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Microfuze: a new generation of devices for "safe, arm, fuze, and fire"

MicroFuze: A new generation of devices for safe, arm, fuze, and firing of weapons

A new generation of hardware — to make conventional weapons safe, more robust, smarter, and ready to fire when they are needed — is under development at Sandia. The new hardware makes use of Sandia’s microelectromechanical system (MEMS) technology: micro-machines acting as sensors. You’ll need magnifiers to see most of these devices.

For America’s conventional weapons stockpile, the concept of "safe, arm, fuze, and fire" is a familiar one. It’s technology designed to keep a weapon safe until it’s armed and fuzed for firing, or detonation. Now Sandia is working to make these technologies more robust and much smaller.

"Microsystem component designers are working on conventional munitions systems as a first step," says Darren Hoke of the Electromechanical Engineering Dept. 2614 and project manager for the effort. "Our aim is to get reliability data on a large scale. That will be important to proving these concepts for future applications."

By way of illustration, Darren carefully spills a small handful of parts across his desk. They comprise the safe, arm, fuze, and firing mechanism for a mortar shell and fit into an egg-sized dome atop the shell. Components like these are typically manufactured by a declining number of vendors, he says. They are sometimes assembled by hand. Dissimilar materials used for different parts of the mechanism create worry in terms of the unit’s shelf life.

Next, Darren picks up a quarter-inch cube. "We want to replace these parts with this," he says. The cube is the first generation of what researchers are calling MicroFuze, MEMS safety and arming device.

Creating the cube

The cube has three silicon wafer layers etched or treated using techniques borrowed from the manufacture of integrated circuitry. The layers are designed so that a sliding plate in one layer is released by the acceleration force on the weapon and clicks into a new position as the weapon begins to spin. In the new position, tiny explosives contained in a third layer are lined up for detonation.

"The device has to react to the acceleration of being shot out of a barrel, which is very different from being dropped on the floor or other shocks," Darren says. This releases the bar, unlocking the sliding plate. "It also has to ‘see’ or sense the spin created by the rifling in the barrel and react to it." Spin forces slide the plate until it snaps into a new location, aligning the fuzing and firing explosives.

Dave Koehler (2614) did the mechanical design of the safe and arm device. "We looked at conventional safe-arm devices to determine their primary functions and thought of different ways we could achieve those functions with MEMS technology," Dave says. He and his colleagues decided on a bulk silicon approach based on size and compatibility, but still had to come up with some new techniques.

"The functional requirements of the design required a combination of fabrication techniques that did not yet exist, so we worked to develop a process that could produce the structures we needed." The design uses a technology called Deep Reactive Ion Etching (DRIE), a two-level mask process, and bonding of pre-processed wafers. The integration of these techniques into a single fabrication process has been the major accomplishment to date, says Dave. "We’ve ended up with an all-new design space."

Currently, devices fabricated at Sandia are undergoing testing as Dave and colleagues identify areas for design improvement and work on manufacturing issues. "We are still working on a total proof of concept at this stage," Dave says.

Processing the MEMS device was far from routine, says Darren. "There were difficult problems in pushing the normal production processes. Randy Shul in Microdevice Technology Dept. 1763 helped develop a workable process. "Getting the mechanical structure to do things in a certain way was very difficult," Darren says.

"Basically, we provided the enabling technology, DRIE, for silicon substrates," Randy says. Working at Sandia’s Compound Semiconductor Research Laboratory, the process involves actually etching a pattern in one silicon wafer and using fusion bonding to put another wafer on top of it and etch a pattern in that wafer. Sarah Rich (1763) was responsible for the etching. Staff members Lauren Rohwer and Andy Oliver (both 1745) handled the wafer bonding. The work enables vertical integration of two wafers, with different structures on each, using bulk micromachining techniques.

"This is one component in a much larger system," says Randy. "The component testing is going fairly well. There are still a few things we don’t understand, but with a few adjustments in the process flow, we’ll be ready for serious prototyping."

Another problem area involves the micro-initiator firing train. That is starting a series of successively larger blasts, beginning with a hole roughly the size of a sharp pencil dot, filled with explosives. "We learned that we are pushing the size and control boundaries for materials in microprocessing," says Darren. (See "The Explosives Train" below.)

Alex Roesler (2614) and Louis Weichman of Firing Set and Optical Engineering Dept. 2612 are working to integrate components to sense and arm an explosive device by providing signals and power to initiate the fuze. The two are looking at some options including a low-voltage capacitive discharge unit with external power or battery and piezoelectric generation, based on impact. In the latter case, impact compresses a piezoelectric stack to generate power. "This has been done, but we are now looking at single-crystal materials that are relatively new and have much higher energy densities than traditional materials," Alex says.

Sandia is also working with Auburn University researchers on a bridge device made of titanium and boron that reacts with a current to produce increasing levels of heat leading to sparks.

As a part of a Department of Energy and Department of Defense (DoD) agreement, Darren works with a fuze technology coordination group to help provide advanced fuzing technologies. Sandia has programs in place to develop technologies (including MEMS), move them to product reliability testing, and ultimately commercialize them, through DoD suppliers.

While many laboratories are looking at MEMS solutions to these weapon issues, Sandia’s microsystems expertise gives it an edge. "We hope to prove the MicroFuze principle by fiscal year end and test it at a DoD fuze laboratory," Darren says. By using artillery shells for proof of principle, a huge database on reliability can be built more quickly. "These shells are manufactured by the tens of thousands," Darren notes. "Then we can move to other weapons."