E-Beam Evaporation and Resistance Thermal Evaporation
Coating process of materials by e-beam
Evaporación por e-beam, una visión general.
During the evaporation process by electron ace “e-beam”, a current passes by a tungsten thread, that for Joule heating effect, an electron emision is obtained. Subsequently, the apparatus applies a high voltage between the thread and the crucible container to accelerate the free electrons towards the crucible witht the material to evaporate. Lastly,a strong magnetic field is responsible to focus these electrons in an ace that transfers the energy of the electrones to the material that is wanted to coat, what causes that this gets evaporated and as a consequence, coats a film in a substrate. See Fig. 1.
Fig. 1. Material coating by e-beam. In the image at the right, Andrea Conzatti Cantú, student of the Instituto Tecnológico de Orizaba, who makes her thesis on pH sensors development, uses the e-beam system of the CIDESI Queretaro Clean room.
Continuous and simultaneous material coatings
At the CIDESI Queretaro clean room we have an e-beam system "one of the spoiled" because it is also equipped with two thermal evaporation systems, which allows it to perform from a single to multiple coatings, or if it is required to coat more than a material consecutively before openning the chamber, and as a consequence, without breaking the vacuum, which gives us incredible adventages and possibilities on devices manufacture.
This system is dedicated for metal coating and some dielectric. The materials that are available are mainly: Gold, Platinum, Silver, Aluminum, Chromium, Nickel, Tungsten, Aluminum Oxide, Silicon Oxide and Chromium Oxide.
This capability to coat different materials allows us to work on the development of sensors from glucose detection to air pollutants, among others. See Fig. 2.
Fig. 2. In the image at the left a crucible with gold pills is observed, in the image at the right the e-beam with the crucible with gold is observed after a coating.
What are the material coating characteristics that these apparatus have?
Besides the options of continuous material co-coating and coating before breaking the vacuum, thanks to the six compartments that the carousel that contains the material in the e-beam has, see Fig. 3, and the two sources of thermal evaporation, the apparatus also has the following capabilities:
- Spinning and warming of the sample holder up to 50rpm and 650°C respectively.
- Simultaneous coating of up to three 4 inches wafers.
- The coating is controlled by software (PID control), also the scanning of the electron scanning is programmable according to the coated material.
- The coated film thickness is monitored by inficon sensor.
- Finally, the apparatus is supported by a cryogenic pump that is able to reach a pressure up to E-8 Torr and guarantees the integrity and purity of the coated material.
Fig. 3. The coating system that CIDESI Queretaro has, is able to perform coatings and co-coatings with temperature on the substrate.
Thermal evaporation, a general view
As mentioned before, the e-beam system has: two thermal evaporation sources.Thermal evaporation of materials is widely used in the micro and nano manufacturing of electronic systems that involve a "lift off" process, due it is a coating considered unidirectional between the source and the substrate. In the thermal evaporation process, see fig. 4, the evacuation of the gases inside of the main chamber in order to reach a high vacuum is crtical and fundamental, because the vapour molecules of the evaporated material inside of the chamber may travel long distances before they collide with gas molecules, the collisions with gas molecules are not desirable , because they may change the direction of the vapour of the material and as a consequence affect the homegeneity of the coating. Coatings with pressures in the order of E-6 Torr guarantee unidirectional coatings.
Fig. 4. Thermal evaporation of aluminum. This process is highly unidirectional between the evaporating source and the substrate where the material is to be coated.
The material coatings by e-beam and thermal evaporation are ideal when applications such as diodes or organic based photovoltaic systems are developed, due the material evaporation is performed at pressure below the -5 power Torr, orginating that the leftovers of oxigen and humity decrease in the main chamber.
Fig. 5. The image at left shows Nickel pellets that will be deposited by e-beam (metallization). The image at right shows a wafer with conductivity and temperature sensors, in which, part of the Nickel film was removed through a process called "lift off". These sensors were designed and manufactured in the clean room at CIDESI Queretaro.