vacuum gauge

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.

for web FLOC Cartoon

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

equation.GIF

first photonic sensor.JPG

 

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.

MKS Handbook.jpg

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.

Granville-Phillips/MKS Announces a novel Wide Range Cold Cathode Ionization Vacuum Gauge

It was an interesting day when I first heard about MKS’s plans to release a new wide range cold cathode gauge. I though to myself, “does the world need another 10-2 torr Cold Cathode Ionization Gauge?” Well, if that is what you are asking yourself, too then take a closer look. This transducer provides a wide measurement range of 10-7 torr to Atmosphere, eliminating the need for multiple gauges.

523 CCG_white background (1).jpg

Revolutionary new CCIG has a range for High Vacuum to Atmosphere. Image courtesy of MKS Instruments, Inc

The Series 523 uses two different types of discharges. The first is the normal pressure dependent discharge found in every CCIG which covers the range from 1 × 10-7 to 1 × 10-2 torr. The second range between 1 × 10-2 torr up to 10 torr is derived from a glow discharge measurements cause by the breakdown of the gas by the electric field.  Above 10 torr, the pressure is measured by the arc discharge current.  The published accuracy of the gauge is +/-50% over the range of the gauge.

The cost is contained by the elimination of expensive ceramic-to-metal seals. The 304 SST electrode structures inside the gauge are unique and made possible by using injection molding technology to build the sensor. The electrodes are held in place by the polymer that forms the vacuum envelope and the electrical feed through materials. The polymer, Polypropylene, was selected for low outgassing rate, mechanical and temperature stability and electrical properties.

The target market for this gauge is that set of processes that do not heavily rely on accuracy but just need basic vacuum level information. For applications that require higher degrees of accuracy, MKS provides a vast selection of vacuum measuring solutions.

Understanding Modern Vacuum Technology describes Cold Cathode Ionization Gauge technology as well as all the other pressure technology used in science and industry today. UMVT  is available through Amazon.com.

You can read about the MKS 523 wide range CCIG on the MKS website.

Measuring High Vacuum Pressure with Goal Posts

BAG.gif

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.

Stabil-Ion-3D-illustration_GS.png

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.

precessing ion.gif

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.

MicroIo ion paths.gif

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.) 

MicroIon Patent image

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.