- One of the most successful MEMS applications is the inkjet printer head, superseding even automotive and medical pressure sensors.
- Inkjet printers use a series of nozzles to spray drops of ink directly on to a printing medium.
- Depending on the type of inkjet printer the droplets of ink are formed in different ways; thermally or piezoelectrically. Invented in 1979 by Hewlett-Packard, MEMS thermal inkjet printer head technology uses thermal expansion of ink vapour.
- Within the printer head there is an array of tiny resistors known as heaters. These resistors can be fired under microprocessor control with electronic pulses of a few milliseconds (usually less than 3 microseconds). Ink flows over each resistor, which when fired, heat up at 100 million ºC per second, vaporizing the ink to form a bubble.
- As the bubble expands, some of the ink is pushed out of a nozzle within a nozzle plate,landing on the paper and solidifying almost instantaneously.
When the bubble collapses, a vacuum is created which pulls more ink into the print head from the reservoir in the cartridge (Figure below). It is worth noting there are no moving parts in this system (apart from the ink itself) illustrating that not all MEMS devices are mechanical.
A piezoelectric element can also be used to force the ink through the nozzles (Figure below).
- In this case, a piezoelectric crystal is located at the back of the ink reservoir of each nozzle.
- Thepiezoelectric crystal element receives a very small electric charge causing it to vibrate.
- Whenit vibrates inwards it forces a tiny amount of ink out of the nozzle. As the element vibratesback out, it pulls some more ink into the reservoir to replace the ink that was sprayed out
- This technology is also used by the majority of the other leading printercompanies.
- MEMS has enabled more and more heating elements and piezoelectric crystals to beincorporated into a printer head. Early printers had 12 nozzles with resolutions of up to 92 dpi possible.
- Today, modern inkjet printers have up to 600 nozzles which can all fire a droplet simultaneously enabling 1200 dpi. Epson, Lexmark, Hewlett-Packard, Olivetti, XeroxandCanon all use a form of these MEMS in their inkjet printers. Over 350 million unitsweresold in 2000.
Fabrication of inkjet print head
- The proposed electrostatic inkjet head is consists of the glass top-layer part and the silicon bottom-layer part.
- They are fabricated using thick-thermal oxidation and silicon micromachining technique such as the deep reactive ionetching (DRIE), separately.
- The top electrode was previously fabricated on glass wafer.
- The aluminum layer is deposited by sputter on the glass wafer, and then dry film is coated by laminator and patterned the hole of the ink ejection.
- The electrode of the hole type is formed using the sand blaster. The fabrication process of the electrostatic inkjet head with the micro nozzle is shown in Fig below.
- The p-type,(100)-oriented double-side silicon wafer substrate is used First, a thick silicon oxide of 2 μm is thermally grown (Fig. a).
- This silicon oxide layer is used as a mask for forming the nozzle and backside reservoir. The front side is patterned by photolithography using the thick positive photoresist. The pole and nozzle for inkjet ejection are formed by oxide dry etching and DRIE (Fig. b).
- The depth of the nozzle outer part is about 70 μm. Next, the back side oxide is patterned by photolithography and dry etching (Fig. c).
- The reservoir for ink storage is then formed using DRIE (Fig. d).
- To penetrate wafer of patterned nozzle, the front side silicon is etched using DRIE (Fig.e).
- Finally, the oxide layer of the wafer is etched by the chemical wet etching included HF solution and cleaned before the next process