MEMS | ARTICLE
THE GENESIS OF A NEWTYPE OF
ELECTROMAGNETIC SWITCH BASED ON MICRO FABRICATION
Stephen Day Ph.D. | Engineering Consultant, Coto Technology Inc. Roger H. Grace | President, Roger Grace Associates
Background and some definitions
Recently, Coto Technology released a new type of magnetic reed switch based on micro fabrication technology. The prototype fabrication method chosen was High Aspect Ratio Microfabrication (HARM), a MEMS fabrication method pioneered by HT Microanalytical Inc. that offers many advantages over conventional planar MEMS fabrication. What drove this decision, given the large number of micro fabrication methods spawned by the MEMS revolution? In this article, we examine various different micro fabrication techniques and discuss the relative advantages and disadvantages of each with particular reference to building electromagnetic devices.
First, we will make a few basic distinctions. We distinguish between serial techniques that produce one item at a time, and batch techniques capable of making a large number of parts simultaneously. Wafer fabrication is a classic example of a batch fabrication technique, and it is the key to the low cost integrated circuits ubiquitous in electronic devices today. We contrast additive processes that build parts up by adding material from a base structure — typically a support wafer — from subtractive processes, which start with a block of material and remove the unwanted material. (Milling, drilling and Michelangelo’s ‘David’ being classic examples.) And finally, moving to the world of micro fabrication, we contrast the planar micro fabrication techniques used to build semiconductor chips from so- called high aspect ratio micro fabrication (HARM) methods, which build components with feature heights considerably greater than their widths. This ratio, called the aspect ratio, is somewhat arbitrary, but generally considered to be >= 20:1. As we shall show, HARM micro fabrication is highly desirable for building a magnetic reed switch.
Reed Switches
What is a reed switch, and why is a smaller one desirable? The magnetically operated reed switch shown in Figure 1 has a unique set of properties that sustain its usefulness in a rapidly shrinking and surface-mounted electronics world. Those properties include tera- ohm isolation when OFF, tens of milli-ohms resistance when ON, zero electrical power operation, around one watt hot switching capability, and immunity to electrostatic discharge. However, the sheer physical size of reed switches is anachronistic when many surface mount electronic components are shrinking to 0402 (1.0 mm x 0.5 mm), 0201 (0.6 mm x 0.3 mm) and even flyspeck sized 01005 (0.4 mm x 0.2 mm) form factors. So why not build smaller reed switches and
keep up with progress? Since its invention 70 years ago, the conventional reed switch has been steadily shrinking, from a 50 mm long glass envelope in 1938 to about 4 mm today. Figure 1 shows the downward trend in the length of reed switches, together with the length of some recently developed MEMS reed switches including Coto Technology’s RedRock RS-A-2515 switch.
This graph is reminiscent of the famous Moore’s Law, which states that the density of transistors on integrated circuits will double every two years. The size of reed switches follows a similar but less spectacular growth pattern, halving every 20 years instead of Moore’s two. However, just like Moore’s shrinking transistors, reed switches are approaching a brick wall, since fundamental manufacturing and physics limitations prevent conventional reed switches from shrinking much below 4 mm. So, is it possible to build a reed switch that maintains its desirable properties — zero power operation, high off- state isolation, extremely low on resistance and hot switching capability, and yet simultaneously break through the 4 mm size barrier? Clearly, micro fabrication is a logical approach — but what technology is best suited to building a reed switch that has very different material properties from a semiconductor chip, including ferromagnetic components and the use of refractory metals such as rhodium or ruthenium to coat electrical contact surfaces.
Challenges in building a reed switch using micro fabrication
The key feature of a reed switch is a flexible ferromagnetic blade that is attracted towards another such blade in the presence of a magnetic field. When the blades touch, electrical contact is made. The blades are enclosed in a hermetically sealed enclosure to prevent contact contamination. Contacts are coated with hard, inert rhodium or ruthenium for long contact life. The challenges for a MEMS fabrication process are therefore:
a) Fabricating a ferromagnetic blade thin and flexible enough to be closed by an external magnetic field, but with enough cross sectional area to carry the required magnetic flux.
b) Putting rhodium or ruthenium on the blade tips. c) Hermetically sealing the blades to protect the contacts.
d) Wafer level packaging after fabrication and before dicing, to ensure low contamination.
e) Getting enough devices on a wafer, and high enough yield, to ensure manufacturing costs are competitive with conventional reed switches.
36 | commercial micro manufacturing international Vol 7 No.1
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52