PHTN1432: Vacuum Systems and Thin Film Technology
Lab #4 - Mind Your P's and Q's: Throughput and Speed (2019W)

Introduction

In this lab you will see the difference between speed (S) and throughput (Q) in a vacuum system. Along the way, you will see how conductance is affected by the design of a vacuum system and how a mass flow controller works.

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

Read chapter 1 of the Varian text specifically the sections on conductance (C) and throughput (Q) - around pages 27 to 36. These sections explain use of the formulae required for the prelab and the lab.

Hand-in the following (worth 20% of the total lab mark):

The pump you will be using (an Alcatel 5081 turbomolecular pump) is rated for a speed of 80l/s. The vacuum manifold (the pipes which carry gases through the system to the pump) are 40mm in diameter and approximately 40cm in length. The conductance, then, dictates that the speed of the system is lower than that of the pump itself (see chapter 1 of the text).

The conductance of a pipe in a vacuum system under molecular flow conditions is given by: C = 12.1 d3/l where d is the diameter in cm, l is the length in cm, and C is the conductance in l/s (ref: Pfeiffer Vacuum 'Know-How' tutorials). This same information may be found in the text pp.307-308.

  1. Calculate the total speed of the system in l/s. Outline all calculations and show all work.
  2. Knowing the speed of the pumping system (from above), and assuming gas leaks into the end of the mainfold at a measured flow of 20 sccm (standard cubic-centimeter per minute) calculate the expected pressure in the manifold. To do this you must convert from sccm to torr-l/s (the standard unit, as you have seen when reading the text), and knowing that Q of the pumping system equals Q of the leak, solve for the pressure at the manifold.

The prelab assignment is worth 20% of the total lab mark and is due at the beginning of the lab period. Late marks are not assigned if the prelab is not received at the beginning of the lab: you lose 20% of the total lab marks immediately with no recourse if it is not received upon entering the lab (extensions will NOT be given to "print it out" in the lab ... be prepared with the hardcopy already printed).

Background

Two things to walk out of this experiment with: an understanding of Q, P, S and the ability to use them to calculate the parameters of vacuum systems and an introducion to a few interesting pieces of equipment which are common in the field of high vacuum technology - the ion gauge and the Mass Flow Controller (MFC).

As covered in the text (and briefly in lectures), Q called throughput when applied to a pump. It is the flow of a quantity of gas. In this experiment the Mass Flow Controller (MFC) is used to regulate the flow of a quantity of gas into the system. As per chapter 1 of the text, be aware that QMFC = Q Pump System where the later consists of Q = P x S (Speed, S, is that of the pump in series with the conductance of pipes and valves in the system). Be sure you understand what Q, P, and S are and how they are related as they are used to calculate important parameters of a vacuum system such as crossover pressure [Text, pp. 73].

Hopefully you are wise enough to realize this is all prime material for a test!

The ion gauge is used in this experiment to verify high vacuum levels and allow the other gauges in the system to be zeroed. It works by bombarding residual gas within the gauge with high energy electrons then counting the ions produced: more ions means higher gas pressure. The gauge, resembling a backwards radio tube, works at moderately high voltages of hundreds of volts and can read vacuum levels from 10-4 down to 10-11. The main problem with an ion gauge is the presence of a hot filament to produce electrons: if the gauge is operated at high pressures (above 10-3 torr), the filament may burn out requiring complete replacement at a cost of over $500! Always be sure the ion gauge is at a moderate vacuum level before attempting to turn it on [Text, pp. 133 - 136].

The MFC is a unique device consisting of an integrated valve and thermal flow sensor - it can regulate a precise flow of gas QUANTITY (Q, as measured in units of sccm or torr-l/s). Basically, the controller applies a voltage representing the desired flow and the MFC returns a feedback signal indicating the actual flow. These devices incorporate electronics to implement a PID control loop ensuring they can accurately respond to changing conditions in supply pressure.

Equipment

This experiment is to be performed on the Alcatel system. The basic procedures to operate this system may be found in the SOP however be aware the system is designed to be reconfigured rapidly for various labs. In this case the system is configured as follows:

Alcatel System - Photo
A photo of the Alcatel system. The pump sits under the bench (the FORELINE valve is atop the mechanical pump).

Alcatel System - Valves
Valves of the Alcatel system. The yellow pipe is the main line from the turbomolecular pump, the orange pipe is the main high-vacuum manifold, and the red pipes the experiment (subject to change). Shown here are valves for this experiment as well as an optical emission spectrum discharge tube.

Alcatel System - Valves
A photo of the valves in the system.

The Experiment

Start the mechanical forepump (under the bench, the power switch is on the rear of the pump), open the foreline valve, MAIN valve, and MANIFOLD valve. Wait until the system pressure decreases to about 10 torr as indicated on the manifold gauge (which reads to 1000 torr ... the manifold gauge will not read at all until pressure is below 1 torr). Start the Alcatel turbomolecular pump. You should observe the manifold pressure decrease on the Baratron gauges. Ensure the type 246 MFC controller is set to OFF while the system is evacuated to ensure gas does not enter the manifold. Continue pumping until the manifold gauge as well as the high-vacuum Baratron gauges both read zero.

Wait a few minutes, (both Baratron gauges should read zero) then turn the ion gauge ON to observe the base pressure (Filament ON) - it should be in the 10-5 torr range by now. Turn the ion gauge OFF (a 30 second ON cycle time will help preserve the life of the gauge). If the Baratron gauges do not read zero, they should be zeroed now (this happens, especially, if the gauge controllers were not left on ... one Baratron has a special heater and should be left on overnight during the term). To zero the gauges, use a small screwdriver to adjust the ZERO setting on each.

Switch the MFC to FLOW mode (left switch) and then ON (right switch). The digital readout will now show actual, measured flow in units of sccm. Using the SETPOINT control increase the flow to 10% of the maximum (the window on the dial control will indicate "1") then record both the actual flow and the high-vacuum manifold pressure (on the one torr Baratron gauge). Increase the flow in 10% increments (recording the manifold pressure each time) until maximum flow is reached.

When complete, turn the gas flow OFF, close the MANIFOLD, MAIN, and FORELINE valves, then turn the turbo pump off and the mechanical pump off.

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!

Each student must submit a unique lab report - no portion other than the results must be shared between lab group members.

    Observations and Results

  1. Using pressure data from the 20sccm pressure reading, calculate the speed (S) of the pumping system as a whole (as you did in the prelab)
  2. Calculate the expected pressure for each flow rate used in the experiment. Show one complete example calculation (at a non-zero flow rate) showing all formulae and substituted values. Use the speed calculated in the previous question to calculate this. Now, summarize the expected and experimental pressure readings in a table in which the first column is flow in sccm (as set in the experiment), the second column is flow converted to units of torr-l/s, the third column predicted pressure in torr, and the fourth column actual observed pressure in torr at that same flow rate.

  3. Application

  4. The same calculations are commonly used to calculate the crossover pressure of a diffusion-pump based system (i.e. the pressure to evacuate the main chamber before the main high-vacuum valve is opened). As per page 72 of the text, a diffusion-pumped system has a 2000 l/s high-vacuum diffusion pump pumps a large chamber. The outlet of that pump connects to the inlet of a mechanical forepump rated at 20cfm. Assuming the maximum tolerable foreline pressure (also the inlet to the forepump) is 150mTorr, calculate the maximum pressure of the main chamber to maintain the foreline below the maximum tolerable pressure. This is the crossover pressure: the pressure to which the main chamber must be evacuated during rough pumping before switching to high-vacuum.