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Micromachined Reed Switches Offer New Solutions to Many MEMS Applications



A microminiature reed switch has enabled a wide range of diverse applications requiring high reliability, small size, and low cost.

Micromachining technology, simply stated, is the utilization of semiconductor manufacturing equipment in such a way that it creates micromechanical systems that serve a specific purpose. MEDER's objective was to develop a microminiature, hermetically sealed reed switch that functions exactly like the larger, standard hermetically sealed reed switch.

The basic critical requirements for reed switches are that the contacts must close when they come near a magnetic field of sufficient strength, and that they must draw zero power when in the off, or open, state. MEDER has developed a microminiature, hermetically sealed reed switch in partnership with the Swatch Group of Switzerland. Outside of the Swatch Group, MEDER is the exclusive worldwide marketing arm for the micromachined miniature reed switch.

By definition, a reed switch is a small, electromechanical device that contains one or more ferromagnetic reeds hermetically sealed in a glass envelope. When the reed switch is brought into a magnetic field, the reeds close, creating a switching function. A typical reed switch is shown in Figure 1. The nickel/iron base metals are relatively soft and would not be good choices for use in a reed switch. This is because high switching loads initiate massive metal transfer, causing the contacts to become stuck in the closed position. However, the switch's ferromagnetic properties are essential for proper magnetic actuation. A more preferable manufacturing option is the use of a hard metal at the switching contact surface. Typically, rhodium and/or ruthenium have been used adding dramatically longer life to the reed switch. However, whether one is plating or sputtering the hard metal layer to the ferromagnetic leads, a transition layer is needed to insure that all the metals metallurgically bond to each other. Gold, copper, and tungsten are often used as transition layers.

Figure 1: A reed switch within its underplating.

In the new MEDER microminiature hermetically sealed reed switch, (see Figure 2) borosilicate glass is used, with a thickness of 0.35 mm at its lower section, and 0.40 mm at its thick upper section. A proprietary metalization process is used on the edges of the glass, so that it becomes hermetically sealed when bathed at a temperature of approximately 320°C. The blade material is rhodium, with a thickness of 0.15 microns, on a nickel/iron base. The contact gap is four to five microns, with argon gas as the cavity medium. To accomplish this, two 4-inch wafers are used, housing over 10,000 devices. The top and bottom are mated together, and the entire wafer undergoes the hermetic sealing process.

Figure 2: A hermetically sealed microminiature reed switch.

Over the years, MEDER has improved its microminiature reed switch by using a continuous improvement philosophy. For example, many laboratories, college research facilities, and semiconductor foundries initially considered an epoxy seal to be sufficient. This was not the case. Epoxies tend to continually out-gas at very low levels, and this is exacerbated by increased temperatures. Any small amount of epoxy-related film on or in the contact area proved to be fatal to the reed switch. This was because these films created an amount of insulation that was sufficient to prevent any voltage, current, or signal transfer through the contacts. Although this did not occur all of the time, it occurred often enough to reduce the long-term reliability of reed switches, thus disqualifying this manufacturing approach.

After some qualification trials and different masking approaches, a truly hermetic microminiature reed switch with a glass-to-metal seal was finally achieved. The reliability of this switch has been demonstrated repeatedly in large volumes, with lifetimes exceeding 100 million operations. MEDER has also found that this reed switch is essentially impervious to shock, as it has been tested at up to 5,000 Gs with no faults detected. This is in contrast to the bigger reed switches, which are manufactured in the conventional manner and can only withstand shocks of up to 100 Gs. Another critical qualification test was regarded long-term exposure to high temperatures with the contacts in the open and closed states. MEDER chose 100°C as the bath temperature and soaked the switches for several weeks in each state. Over 1,000 switches were tested in this environment with zero failures. The reed switches produced for commercial applications undergo a 16-hour quality control screening, during which they are temperature cycled and temperature screened from -40°C to 125°C with a dwell time of one hour at each temperature.

After significantly improving the micromachined reed switch, MEDER developed different packages which make it easier for customers to mount the switch on PCBs (printed circuit boards). MEDER added a ferromagnetic lead frame in one packaging scheme which dramatically adds to its magnetic sensitivity (see Figure 3). This allows customers to sense and activate from greater distances.

Figure 3: Microminiature reed switch over-molded on a gold-plated lead frame.

The use of stringent qualification testing has allowed MEDER to design of the microminiature reed switch into applications requiring high reliability. For example, it has been designed into medical applications where proper operation of the switch can often be a matter of life and death. The reed switch also operates successfully up to 200°C, which could be very useful in certain applications such as automobiles and small home appliances where high temperatures may be involved.

Applications

MEDER's microminiature reed switch has allowed designers to develop new products that were not previously possible. This patented reed switch is the smallest ever invented and has passed the test of time, having been successfully manufactured and sold for the last 15 years. Many competitors have tried producing similar reed switches and failed over the past 25 years.

The following are some examples of applications that MEDER has worked on and the applications in which the microminiature reed switch has been designed.

In-the-canal hearing aids

Hearing aids in the past were worn on the outside of the ear. They hooked over the top of the ear and rested behind it. Many people felt self-conscious wearing these hearing aids in a public environment, and some would not wear them at all. These large, behind-the-ear hearing aids used a rotary mechanical thumb switch to regulate volume. Designers have been working on developing a smaller, better hearing aid that would fit in the ear canal. These new high-tech, miniaturized hearing aids have been made possible by the continued development of IC technology along with the fact that less sound amplification is necessary due to their position closer to the ear drum. In designing these smaller hearing aids, it is crucial to have a way to adjust the volume and to program the microelectronics. MEDER's microminiature hermetically sealed reed switch offered the perfect solution. A small wand, similar to but smaller than a pencil, with a magnet mounted on its end activates the reed switch when brought close to the ear. This initiates the setting of the various modes and volume controls. This remote activation offered by the reed switch was the essential ingredient for the implementation of smaller, in-the-canal hearing aids.

Pacemakers and implantable defibrillators

With the invention of ICs over 40 years ago, the first pacemakers were introduced. These were very large by today's standards, and could not be implanted in the human body. Over the years, they have undergone dramatic size reduction due to the increased miniaturization of all components, making it possible to implant the pacemaker in the human body (see Figure 4).

Figure 4: An example of an implantable pacemaker.

Reed switches play a big role, as they allow communication with the device after it is implanted. When an external magnet is placed near the chest cavity, it causes the reed switch contacts to close, allowing communication with the implanted device. After the microminiature reed switch closes, information in the pacemaker can be wirelessly downloaded, heart rate can be adjusted, calibration can be initiated, and different modes can be set. When the reed switch is in the open position, only a minimal amount of battery power is used, thus lengthening battery life. When the reed switch is closed or activated, such as for programming, battery draw is increased only momentarily. Thus, battery power is greatly conserved. Using the microminiature reed switch, pacemakers became only the size of a thick silver dollar. Now the pacemaker and defibrillator are combined in a single unit and still not much bigger than the silver dollar sized design.

Micro-glucose detection and administration systems

As anyone diagnosed with Diabetes Mellitus will tell you, pricking a finger for blood samples and injecting insulin with a needle up to eight times a day is far from optimal. The traditional method for diabetes treatment is to check blood sugar four times a day, then administer a specified dose of insulin based on the blood sugar level. Each insulin shot helps to control blood sugar, but it also causes a shock to the system and these can cause a cumulative, negative effect on the organs of the body. Doctors and medical electronics designers have teamed up to develop a better, unified system for insulin administration that mimics the natural function of the pancreas. Specifically, a sensor implanted in the waist area is used in conjunction with a small insulin reservoir and dispensing system, worn externally at the waist (see Figure 5). A microminiature reed sensor in the implant is used for calibration and mode changes in a manner similar to the pacemaker. With this system, when very small changes in the blood sugar level occur, a small amount of insulin is administered that corrects the sugar level. This minimizes the shock to the system, allowing people with diabetes to live longer, healthier lives.

Figure 5: A glucose monitoring system worn at the belt level.

Pills that are ingested for video filming of internal organs

A few years back, the FDA approved a pill for ingestion that contains a video camera attached to its microcircuit (see Figure 6). As this battery-operated device is swallowed and moves through the digestive tract, it takes a video of its descent into the stomach and through the small intestine. The tiny camera records video images of areas of the small intestine that are unreachable by endoscopy or colonoscopy. These images are transmitted wirelessly for viewing outside the body. The microminiature reed switch plays a key role in this pill. After being manufactured, the pills may sit in inventory and later in a hospital stocking area for many months. The pill's battery life is only a few hours. The designers have developed a solution to this problem by using the microminiature reed switch in conjunction with a magnet. In the shipping container, each slot has a magnet dedicated to each pill, as the battery is not yet activated. Once the pill is removed from the proximity of the magnet, the microminiature reed switch opens and activates the battery, in turn applying power to the circuitry and video system. Because of these pills, many previously undiagnosable digestive tract issues have been discovered and treated.

Figure 6: An ingestible pill that records a video of its entry into the stomach and the small intestine.

Animal tracking devices

Several animals are on the endangered species list. To prevent their extinction, the government and private donors have set up programs such as the development of tracking devices that are worn on a collar or implanted in the animal (see Figure 7). The implanted device has proven more reliable, as the collars wear out over time or can be torn off by their hosts. These tracking devices must be very small, and of course, are battery operated. When the implanted tracking device is used, after the animal has been tracked for a given period of time, it is caught and tranquilized. Then a permanent magnet is placed near the implanted device, closing the micromachined, hermetically sealed reed switch, which wirelessly transmits all of the tracking information to a receiver. The tracking information can then be analyzed and decisions made on how to help the animal better survive.

Figure 7: The removal of information from an animal that has been tracked.

Fish tracking devices for migration studies

In a manner similar to the animal tracking devices, an electronic device with a microminiature reed switch is implanted in migratory fish (see Figure 8). Salmon are the primary fish tracked, since they are so widely used as a food source. When the fish are caught, a permanent magnet is used to close the reed switch, and then the vital information is wirelessly transmitted to a receiver where it is recorded for later analysis.

Figure 8: Microelectronic circuits implanted in fish.


Carotid artery plaque detection, implantable muscle stimulation and incontinence prevention systems

The number of tiny, implantable electronic systems that are being designed and placed in the human body is growing at a rapid rate. These devices have the following in common: they serve to detect a fault in the human body, they need to perform a function when the fault is detected, they often remain in the human body for several years, they are battery operated, they use minimal battery power, and they need occasional adjustments or mode changes. The micromachined, hermetically sealed reed switch is used to perform the adjustment or mode changes. Importantly, the reed switch draws no power in its "off" state, and after a brief period of being energized, it can carry out its intended function, such as wireless transfer of information, adjustments, or mode changes.

The micromachined, hermetically sealed reed switch is being used more and more wherever the need for sensing does not require physical contact, but rather requires remote sensing by a microminiature device.

Summary and future direction

Research facilities, colleges, universities, and businesses have devoted a lot of time and energy using semiconductor manufacturing techniques to develop microminiature mechanical systems that can carry out a specific function. The vast majority have found disappointment and failure. MEDER is one of the few companies that has had success with the development of a micromachined, hermetically sealed reed switch, simply because of the amount of time, effort, and dedication that went into the undertaking. In addition, MEDER has expanded its manufacturing by using a more mechanized, automated approach, resulting in a higher-volume output at lower costs. Meder is now working on developing an even smaller microminiature reed switch, which will initiate new requirements and therefore stimulate new applications.
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