A new type of sensor based on porous silicon and a unique metallisation process could offer enhanced sensitivity, reduced power demands and lower cost compared to existing technologies for detecting gaseous compounds important in environmental, food and biomedical applications.
Because they are based on silicon wafers, manufactured using integrated circuit production techniques and operate at room temperature using relatively low voltages, the new sensors could be integrated into electronic equipment and used to build sensing arrays.
‘The sensors show a rapid and reversible response to low concentrations of these gases at room temperature,’ said James L. Gole, a Georgia Tech professor of physics. ‘They operate on a voltage much less than that of a watch battery and would be small enough to be taken into the field with a troop contingent or any other group concerned about the presence of harmful gases. The sensors are so simple that they could ultimately be mass-produced for pennies apiece.’
Sensors based on porous silicon have been built before, but their practicality has been limited because of high resistance in the electrodes connected to the porous silicon and the power requirements of as much as 5 volts. Using a unique metallisation process, however, Gole and his collaborators dramatically reduced the resistance of the electrodes built into the silicon, allowing their sensors to operate at between one and 10 millivolts.
The new devices can detect ammonia, hydrochloric acid and nitrogen oxides at concentrations of between 10 and 100 parts-per-million compared to 100 to 1,000 parts per million for the higher-voltage sensors. Because the chemical reaction they use to detect the gases can be rapidly reversed, the new devices are reusable. And after long-term use, they can be regenerated with a simple chemical treatment.
The introduction of gases onto sensitive porous silicon surfaces causes dramatic changes in their conductance. Simple and inexpensive electronic equipment can be used to measure these changes. That could allow the sensors to be integrated onto a microelectronic chip and used as part of an ‘artificial nose’ to detect a range of potentially toxic compounds.
Production of the new sensors begins with a silicon wafer that is coated with a silicon nitride film deposited at about 250 degrees Celsius. Using integrated circuit fabrication technology, a pattern is then applied on the film using a photoresist. Reactive ion etching then selectively removes the silicon nitride, leaving a pattern of exposed silicon.
Next, a hybrid electrochemical process that Gole describes as a ‘semi-hydrous’ etch forms micropores in the silicon with diameters of 1-2 microns and aspect ratios of up to 400. Multiple applications of the etch treatment can create pores of consistent size. On top of these micron-scale pores, the Georgia Tech researchers then fabricate a layer of material with pores of a nanometer-scale size.
A final electroless metallisation process takes advantage of a unique property of porous silicon to produce low-resistance (~ 20 Ohm) contacts to which electrical leads can be attached. This reportedly overcomes one of the existing challenges in the fabrication of porous silicon devices: establishing low resistance electrical contact to the porous silicon structure.
Porous silicon is said to have attracted considerable interest because it produces a strong orange-red photoluminescence when subjected to ultraviolet light. The Georgia Tech team believes the same electronic excitation mechanism that creates the orange-red glow also helps improve the efficiency of the electroless metallisation process thereby improving the conduction of the contacts.