Developing low cost vacuum pump to make electron microscope accessible to kids in classrooms

Methods

Summary

         A scanning electron microscope (SEM) is an easy-to-use microscope that gives lifelike images of samples with a resolution and depth of focus far superior to those of a light microscope. However, current models are expensive (>=$70000) and hence way beyond the means of high school students. For several years there has been a program to realize an SEM that costs less than many smartphones (ref). Key to this reduction in cost is eliminating the need for a high vacuum pumping system (these cost about $5000). This has been achieved by having the electron column in a sealed-off vacuum and bringing the electrons out through a thin (20nm) window and then onto the sample. If the sample is in the air at atmospheric pressure then the pencil beam of electrons will be defocused by scattering by the air molecules unless the sample is impractically close to the window: about 1 micron. However, if the pressure in the sample chamber can be reduced 1000-fold to about 0.1 Torr we will have a much more practical microscope. Note that 0.1 Torr is still a factor of 10,000 higher than is required in the electron column and can easily be reached with a single pump. For example, a 2-stage rotary-vane pump can reach 0.02 Torr, but even these pumps cost about $850 and require 120V, 10A power.  So we are seeking a pump that costs $50 or less can pump a 1-liter volume to 0.1-Torr in about 10 minutes and can be manually or battery operated.   

SEM Design

Challenges

One of the major challenges in this project is to find a tubing material that is both elastic enough for a peristaltic pump and has good absorption resistance, so it does not leak molecules into the chamber.

Solution: Some companies make special tubing for peristaltic pumps with two or three different material layers to achieve these results. We will first try to find a common tubing material and then consider these custom options later.

one other problem we are facing is with leaks in the pumping and chamber system, we plan to use different types of gases to detect the leaks in the chamber as the gas enters the system through a leak the plasma in the chamber will change its color depending on the gas.

Pre Analysis Plan

We plan to do pressure measurements in a sample chamber and try to achieve an electron mean free path of 1cm this will mainly depend on the chamber size, temperature, and pressure, we will optimize these parameters to achieve maximum output.

As we had a low budget for this project we are using a GEISSLER TUBE CROSS VACUUM SCALE to estimate our chamber pressure by making our own Geissler tube and using our pump to pump down the chamber and comparing the results with the already-known results from the vacuum scale.

finally, we plan to use this pump to do real electron microscope tests, and this is the best way to know our output and results.

Here is the vacuum tube

Here are the results of the vacuum scale with respect to tubing material