Micromirror With Electromechanical Pulse Width Modulation

A novel micromirror design is presented that can be used as a pixel in a light valve. This mirror allows grey levels to be produced without the need for binary subframes. Analog pulse-width modulation (PWM) will be achieved without needing extra transistors in the CMOS layer below the mirror structure. This can be done by exploiting an electro-mechanical phenomenon known as 'pull-in'. It will be shown that it is possible to make the mirror switch from a positively tilted to a negatively tilted state and back with a duty ratio that is dependent on fixed voltages on two out of four addressing electrodes underneath the mirror. The other electrodes are driven with two independent triangular waveforms. The mirror thus performs an analog to PWM conversion at the MEMS level, not at the CMOS level. The number and the distribution of grey levels does not depend on the number of binary subframes or bitplanes, resulting in less severe speed requirements for the on-chip electronics.