Solving the burden of Bourdon tubes
Mini diaphragm gauges offer a new alternative to Bourdon tubes.
BY BRIAN SULLIVAN, Valin Corporation, San Jose, CA
Fabs and OEMs in the semiconductor industry face a number of difficult challenges today, specifically in the etch and deposition/thin film processes. These incredibly specialized processes require extremely clean gases and vaporized chemical sources. The fabs and their process tools utilize gas delivery systems to provide these ultra-pure materials from their bulk sources to their process tools and systems. The increased use of highly aggressive and reactive gases in these processes has caused one very specific problem. These aggressive gases are picking up moisture (through leaks, flawed component installations, improper purging, poor PM practices, etc.) and then attacking and corroding the bourdon tubes located within the pressure gauges in the impacted lines. In a few instances, leaks have been created through these stressed system components.
Millions of dollars are spent inside the fabs and by OEMs to have a highly electropolished finish on the internal wetted surfaces of the many components that comprise their gas delivery systems. The gauges themselves have not been found to be the originating source of the leaks. Instead, the leaks form elsewhere, and the moisture laden and now highly corrosive gas immediately attacks the least corrosion resistant components found within the delivery line. Unfortunately, the bourdon tubes found in most “ultra-high purity (UHP) gauges” today are a principal target. When this type of event occurs, it doesn’t take long for the exposed gauges to fail.
The root of this problem lies in the fact that a standard pressure gauge’s main functioning component is typically an un-passivated, or only marginally passivated, bourdon tube. This tube is open to pressure on one end and welded closed at the other, a design invented by Eugene Bourdon more than 165 years ago. This is the principal weakness and ultimately leaves these gauges subject to corrosion.
As pressure enters this thin, hollow, C-shaped bourdon tube, it causes the tube to flex outward from its relaxed, round shape, stretching it up and away from its original form and position. The tip of the bourdon tube is connected to linkage that moves a pointer around the internal dial (or face) of the gauge, indicating the pressure the gauge is currently measuring. Of course, flexing components made of stainless steel – particularly if they aren’t fully electropolished like the tubing, fittings, valves, regulators, and other components in the delivery system’s line – become vulnerable to chemical attack through the micro fissures formed by the flexures they experience. Each time a bourdon tube flexes, it can suffer the creation of micro fissures. Over time these can then grow into macro fissures, and then ultimately create internal cracks or complete breaks in the bourdon tube’s integrity. Throughout the life of a typical pressure/vacuum gauge in a dynamic system, going through gas source changes, pressure spikes, cycle purge sequences, and other events, the flexing bourdon tube will be subjected to the formation of countless micro fissures. If they are then exposed to a corrosive gas that has become aggressive through the introduction of moisture, it should be no surprise that the bourdon tubes will be aggressively assailed and damaged in the process.
It is well known throughout the industry that aggressive corrosive gases transported through the gas lines increase the likelihood of both internal particle generation and outbound leaks from any vulnerable component. Of course, the presence of any entrained moisture compounds the probability greatly. Any time a minimal quantity of atmospheric moisture makes its way into these corrosive gas lines, it will convert the corrosive gasses into corrosive acids. The bourdon tube acts as a dead leg in the system and is an ideal place for the corrosive gas to enter but does not allow it to get back out. Once the gas forms an acid, the acid will corrode any susceptible surface and generate an exit path by eating its way through the material. Many of the most vulnerable areas for this activity in a gas delivery system are the micro fissures found inside of bourdon tubes.
Although the process connection of a pressure gauge (typically a face seal fitting for semiconductor applications) will be fully passivated and electropolished and is clearly identified in the literature as such. The surface finish and Ra Max or Ra Average values of the bourdon tube itself is usually not provided. Gauge manufacturers measure their gauge connection’s wetted surfaces, but when they are asked about the bourdon tube, there is usually not a clear answer. The surface finish and passivation level of the bourdon tube inside the gauge is not disclosed in most cases. The reason for this is simple. Gauge manufacturers do not make a bourdon tube of electropolished and fully passivated stainless steel because the electropolishing process would damage the bourdon tube due to its thin, spring-like design. A bourdon tube must be able to flex to properly function and to do that it has to be made from thin metal.
Originally the industry used these “standard” bourdon tube gauges in non-critical applications because, compared to their more expensive transducer cousins, they were inexpensive, simple to use, and easy to obtain. However, as the industry has continued to evolve, and the processes used in the OEMs systems have required more aggressive and reactive gases, the use of these gauges has continued. Today, if decision makers want the best running and safest fabs their money can buy, they have to make a change.
The solution: mini diaphragm gauges
Engineers have been searching for a solution to the burden this issue presents to the fabs, and fortunately, a solution has been found and has proven itself to be both long lasting and resilient.
Mini diaphragm gauges for both pressure and compound applications are now available that eliminate the bourdon tube completely. These mini diaphragm gauges employ a diaphragm made of Inconel®, which is highly flexible and extremely corrosion resistant. In an accelerated corrosion study, it exceeded the lifespan of a standard “UHP gauge” using a bourdon tube by a factor of twenty. This means that a gauge that would have lasted only six months in a corrosive application can now last up to ten years.
This Inconel® diaphragm will not suffer the effects of corrosion that its weaker, stainless steel bourdon tube counterpart does. It also removes the dead leg of the bourdon tube itself within the gauge. And all the wetted surfaces of these mini diaphragm pressure gauges are made of either fully electropolished 316L Stainless Steel (Ra <0.25 μm) or Inconel® 718. They also comply with SEMATECH and SEMI Standards.
In the mini diaphragm gauge, the Inconel® diaphragm is welded directly to the solid, stainless steel body which is machined out of a piece of 316L SS bar stock. This seals the wetted surfaces away from the atmosphere and the linkage used to actuate the gauge’s pointer.
Standard (bourdon tube) gauges are made with two separate assemblies. The outer case that holds the dial and outer face is usually made from a very thin sheet of stainless steel and formed into a cylindrical cup-like shape. Its whole function is to hold and protect the dial, the window, the gauge’s bourdon tube assembly and the associated linkage inside of it. The bourdon tube assembly is made of the process connection socket, welded to the bourdon tube, and welded to a tube end-piece. Those are then connected to the linkage and movement pieces that connect to the pointer. Additionally, there are usually a pair of screws that hold the housing onto the gauge’s internal assembly and a couple more that fix the gauge’s dial in place.
The mini diaphragm gauges are made in a manner similar to that of a UHP valve or regulator where the process connection and the case (body) are machined from one solid piece of 316L stainless steel. The Inconel® diaphragm is then welded in place, sealing the wetted surfaces away from the atmosphere, the linkage used to actuate the gauge’s pointer, the face of the gauge, and its outer window. Additionally, the linkage inside the mini diaphragm gauge is not the simplistic linkage of a regular gauge. It is more like a swiss watch in its complexity.
The mini diaphragm gauges are currently only available in 1” and 1.3” dial sizes (hence the “mini” in the name) with ¼” face seal connections. As aggressive gases in gas delivery systems are typically run in ¼” inch lines, there is not a need for larger gauges for these applications, meaning a mini diaphragm gauge should suffice. Another benefit is these can also be used in surface mount applications common to the industry today. If there is a need for a gauge to be installed into a 1.125” or 1.5” surface mount application, this is a perfect fit.
Moving away from using a flexible un-passivated stainless steel internal component to a highly corrosion resistant diaphragm is the exact same technology path taken years ago when diaphragm valves overtook bellows valves for use in reactive and corrosive process gas applications and in nearly all UHP systems. It is a simple, fully-established, and well-proven solution for safer and cleaner gas delivery systems.
Having gauges follow this technology path is one that many OEMs and fabs are just beginning to move toward. This is especially true in the applications and processes where a costlier pressure transducer is not required.
BRIAN SULLIVAN is the Director of Sales - Technology for Valin Corporation, San Jose, CA.