PHTN1432: Vacuum Systems and Thin Film Technology
Lab #3 - Residual Gas Analysis (2019W)

Introduction

Mass Spectroscopy Laboratory, Photo by Andrew Klapatiuk at Niagara College
A lab group is seen here observing the mass spectrum of a gas sample. In this case air is used as the sample and is admitted into the sampling manifold via a needle valve. A small portion of the manifold gas is leaked through a sampling valve and into the RGA which is kept below 10-5 torr. The mass spectrum is displayed on the PC to the right and output from the computer may be saved on a USB key or to the network for later analysis.

Pre-Lab (Do this before your assigned lab period)

The Experiment

Begin by ensuring all valves are closed except for the evacuate valve and starting the pumps on the EXC controller. Pressure on the high-vacuum side of the system may be monitored via the TC gauge at the intake of the turbo pump (the Varian 843). Wait until this gauge begins to read (i.e. below 1 torr) then open the main valve to evacuate the entire manifold. The TC gauge will 'bottom-out' at which point the system is below 10-4 torr. Continue pumping for 10 minutes or more to clear the system of impurities. Start the RGA program on the PC and turn on the unit. Using the utilities menu 'connect' to the RGA head. Select ANALOG SCAN mode and perform a scan of the residual gases left in the system. Be sure to turn the TOTAL PRESSURE indicator on. You may use the AUTO-SCALE feature on the GRAPH menu to scale it for proper display. Save this trace and data two ways: First, by using the "Save as Windows MetaFile" option in the file menu (the image is to be included into the report using a word-processor and is small at about 12K per image ... it is optional, though, as you may recreate it in any spreadsheet program) - save to a directory (and later to a USB key) for later printing and inclusion into the lab report (Make sure you label it 'Background'). Second, and MOST IMPORTANTLY, save the PRESSURE DATA from the trace using the "Save as ASCII Data" option - this is a data file with two columns, one amu and one pressure in torr - this is required for each sample since the trace is not good enough for quantitative analysis. This file may be opened or imported into any spreadsheet program.

 17.30,   7.79E-007 
 17.40,   1.30E-007 
 17.50,   2.86E-007 
 17.60,   1.65E-006 
 17.70,   4.20E-006 
 17.80,   5.24E-006 
 17.90,   6.32E-006 
 18.00,   6.08E-006 
 18.10,   6.20E-006 
 18.20,   5.13E-006 
 18.30,   3.22E-006 
 18.40,   1.34E-007 

Ensure you understand why peaks occur only at integers for atomic mass and why an element with an atomic mass listed as "35.45" will NOT show-up at 34.5 amu (!)

First, obtain a BACKGROUND trace showing gases in a system under high vacuum. Open the evacuate valve and pump the mainfold down to high vacuum. After five minutes of pumping, turn the RGA on and run a trace (collecting pressure data in a file) showing gases in the system. After the trace is stable (i.e. it remains unchanged after several sweeps through the range of atomic masses) stop the trace and turn off the filament using the proper icon. Save this trace as "backgnd".

Now admit air into the manifold and sample it as follows:

Again, stop the scan, turn off the filament, and save this trace on disk as 'Air'.

We must now pump the air out of the manifold before using the next gas. FIRST, you must TURN OFF THE RGA FILAMENT! Stop the scan then turn off the filament using the filament icon if not already done. Check it twice - these are $360.00 each! Now that the RGA is off it is safe to bring the pressure higher than 10-4 torr. If the filament is left on above a preset limit an interlock system will trip and power to the RGA will be cut - if this occurs the pressure at the RGA head must be reduced again and the RGA re-initialized from the connect menu. Now open the evacuate valve and pump the residual gases out of the manifold for five minutes.

With a clean manifold, close the evacuate and sample valves and admit 2 torr of PURE HELIUM GAS into the manifold. Using the same procedure as before (and optimal sample pressure of between 1 * 10-5 and 3 * 10-5 torr), sample the helium gas and save a trace as 'Helium'.

Now, let's see what happens if the sample pressure _is_ too low. Repeat the above trace using helium but this time set the total pressure much lower than optimal, at between 1 * 10-6 and 3 * 10-6 torr.

When done, stop the scan, turn off the filament, and save this trace on disk as 'He-low'.

We must now pump the helium out of the manifold before using the next gas (pump for another five minutes). With a clean manifold, close the evacuate and sample valves and admit 2 torr of NEON GAS into the manifold. Using the same procedure as before (and optimal sample pressure of between 1 * 10-5 and 3 * 10-5 torr), sample the neon gas and save a trace as 'Neon'.

Finally run another trace on carbon dioxide gas.

You need the following observations:

When done, evacuate the manifold, close all valves, and shut down the vacuum system.

Lab Report

For this experiment, an abbreviated lab report is required (word processed, never hand-written) with the same format as PHTN1300. Answer each question in the form "4a., 4b., 4c. ..." with each new question (#4, #5, etc) beginning on a new page. Do NOT answer an entire question (e.g. question 4) as a single paragraph without identification of sub-parts ('a', 'b', etc). Submit the lab report in a bound folder NOT simply a pile of loose, stapled papers nor a thick binder!

  1. Background Trace Analysis:
    1. Present a background trace (a graph with amu on the x-axis and pressure on the y-axis) showing residual gases in the system after pumping (i.e. with the manifold at high vacuum).
    2. A table analyzing each peak in the background gas trace and showing the atomic/molecular species (e.g. O+, CO2, be as specific as possible in identifying fragments). The table must include four columns: atomic mass (amu), observed pressure (torr), elemental species (e.g. Oxygen), and classification ('atomic' or 'molecular'). If it is an isotope, be sure to show this as well under elemental species (e.g. 14C). All peaks over 0.1% composition (i.e. all seen on the graph) must be identified for all analyses in this lab and be sure to change all masses to integers (even though a peak might be found at, say, 5.9 the actual mass is 6 amu).
  2. Air Sample Trace Analysis:
    1. A trace (graph) showing the composition of air.
    2. A table analyzing each peak on the air trace and showing the atomic/molecular species generating each peak (again, like the background trace, produce a table with at least four columns being specific to the species which produces the peak).
    3. Ratio the partial pressure of the peaks for common air gases (using the data file you captured and saved, NEVER measuring from the graph) in the air trace (using the top three gas components of air but not water vapour) and compare to the known ratio for air. Show the calculations here and the pressures observed.
    4. Based on what you have observed from the air and background traces, explain what the trace (i.e. the composition and ratio of gases seen in the system) for a LEAK would look like as opposed to OUTGASSING.
  3. Helium Trace Analysis:
    1. A trace showing the composition of helium at normal sample pressures.
    2. A table anaylzing each peak on the helium trace. In the table include the four columns as previously used plus add two extra columns: a fifth one identifying peaks attributed to the BACKGROUND gases in the system, which are not part of the gas sample at all (e.g. water vapour, etc) nor part of the actual helium sample itself (which is ultra-pure). The sixth column is an evalulation of the concentration of those background peaks as a percentage of helium pressure (ratio the pressure of the peak to that of helium, not the total pressure).
    3. A trace showing the composition of helium at abnormally low sample pressures.
    4. A table anaylzing each peak on the low-pressure helium trace the same as above (i.e. with six columns).
    5. An explanation of the difference between the traces in (a) and (c) and why the changes in the concentration of background elements is seen when the pressure of the sample is lowered.
  4. Neon Trace Analysis (the concept of isotopes):
    1. A trace showing the composition of neon.
    2. A table anaylzing each peak on the neon trace. In the basic table of four columns, add two extra columns: a fifth one identifying the specific isotopes and a sixth attributing peaks to the BACKGROUND gases in the system, which are not part of the gas sample (e.g. water vapour, etc) or to the actual neon sample itself (which is ultra-pure).
    3. Ratio the partial pressure of the neon isotopes to the total neon pressure using the data file you captured. Compare to the known isotopic composition of 'natural' neon. Show the calculations here.
  5. Carbon-Dioxide Trace Analysis (the concept of fragmentation):
    1. A trace showing the composition of carbon dioxide.
    2. A table analyzing each peak on the carbon dioxide trace (like neon, six columns). Be sure to identify the specific composition of fragments of the carbon-dioxide gas sample as well as background peaks again.
    3. Ratio the partial pressure of each of the CO2 fragments found to the total CO2 pressure (at 44amu) using the data file you captured. Present the data as a table with columns as follows: fragment composition (e.g. "CO"), Pressure (in torr), pressure ratio.