrication technology, thin metal films are deposited as seed layers for adhesion and electrical continuity for electroplating processes. In various surface science and microbiology applications, noble metals are useful as completely inert background surfaces that can be chemically coupled with alkanethiol-derived molecules. In the various forms of oxides, sulfides, and selenides, metals make interesting compound semiconductors that are the subject of intense study. Many metal oxides are useful to support custom-designed catalyst materials (also typically made of metals and their oxidized counterparts) in high temperature reactors. Clearly, metal surfaces are useful for an incredibly large number of technological reasons.
Transmission electron microscopy (TEM) is an highly useful and versatile technique that allows visualization and characterization of extremely small objects, such as individual molecules and atoms. Because the image is formed by the transmission of electrons through the sample, any object that is directly behind or in front of the sample to be studied will be incorporated into the final image or analysis. In transmission electron microscopy there is no “microscope slide” analog because all known solid materials interact with the electrons emitted by the electron microscope. No ordinary surface of any practical thickness is transparent enough to electrons to allow immediate viewing in the microscope. Thus, the sample must be specially prepared for TEM. Sample preparation procedures vary dramatically from sample to sample, and virtually all are time-consuming and destructive, typically requiring significant time to acquire the requisite skill.
The ideal solution would be to have customized surfaces that are thin enough to be placed immediately in the transmission electron microscope without the need for sample preparation. However, such a surface must still be easy to handle physically, robust enough for a wide variety of applications, provide the needed electron transparency, and yet be mechanically stable under the intense electron beam and associated radiation. Commercially available surfaces are derived from two basic elements: carbon and silicon. Carbon-based films are either polymer or pure carbon films, silicon-based films are based on silica (SiOx) or silicon nitride. These two elements represent the current diversity available, but clearly the most useful materials, metals, are needed to allow more sophisticated samples to be produced on the full spectrum of material surfaces which can benefit from the tremendous characterization power of the transmission electron microscope.
The conventional technique for making electron transparent metallic films involves depositing of a metal film onto a sacrificial surface, dissolving the sacrificial surface to release the metallic film, and then picking up the released metal film onto a blank TEM specimen grid. Although the conventional technique for making metallic films for TEM has been around for some time, the procedure requires careful and tedious attention, making the procedure a highly labor-intensive process available in studies where only a limited number of surfaces are necessary for study. Another standard technique involves depositing the metal onto nitrided silicon, then back-etching to the nitride. This process generally requires films too thick for good electron transparency and suffers from poor yields.
There exists a need for a method for making unsupported, electron transparent films that enables high throughput, hands-free preparation so as to render such unsupported, electron transparent films commercially viable products to relieve end users of the need to make them on their own. The present invention seeks to fulfill this need and provides further related advantages.