PHTN1432: Vacuum Systems and Thin Film Technology
Lab #2 - Pressure Measurement (2018W)
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.
PART A: Roughing Gauges |
PART B: Residual Gas Analysis
This is a two-part lab in which student will be divided into two groups with each group performing parts A and B alternately in two consecutive weeks.
Part A: Roughing Gauges
The effects of various gases on the accuracy of gauge readings will be investigated by injecting low-pressure gases (between 2 and 10-3 torr) into a mainfold which features capacitance manometer (BaratronTM), pirani, thermocouple (TC), and ConvectronTM gauges.
Pre-Lab (Do this before your assigned lab period)
- Familiarize yourself with the operation of the Alcatel turbomolecular pumping system (read the SOP)
- Research, using the textbook and web resources, the operational principles of various roughing gauges and the effects of various
gases on their accuracy.
Start the Alcatel turbomolecular system with the MAIN valve closed. Monitor the turbo inlet pressure using the TC1 channel of the Varian multigauge (TC1 is mounted directly on the pump inlet tee replacing the Convectron gauge shown on the diagram in the SOP, TC2 is mounted on the experiment manifold). When the pressure falls below 1 torr, the turbo pump may be started and the inlet pressure will fall rapidly. When the pressure has fallen below 0.1 torr, slowly open the manifold valve to evacuate the manifold completely (below 10-4 torr).
Figure 1: Configuration of the Gauge Lab Manifold.
Now, close the manifold valve, open the gas valve (with the flip valve), and slowly admit air via the needle valve until the Baratron gauge achieves maximum reading (10.0 volts). Read the pressure on each of the four gauges as follows:
Figure 2: Roughing Gauges on the system.
- The Baratron gauge (with a 2 torr full scale so that P(torr) = Full Scale Range * Voltage Read / 10)
- The Convectron gauge
- The TC Gauge (Channel 2 on the Varian Multigauge)
- The Edwards Pirani gauge (which reads in mbar and must be converted to torr)
Reduce the pressure by opening the manifold valve slightly and increase the pressure by opening the needle valve. Read the gauges at each of the following approximate pressures (2, 1, 0.5, 0.2, 0.1, 0.05, 0.02, 0.01, 0.005, 0.002, 0.001 torr - as read on the Baratron gauge) and record each gauge reading in a table format. Take the Baratron reading to be the reference against which all others will be compared. The table must show the "true" pressure (as read on the Baratron), and for each of the other gauges the indicated pressure reading.
Now, repeat the above experiment except with propane as the gas. Connect the outlet tube from the propane cylinder to the second needle valve inlet, open the propane cylinder valve, evacuate the manifold completely, and admit propane gas to a pressure of > 2 torr (as read on the Baratron). Again, reduce the pressure in stages, and record results in the form of a table (actual pressure vs. pressure read off each gauge).
Finally, utilize helium to perform the same set of observations. Connect the helium supply (leak test helium) to the needle valve inlet, open the main cylinder, and set the regulator for minimal output pressure.
When done, evacuate the manifold, close all valves (including the propane supply), disconnect the propane and helium supply from the system, and shut down the pumps.
Part B: Residual Gas Analysis (Mass Spectroscopy)
In this part of the lab, mass spectroscopy will be used to analyze gas samples for isotopic components. In the next lab, mass spectroscopy will be used to analyze the gas composition of a helium-neon laser mixture.
Pre-Lab (Do this before your assigned lab period)
- Research, using the web and other sources, the concept of mass spectroscopy, atomic and molecular weights, and the concept of an isotope.
- Research how a quadrapole mass filter works - this is the method by which our mass spectrometer operates.
- Review the operation of our system from the COURSE NOTES section on the course home page
- Obtain the mass spectrum for air, neon, and carbon dioxide and identify the isotopes and components for each peak in the spectrum (e.g. H, H2, OH, HO, etc).
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.
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:
- CLOSE both the evacuate and sampling valves to isolate the manifold from the system
- OPEN the vent valve and admit 2 torr of air into the manifold (this is a high-precision valve and may require several turns) then CLOSE the valve
- Now, sample the manifold gas (air) by opening the sampling valve slightly to leak gas into the RGA. Open the valve ONE-SIXTH to ONE-HALF of a turn counter-clockwise only! Turn on the RGA and ensure the total pressure is between 1 * 10-5 and 3 * 10-5 torr. This is the target total pressure for all samples (allowing comparison between them) - don't push it higher as the filament may burn out and do not use sample pressures significantly lower (or the RGA will be reading residual gases from outgassing in the chamber walls, etc, instead of the sample gas) either.
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 from Part B:
- A background trace AND data file showing residual gases in the system after pumping (where do they come from?)
- A trace AND data file showing the composition of air.
- A trace AND data file showing the composition of helium.
- A trace AND data file showing the composition of helium when the sample pressure is too low.
- A trace AND data file showing the composition of neon.
- A trace AND data file showing the composition of carbon dioxide.
When done, evacuate the manifold, close all valves, and shut down the vacuum system.
Submit one lab report for both parts of the lab, only after both parts are complete (i.e. after two weeks of labs).
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!
Roughing Vacuum Gauges (Part A):
- Tables of data for air, propane, and helium. Show, explicitly, any pressure conversions required.
- A calibration graph for each gas used showing the indicated pressure vs. real pressure. The graph must feature the indicated pressure on the x-axis, true pressure on the y-axis, and both axes are to be logarithmic (i.e. it is a log-log graph) ... an example of the required graph may be found on page 3 of the supplementary notes on gauges (available on the course home page). Three graphs are required: one for each gas used. Each graph must show multiple curves, one for each gauge (including one for the Baratron which will, of course, be absolutely linear so that the indicated pressure is the same as the real pressure).
- Describe WHY the Baratron is taken to be the "true" pressure in terms of the physical principles it uses to measure gas pressure (i.e. why does the design of the Baratron gauge make it the most accurate of the gauges surveyed ?). In answering this question consider the physical definition of pressure and how this relates to the way in which this gauge works.
- Describe why various indirect-reading gauges have a pressure reading which varies with the type of gas.
- WHAT physical property of these gases (thermal _what_?) brings-about this situation?
- Give example numbers of this property for helium, propane, and air. For ALL physical constants referenced, provide a footnote as to WHERE you obtained it.
- Does the discrepancy found with each gauge (i.e. reading higher or lower than air) match that expected based on this physical property (i.e. would you expect a particular gas to read "high" or "low" based on the specific value of this property ?). Detail, explicitly, how the filament is expected to operate in a helium and propane atmosphere, as opposed to air: will the filament run hotter or cooler with these gases than with air and doesthis match what is seen. To answer this you need to explain what was observed (e.g. that the gauge consistently read higher or lower with a specific gas than with air at, say, 100mTorr) and explain this in terms of the filament temp.
RGA / Mass Spectroscopy (Part B):
- Summarize the concept of atomic and molecular mass
- Explain how both atomic and molecular mass are calculated and give examples for both an atom an a molecule: for example show how oxygen achieves a mass of 16 amu and how water has a mass of 18 amu - show exactly how you get this. A good chemistry book or site may help here.
- Explain the concept of isotopes (neon is a good example, also oxygen - be sure to use REAL isotopes here as examples).
- Explain, in a page or two, how a quadrapole mass spectrometer works. A diagram is required for the mass spectrometer as is an explanation of how the unit works. Topics must include ionization of the gas sample, selection of mass, and detection of pressure.
- Background Trace Analysis:
- A background trace showing residual gases in the system after pumping (i.e. with the manifold at high vacuum).
- 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).
- Air Sample Trace Analysis:
- A trace showing the composition of air.
- 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).
- Ratio the partial pressure of the peaks for common 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.
- Explain what the trace (i.e. the composition of gases seen in the system) for a LEAK would look like as opposed to OUTGASSING. Provide a reference showing the expected composition of a system under high vacuum (without a leak).
- Helium Trace Analysis:
- A trace showing the composition of helium at normal sample pressures.
- 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).
- A trace showing the composition of helium at abnormally low sample pressures.
- A table anaylzing each peak on the low-pressure helium trace the same as above (i.e. with six columns).
- 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.
- Neon Trace Analysis:
- A trace showing the composition of neon.
- 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).
- Ratio the partial pressure of the 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.
- Carbon-Dioxide Trace Analysis:
- A trace showing the composition of carbon dioxide.
- 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.