Micromechanical display uses interferometric modulation

Iridigm Display Corp. (San Francisco, CA) has developed a new way of using micromechanics that may eventually provide an alternative to liquid crystal displays (LCDs). The company says that it has already demonstrated reflectivities of more than 80 percent using Digital Paper technology, and that contrast ratios of 12:1, viewing angles of ±60 deg., and drive voltages <5 V have been achieved. In addition, the new displays can be made using fabrication processes that have already been developed for the LCD industry. According to Iridigm, major companies have already expressed interest in using the new technology, including Palm, Motorola, and Handspring.

The basic component of the new display is the Interferometric modulator, or IMod.¹ Previous micromechanical devices have used interference,² but as the result of diffraction from surface relieve structures. This is not unlike the way embossed holograms (such as those on credit cards) work. To combine the desired diffractive orders and eliminate unwanted colors, these systems require a relatively sophisticated optical system. Plus, the angle of view is narrow. For these reasons, development of this technology has been geared around the projection display market where the cost of the optical system and the geometrical restriction can be accommodated.

Figure 1. The color reflected by the Interferometric modulator (IMod) device is set by the optical distance between the thin-film stack and the metallic membrane. The thickness of a protective insulating layer between the two determines the color when the two mirrors are in contact (right). To set the color reflected when the two surfaces are separated (left), the designer must calculate the additional distance required and set the height of the membrane accordingly.

The IMods, on the other hand, use interference in a way that has much more in common with reflection holography (Figure 1). The device consists of a conducting metallic membrane, a thin-film stack protected by an insulating layer, and a transparent substrate (in this case, glass). The stack and metal membrane act as two mirrors in an optical resonator. The optical distance between them determines which wavelengths constructively interfere with each other -- and so are reflected by the device -- and which destructively interfere and are absorbed.

Where no current is flowing, the optical distance between the two mirrors is determined by the thickness and refractive index of the insulator, and the height of the membrane. When the device is switched, however, the current causes the membrane to be electrostatically attracted to the insulator, stack, and substrate. When it makes contact, the optical distance drops to that through the insulator, and the color changes. By using this method, some of the problems inherent to diffractive systems are immediately solved. In particular, there is almost no "rainbow effect," where the color changes with viewing angle.

Figure 2. Measured and theoretical hysteresis curves for a second-generation IMod design. The curves seem to agree well, despite idealized assumptions about stress and boundary conditions used in the model.

One of the other advantages of the IMod device is that it has an inherent hysteresis (Figure 2). This is caused by the fact that restoration force of the membrane is linear whereas the electrostatic force opposing it is nonlinear. As a result, if the voltage is kept at the correct level (here, somewhere between 3 and 5 V) the device only needs to be switched when its state is actually required to change; it will then maintain that state until it is switched again. This bistable operation means that the device has inherently low power requirements.

Iridigm has produced a number of different test displays using their first-generation design, including a Palm-Pilot-sized screen. In these systems, tens or hundreds of 40- X 30-µm IMods were used within each pixel.Though most of the demonstrations so far have been monochromatic, the company says that, using conventional RGB additive color mixing, full-color displays are a straightforward extension of the existing technology.

Figure 3. The second-generation IMod supports a large membrane on posts, giving a fill-factor of 95 percent.

Among the improvements that Iridigm has been working on is its second-generation design (Figure 3), which has already been fabricated and tested. Here, a large membrane is raised up using a number of support posts, rather than having lots of small, separate devices. The new design not only supports a fill factor of up to 95 percent, but it has also has good long-term behavior. In tests, it has been shown to be reliable for a number of switch cycles equivalent to using a Palm Pilot four hours a day for 18 years.  

References:

1. M.W. Miles, Digital Paper: Reflective Displays Using Interferometric Modulation, SID Digest 5.3, 2000.

2. Sunny Bains, Grating Light Valve speed exploited in new architecture, OE Reports, August 1998.