Since the inception of thermal fluid ejection devices, the size of droplets ejected by the devices has continually decreased. For the production of printed images by the ejection of inks, the droplet size need not be decreased below about 10 femtoliters (0.01 picoliters) as the spot size provided by such droplet is about 3 microns in diameters. Human vision measurements have shown that spot sizes of 42 microns are easily detectable, whereas spot sizes of less than 28 microns were substantially undetectable. Only about 0.07% of people can detect a spot size of about 20 microns, and less than 1 person per million can see a 3 micron spot. Nevertheless, fluid droplets of 10 femtoliters or less may be suitable for other non-printing applications including, but not limited to, pharmaceutical applications, electronics fabrication, and other applications where visual detection of spots of fluid on a media are not required.
One of the challenges for producing micro-fluid ejection devices for ultra-small droplets is the ability to provide high frequency droplet ejection without a substantial increase in wasted heat energy. For example, an ejection head containing 9000 nozzles operating at a frequency of 200 KHz and requiring 0.08 microjoules of energy per activation may require 144 watts of precisely regulated power resulting in about 0.125 picloliters per microjoule of energy. Such a power requirement results in a significant amount of wasted heat energy.
In order to reduce the amount of wasted heat energy for micro-fluid ejection devices for ultra-small fluid ejection, unique ejection devices and manufacturing techniques are needed.
With regard to the above, embodiments of the disclosure provides a micro-fluid ejection device for ultra-small droplet ejection and method of making a micro-fluid ejection device. The micro-fluid ejection device includes a semiconductor substrate containing a plurality of thermal ejection actuators disposed thereon. Each of the thermal ejection actuators includes a resistive layer and a protective layer for protecting a surface of the resistive layer. The resistive layer and the protective layer together define an actuator stack thickness. The actuator stack thickness ranges from about 500 to about 2000 Angstroms and provides an ejection energy per unit volume of from about 10 to about 20 gigajoules per cubic meter. A nozzle plate is attached to the semiconductor substrate to provide the micro-fluid ejection device.
In another embodiment there is provided a method of ejecting ultra-small fluid droplets on demand. The method includes providing a micro-fluid ejection device containing a resistive layer and a protective layer on the resistive layer. In combination, the resistive layer and protective layer define a thermal actuator stack. The thermal actuator stack has a thickness ranging from about 1000 to about 2500 Angstroms and a thermal actuator stack volume ranging from about 1 cubic micron to about 5.4 cubic microns. An electrical energy is applied to the thermal actuator stack sufficient to eject less than about 10 femtoliters of fluid from the micro-fluid ejection device with a pumping effectiveness of greater than about 125 femtoliters per microjoule to provide a fluid spot size ranging from about 1 up to about 3 microns on a substantially non-porous surface.
An advantage of embodiments of the disclosure is that apparatus for delivery of ultra-small volumes of liquids may be provided for use in electrical fabrication, pharmaceutical delivery, biotechnology research applications, and the like. Another advantage of the embodiments is that the methods may provide ultra-small volume delivery devices that may be fabricated in existing micro-fluid ejection device fabrication facilities.