ECE researchers have built three different types of record-breaking micro scale vacuum pumps that could greatly extend the capabilities of electronics and sensing devices that use these devices, such as gas analyzers for homeland security, healthcare, search and rescue, and other applications. They have also taken an important step towards building an integrated, easily manufactured, micro gas chromatography system that incorporates a vacuum micro pump.
The microdevices created at Michigan are each particularly suited to specific applications, and include: 1) a sputter-ion pump for extremely low pressures, 2) Knudsen pumps with high reliability, with one specially designed for on-chip integration, 3) and a low-power mechanical pump for medium to high pressure applications.
Sputter-ion pumps (SIPs) are vacuum pumps that operate by ionizing gases within the pump. They are particularly suited for use in microsystems that require long-term or precise control over package pressure, such as portable mass spectrometry systems, resonators, and frequency references. These applications typically require vacuum levels of 1 Torr and below.
Prof. Yogesh Gianchandani, Dr. Scott Green, and graduate student Ravish Malhotra designed a miniaturized, chip-scale Penning cell array for sputter-ion pumping that is capable of operating at a pressure at least as low as 1.5 µTorr, which is at least 6 orders of magnitude lower than previously reported microfabricated SIPs. The chip-scale Penning cell array was specially designed to overcome some of the inefficiencies caused during the ionizing process at very low pressure levels. The pump was shown to reduce the pressure from 1 Torr to below 200 µTorr, and remove molecules in this range of pressure at the rate of 0.09×1013 to 1.17×1013 molecules/s.
Knudsen pumps operate by thermal transpiration to control gas flow. Like SIPs, they have no moving parts which allows the vacuum pumps to be highly reliable as well as miniaturized. They are ideally suited for chemical detection and breath analyzers, among other applications.
Prof. Gianchandani, graduate student Seungdo An and Dr. Naveen Gupta developed a Si-micromachined 162-stage two-part Knudsen pump for on-chip vacuum. Experimental evaluation shows that, for a pump measuring 12×15 mm2, using an input power of ≈0.39 W, the evacuated chamber is reduced from 760 to ≈0.9 Torr, resulting in a compression ratio of ≈844. Another version provides about 200 sccm flow. These pumps can operate continuously for years without failure.
In related work, Prof. Gianchandani and Yutao Qin co-designed and co-fabricated a Knudsen pump for a micro gas chromatography (μGC) system along with three additional devices. The additional components are a preconcentrator-focuser (PCF), a separation column and a gas detector. Miniaturizing all four components and fabricating them in a compatible process greatly facilitates eventual commercial production.
The four components fit within a footprint of 1.8 x 1.8 mm2 and were each fabricated by a 3-mask lithographic process. The Knudsen pump provides 0.4 sccm air flow rate with 1 W input power; the preconcentrator demonstrates a fully desorbed heptane peak at 170° C; and the column and the detector demonstrate successful separation and detection of three alkane species. In a stackable architecture, it is expected that the components will ultimately be assembled into a 4 cm3 system.
The third type of miniaturized vacuum pump recently developed at Michigan is a 24-stage peristaltic microscale rough pump, developed by Prof. Khalil Najafi, Dr. Ali Besharatian, Dr. Karthik Kumar, Prof. Rebecca. L. Peterson, and Prof. Luis P. Bernal. Called the Honeycomb Michigan Pump, it can be mass produced at low cost by the use of high throughput conventional microfabrication techniques, enabling integration of the device with a variety of consumer-based microelectronic devices, a feature not available in most of previously reported similar works.
This vacuum pump uses tiny micromachined hexagonal membranes in series configuration. Each element of the array is either a pump or a valve. These ultrathin electrostatically vibrating hexagonal membranes along with their tiny vacuum chambers form a scalable honeycomb configuration to pressurize or depressurize gases in a few seconds using only few milliwatts of electric power. It is the fastest gas micropump reported to date. The scalable, resonant design produced a large flow rate of 0.36 cc/minute and evacuated a sample chamber to about 97 kPa from the atmospheric pressure of 101.3 kPa. The theoretical limit on the performance of such a pump is to reduce the pressure to around 1.5 kPa.
The three vacuum micro pump devices described above were funded by DARPA under its Chip-Scale Vacuum Micro Pumps (CSVMP) program.