Understanding Modern Vacuum Technology

Measuring Gas Pressure with LASERs

I have just returned from the American Vacuum Society Symposium in Tampa, Florida. During the Vacuum Technology Division Sessions, we were updated on the NIST project to develop photonic based vacuum measurement standards. In order to introduce the topic, below is an excerpt from my book Understanding Modern Vacuum Technology, 2nd Ed.

As this work is being compiled, there is groundbreaking work being done at NIST in the Thermodynamics Group. This is in alignment with the “NIST on a Chip” program which has a goal of having reference standards based on quantum phenomena that can be scaled down to extremely small packages.

The pressure standards that are in use today are manometers that are evolutionary advances of the mercury barometer that Torricelli developed in 1643. While the principle behind the primary standards is simple, the manometers are operationally complex, requiring accurate determination of temperature, density, gravity, speed of sound, and ultimately column height. The manometers are large instruments (up to 3m tall) and contain up to 250kg of hazardous mercury. Mercury manometers are still used at eleven National Metrology Institutes, including NIST.

The NIST 13 kPa, 160 kPa and 360 kPa Ultrasonic Interferometer Manometers (UIMs) use ultrasound pulses to determine column heights with a resolution of 10 nm or a pressure resolution of 3.6 mPa and provide the world’s lowest pressure uncertainties. Once the instrument is set up, it takes about a minute to take a data point.

NIST mercury interferometer pressure standards.

Figure 4 45 10 kPa Ultrasonic Interferometer Manometer in the foreground and the 360 kPa UIM in the background. The 260 Pa UIM stands three meters tall and provides the nation with best-in-the-world capabilities for measuring absolute and differential pressure from 1 Pa to 360 kPa.

The goal is to replace the oil and mercury manometers with a standard that can be easily transported between national labs, researchers and industrial users. Once the photonic based standard is developed, NIST will be able to export a primary standard to an end user.

The photonic based method is to make vacuum and pressure measurements using Fabry–Pérot optical cavities to measure the interaction of light with a gas. This represents a disruption in pressure measurement technology and a way of realizing and disseminating the pascal, the SI unit of pressure.

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Figure 4‑46 A fixed length optical cavity realizes pressure by comparing the beat-frequency, f1-f2, as the pressure in gas cavity is increased while the vacuum in the reference cavity is held constant. This standard is simple (has no moving parts) and requires that the temperature of the cavity and molar index of the gas in the cavity is known from either measurements (for example nitrogen) or from theory (helium). (Hendricks, et al., 2015)

The basis of the optical technique is a Fabry–Pérot cavity, where two highly-reflecting mirrors face each other. Two Fabry–Pérot cavities built on a single low-expansion glass spacer, with one of the cavities serving as a vacuum reference. Figure 4‑46 shows a schematic for a dual Fabry–Pérot device. A laser shines in the end of one cavity and the transmitted light is detected at the other end, after bouncing back and forth inside the cavity multiple times (depending on reflectance of the mirror coatings). When the laser light entering the cavity has a wavelength such that an integer number of half wavelengths exactly fits between the two mirrors, a resonance occurs along with a detected signal intensity maximum at the detector, which is used in a stabilization feedback loop to lock the laser into resonance with the cavity.

The Fabry–Pérot cavity is used to determine the pressure by measuring the refractive index n or equivalently, the refractivity n-1. The first approximation, the refractivity of a gas is proportional to the number density of molecules

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MKS Semiconductor and Process Technology Handbook

MKS Instruments just released a new handbook that describes many of the basic design structures and processes for semiconductors. The first section has articles describing the basic transistor structures for the devices being built today including planar and 3D structures. It starts off with a discussion of the simple P-N junction and goes right up through the transistor technologies up to the cutting edge 3D FinFET structure. It has definitions of all of the common diode and transistors.

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Click the image to go to the MKS Semiconductor Devices and Process Technology ebook download page.

The handbook goes into the silicon, dielectric and interconnect materials used and their fabrication methods. It has a wealth of information of the process sequence from raw wafer to finished product.

The second section of the handbook includes vacuum technology; some vacuum basics, pump information, gauging and components. MKS has a wealth of vacuum metrology solutions and the majority are mentioned in the handbook. What is very helpful is that the articles show how all of the MKS technology solutions can be used together as subsystems within a larger system.

If you work in a section of the industry and would like to be introduced to other processes in the industry, then I would encourage you to check this out. It will give you a good encyclopedia level education in semiconductors as well as see all of the MKS technology solutions.

In other news, in case you missed it, the second edition of Understanding Modern Vacuum Technology is available. There is a discount available for readers of this blog.

The Second Edition of Understanding Modern Vacuum Technology is Now Available!

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Cover art for the book Understanding Modern Vacuum Technology, 2nd Edition 

The second edition of Understanding Modern Vacuum Technology is now available! This book contains all the information found in the first book PLUS information that is not found in any other book to date.

What’s new? There is information on the cutting edge technology being done at NIST to develop new quantum based pressure standards. This is so new that the project is moving from the concept and feasibility phase to developing the standards that will replace the mercury manometers that are the primary standards used today.

There is an extensive section describing the Granville-Phillips VQM™  (Vacuum Quality Monitor). This is a revolutionary method for measuring the gas composition in a high vacuum system based on ion trap technology.

The second edition contains an introduction to leaks and their detection.

There are now knowledge check questions at the end of each chapter.

The book is available through Amazon.com and Amazon’s European websites including Amazon.co.uk, Amazon.de, Amazon.fr, Amazon.it, and Amazon.es. For larger quantities, it is recommended to order from the Understanding Modern Vacuum Technology book store.

Working principles of a turbomolecular pump

Turbomolecular pumps are found in every semiconductor fab, the vast majority of helium leak detectors and research laboratories. Yet most never give them much thought, one simply roughs out the vacuum chamber and spin them up. In a few minutes, your chamber is in the high vacuum range.

Today I want to introduce to you the working principles of these pumps. First is a video that shows how a Pfeiffer HiPace 2300 moves gas molecules from the chamber to the exhaust port of the turbomolecular pump. As you watch gas flow through the pump in the video, get the idea that the pumping principle is to increase the probability that the gas molecules will be impelled by the rotor (the spinning blades) into the stator (stationary) blades towards higher concentrations of gas. Pumping is achieved by directing the gas molecules from the low pressure inlet to the higher pressure exhaust port.

Turbomolecular pumps do not have liquid or contact seals. The rotor blades are typically spinning a few tenths of a millimeter from the envelop of the pump and the rotor shaft is a few tents of a millimeter from the stator blades. These gaps are necessary for the operation of the pump, however they do allow a small fraction of the gas to backstream (flow backwards, if you will) through the pump. This is one of the reasons that turbomolecular pumps must have a mechanical pump providing a rough vacuum pressure on the exhaust port. In other words, the turbomolecular pump cannot provide sufficient compression to move gas to atmospheric pressures and they need to be “backed” by a roughing pump.

The video below was produced by Agilent. It shows how a typical turbomolecular pump is built.

I hope this gives you some basic understandings of turbomolecular pumps. Understanding Modern Vacuum Technology has a section devoted to turbomolecular pumps. It covers the development of the pumping principles, the operation and safety considerations when implementing turbomolecular pumps. UMVT  is available for $59 at Amazon.com. You will find information UMVT that is not available in any other book to date.

Get an Intuitive Feel for Gas Properties with this Simulator

Today I want to bring attention to the PhET Gas Properties Simulator. This is a wonderful tool that allows a learner to interact with all of the levers in a gas system. The tool is set up so that the learner can add gas particles into a chamber and then see the results as the system’s parameters (temperature, volume, etc.) are changed.

In the figure below, I introduced equal amounts of a heavy gas and a light gas (60 of each) and then opened the lid on the top of the chamber a bit to see what happens. After letting the simulation run for a short while I could see that the light gas was escaping faster than the heavier gas, the temperature was dropping in the system along with the pressure.

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60 heavy gas molecules and 60 light gas molecules were introduced into the chamber. The lid was cracked open and gas was allowed to escape. At the time of the screen shot, 55 heavy gas molecules and 50 light molecules remained in the chamber.

There are also some graphs that give insight of the molecular speeds and energies of the molecules. Lessons and exercises are available on the PhET website.

As a vacuum technologist, I think in terms of molecular flow where the molecules interact with the walls of a conduit rather than collide with each other. When opening the lid to allow that gas to expand out into space, it occurred to me that this is also a good tool to help convey the idea that in order for a pump to remove a gas molecule from a chamber, the molecule has to enter the pump’s inlet. Since the lid has a variable aperture, it can be use to introduce the concept of how a high vacuum pump’s pump speed is dependent on the inlet diameter .

The Gas Properties Simulator is suitable for high school, college, and continuing education students. I have loaded it onto my work laptop and will be using this in my corporate vacuum lectures in the future. I hope you will find it useful too.

Understanding Modern Vacuum Technology discusses the gas properties that can be explored with this simulator. UMVT also discusses vacuum pumps and pressure measurement.

Take a close look at the construction of the Edwards GXS Screw Roughing pump

In this video, we get a close look at the construction and design considerations of an Edwards GXS screw pump with a Roots Booster. While watching this video, I am reminded that vacuum technology is a multi-disciplinary activity. These pumps are the result of mechanical engineers, electrical engineers and vacuum scientists.

I hope this video is helpful.

You can find the basics of roughing pump technology in Understanding Modern Vacuum Technology. Just click on the link in the right banner which will take you to Amazon.com.

Measuring High Vacuum Pressure with Goal Posts

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Most vacuum technologist are familiar with the construction of a Bayard-Alpert gauge. A filament  emits electrons that are accelerated into a grid where ions are formed. Ions striking the collector cause a current to flow through an ammeter which is proportional to the pressure inside the gauge.

Back in the 1990s, Granville-Phillips embarked on a project to create an inexpensive OEM gauge for the semiconductor market. We had just finished the development of the Stabil-Ion® gauge. The Stabil-Ion was designed to be an extremely stable and reproducible gauge (Arnold et. al. 1994) to serve customers with the demanding vacuum requirements. We learned many techniques to make a gauge stable and reproducible, but it was not attractive to the OEM market because of the price.

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Granville-Phillips Stabil-Ion gauge. (Arnold et. al. 1994.)

We turned out attention to a lower cost, compact gauge that could be offered at a lower price. One of the problems with all compact gauges is that volume inside the grid where the ions are formed is much smaller than a conventional B-A gauge. This means that the number of ions that are formed in the region where they can be collected is greatly reduce. So it is important that we collect as many ions as possible.

Inside a B-A gauge grid, ions  formed with certain angular momentum values cannot be collected because they form stable orbits around the collector as shown below.

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A dual collector design was introduced to solve that problem. Since the grid is at 180 volts and the collector is at 0 volts the electric field well has as saddle shape. The analogy would be like trying to place a basketball on the back of a horse. It will try to fall off either side of the horse rather than staying on its back.

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Ions formed on on the saddle shaped potential field are driven towards the the collectors. (Understanding Modern Vacuum Technology, pg. 101. Courtesy of MKS Instruments, Inc.) 

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Cutaway drawing of a Granville-Phillips MicroIon® gauge. The dual collectors are labeled 140a and 140b. (Knott 2008)

The result of this design was that the sensitivity for this gauge is 3 to 4 times the other gauges in its class.

You can lean more about Bayard-Alpert gauges and other vacuum metrology devices by reading Understanding Modern Vacuum Technology. 

Arnold, Bills, Borenstein and Borichevsky (1994) J. Vac. Sci. Technol. A, 12, 568

Knott (2008) Patent No. US 7,456,634 B2

MicroIon and Stabil-Ion are registered trade marks of MKS Instruments, Inc.

First blog post: Confessions of a long time blogger

It has to start somewhere. I am not new to blogging. I have been blogging for over 10 years. My current photography blog has been in publication for 9 years and my wife and I ran a wine blog for a couple of year before that. Blogging is getting to be a little old school, but still, it is an effective method of communication about a topic.

I bring all this up because one is tempted to exclaim from the mountain tops, “This is my blog! I am going to say really important things and it is going to__” and then lay down a bunch of ideas of what the blog will be only to have it evolve into something else. I must truly confess, I have ideas for the blog, but honestly, after seeing so many blogs come and go, I think that I will just sit back and relax today and not worry about what it will be tomorrow.

So about this Understanding Modern Vacuum Technology book. I had been toying with the idea of writing a manuscript of the vacuum principles that I use frequently. I have lots of books and references, but every time I want to find that handy-Jim-dandy reference that solved that problem five years ago, I would have to figure out where I found it before.

The next problem I faced, after becoming a corporate vacuum guru, was that folks would walk into my office and ask me questions vacuum technology. “Now where was that handy-Jim-dandy reference? Oh, it is in Dushman’s† second book. Oh Lord, I can’t send Pete away with that book, how will he separate out the old that applies to the new?”

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Dushman, 1949, Scientific Foundations of Vacuum Technique, John Wiley & Sons, Inc.

So I decided to write my own book. Feeling good about that decision, I put it off for about six years. You don’t want to go rushing into these things, now do you? Then one fine day, or maybe it was during a nor’easter‡ and I was getting cabin fever, I decided that the I just needed to get started on the book and get it done. How long could it take?

I bumped into my old friend and mentor, Paul Arnold from Granville-Phillips. Still feeling a little wobbly on my pins about writing a text book, I figured that if I actually announced to Paul that I was writing a book, then that would be like a promise or a commitment to follow through and get it done. “Paul, I’ve decided to write a vacuum technology book.”

“Well, then,” Paul interjected, “that will keep you busy.”

He was right, but I was thoroughly committed. Sometimes when you are “stuck in a maybe”, you just have to make a decision. I now had my “winter project” to get me through the next season of nor’easters.

So that was the genesis of Understanding Modern Vacuum Technology. I has burned up 800 to 1000 hours of spare time. (Paul was right.)

Oddly enough, the little book has been doing okay. I am surprised at who is reading it. I expected engineers types, but there has been feedback from executives and sales folks in the industry, too.  I had a Ph.D student stop me to tell me that it helped her with a problem in her thesis.

And all that makes me happy because I know that finding that handy-Jim-dandy reference can be difficult enough for a died-in-the-wool “Torr-head” like me. At least I can make it easier for others.

So this is the start of this blog. My next posts will be of more significant content.


Saul Dushman: vacuum technology pioneer and former Assistant Director, Research Laboratory, General Electric Company Schenectady, NY

nor’easter‡: A nor’easter (also northeaster) is a macro-scale cyclone. The name derives from the direction of the strongest winds—as an offshore air mass rotates counterclockwise, winds tend to blow northeast-to-southwest over the region covered by the northwest quadrant of the cyclone.