Cartoon detective Dick Tracy has been calling on his wristwatch radio to get out of sticky situations since 1946. Back then, when a two-way communication was about as portable as a filing cabinet, it was pretty amazing stuff--and it had the comic strip's young fans mumbling into toy watches for years.
These days, Tracy is still alive and well and battling the bad guys. But the idea of a cellular phone the size of a watch is no longer looking quite so futuristic. Using the new methods of micromachining, which borrow technology for making computer chips to carve out and build up microscopic structures on silicon wafers, Peter Gammell and his colleagues at Bell Labs/Lucent Technologies have reduced three of the critical devices of a cell phone to Lilliputian size.
Moreover, all the components could be constructed on a single chip. Reporting on their feats of miniaturization at an American Physical Society meeting held at Bell Labs in late April, Gammell predicted that a cell-phone-on-a-chip would be reality by 2005. Here's a look at the work so far:
MINI-MICROPHONE. This tiny pyramid of silicon is actually a highly sensitive microphone constructed on top of an integrated circuit. The hatch marks on the base are approximately 100 microns apart. This is the first microphone built by surface micromachining techniques, in which various thin films are deposited on a silicon wafer and some of the features are then etched away to result in movable parts.
FINDING THE FREQUENCY. What's known as a "tank circuit" consists of a device called an inductor, combined with a capacitor to store electric charge. The two together determine a frequency that is proper for communications. For this micromachined structure, the frequency is equal to the 1960 MHz required for communications using the worldwide cellular PCS network. An inductor is a loop of wire that sets the frequency. In this version, it's a tiny spiral warped away from the surface of the silicon chip. The two flat plates to the left are a micromachined version of a capacitor.
INVISIBLE SHIELD. An essential part of cell phones and radios is a device known as an rf (radio frequency) filter, which blocks out unwanted radio frequencies and protects the receiver on a phone from the transmitter on the phone. In current cell phones, the rf filter, made of a ceramic material, is not much bigger than a penny but is, by far, the largest single component (upper left). Compared to the Bell Labs version (top right of the penny), made of aluminum nitride on a silicon surface, it's a behemoth.