Introduction
MEMS-based technologies are already commonplace in inkjet print heads for many years. The actuation techniques used can be divided into three main categories:
- Thermal
- Piezo
- Capacitive (or electrostatic).
Whilst all three have a long history, only the first two are being used in commercially available heads. In this article:
- We will explain why the capacitive actuation technique is an excellent alternative to the more mature thermal and piezo actuation techniques;
- We will demonstrate how the technology has been matured in a completely different application field (medical ultrasound imaging), and show its feasibility for inkjet applications.
Capacitive actuator basics
The basic operating principle of a capacitive actuator is fairly straightforward: a flexible freestanding membrane is displaced by a voltage between one electrode on the membrane and one on the underlying substrate. This displacement can be used to move fluid.
The main advantages of capacitive actuation compared to piezo-based are:
1. Manufacturing
- Easier integration with electronics (“CMOS compatible processing”)
- Better reproducibility
- Environmentally friendly (no Pb)
- Lower production cost
2. Performance
- Higher output pressure
- Larger bandwidth
- Lower power consumption
CMUT for medical applications
The capacitive actuation (and detection) technology has been developed and matured inside Philips first for the application in medical ultrasound products, so-called CMUT (Capacitive Machined Ultrasound Transducer). Next to the potential for replacing the piezo-based transducers in conventional ultrasound probes, the ability to miniaturize the transducer allows it to be used on top of catheters, for in-body ultrasound imaging.
Detail of a CMUT array. The circular structures are the flexible membranes
CMUT mounted on the tip of a catheter, for use in in-body imaging
Using the medical imaging application domain, we have been able to develop CMUT technology into a mature, manufacturable process.
CMUT technology for inkjet
The inherent advantages of the CMUT technology mentioned above are also relevant for inkjet applications. We have therefore conducted a simulation study to assess the ability of our capacitive actuation technology to generate droplets from an aqueous solution. The picture below is a snapshot of this, showing the feasibility.
If you want to know more about this technology please visit my presentation at the Digital Printing Conference in Barcelona, or visit the website: https://www.innovationservices.philips.com/looking-expertise/mems-micro-devices/mems-applications/capacitive-micromachined-ultrasonic-transducers-cmut/
Erwin Hijzen, Program Manager
MEMS & Micro Devices, Philips Innovation Services