Spatial Light Modulators

One of the more complex photonic devices one will encounter as an optical engineer is the spatial light modulator. This is a device that (usually) converts a computer-generated signal into a spatially controlled distribution of intensity or phase, depending on the type of modulator.

A very well known device for intensity modulation is the DLP or DMD from Texas Instruments. At first glance it is a very impressive device, and for its purpose as a modulator for projectors it is indeed impressive.

In the phase modulation corner, we find more niche-oriented LCOS (Liquid Crystal On Silicon) from Holoeye or Hamamatsu. These spatial light modulators will be found in optical tweezers or wavelength selective switches (WSS), microscopes or virtual (near-to-eye) displays.

All LCOS modulators are too slow for high-end/high-speed imaging. The DMD has taken over this task in its entirety. Pattern generators producing 8-bit grayscale images at a rate of 5 giga pixels per second can be designed, although it is a lot of work to get there in order to get around the shortcomings of these devices.

So what are these shortcomings? The casual SLM buyer may initially focus on tangible (external) properties such as size, bus speed, and number of pixels, which say nothing about image quality. One property that is not talked about often enough is optical flatness, short or long range.

One of the early 512x2048 Fraunhofer SLM operating at 2 kHz

A device that possesses the aforementioned property is the Fraunhofer IPMS MEMS-based spatial light modulator. In the field of high-quality pattern generation, it is a common practice to write the same portion of a mask multiple times, using different segments of the modulator to reduce various imaging issues through averaging. However, this approach can be quite expensive since the imaging errors reduce with the square root of the number of images used for averaging. Thus, the image quality prior to averaging is of great importance.

The Fraunhofer IPMS MEMS-based spatial light modulators were developed in collaboration with Micronic in the early 2000s. The initial single-layer aluminum MEMS devices had some limitations, which were overcome by the development of a two-layer MEMS that resolved all mechanical issues while preserving the optical properties. Unfortunately, the industry opted for e-beam technology, believing that multi-beam systems would address write time challenges, which resulted in the end of the production of high-end optical mask writers. However, as the hopes for high-capacity e-beam mask-writers faded and the volume of mask suitable for these types of pattern generators increased, another opportunity for this technology emerged. Micronic and ASML developed a step-mirror device that could write the equivalent of an alternating phase-shifting mask. Both devices enabled individual calibration of each of the millions of tilt mirrors to the theoretical limit set by the optical flatness of a few nanometers.

While the ideal laser source for this modulator is the excimer laser, due to its etendue and short pulse lengths that enable sharp exposure of images onto a continuously moving workpiece, the cost of the excimer laser restricts the potential application space of a prospective product. Solutions using single-mode solid-state lasers exist, but they have yet to be fully utilized.

Given the industry's success with multiple patterning and optical proximity correction, achieving 50nm or even lower is possible at the mask, representing a 4x magnification relative to the wafer. This technology can write a substantial number of masks, but for OPC, phase-shifting, and pixelated mask magic, the MEMS must be optically flat.

Regardless of which spatial light modulator technology meets external requirements, the manufacturer does not know how it performs in a particular system because, generally speaking, this is not what they do. However, physical optics modeling will easily capture their properties and their efficacy can be determined through simulations.