HowTo: Spatial Light Modulators

About This Tech-Talk

Spatial light modulators are a class of active optical components that can alter the amplitude, phase or polarization of a beam of light and for this tech-talk we will focus on those that can do this using a pixelated, two-dimensional array. This wouldn’t capture all spatial light modulators but it is a sufficiently large and interesting subset to talk about. Since this site is geared toward system integrators, I will also focus on what to expect and think of should you decide that you need one.

Reflective or Tranmissive

To begin with, we have two major sub classes, reflective and transmissive. Most of the industrially interesting SLMs are reflective for the simple reason that high performing devices require a CMOS wafer to distribute the data and the simplest to arrange that is to put a MEMS of a metallic layer directly on top of the wafer. It is possible to arrange an optically transmissive surface that can distribute an electric signal but that implies free electrons and those tend to interact with the transmitted light. The trade-off that this implies typically affects device speed or optical transmission.

That said, there are still many technically interesting products that use this principle, which will be found in both consumer (LCD displays, and some LCoS based projects) and industrial applications.

Reflective Amplitude SLMs

This is a major class of spatial light modulators. Notably, we find the The Texas Instruments DLP in this class. Even if this type of SLMs alter the phase of the reflected light, they are not used as such. Let me explain. The DLP is a purely reflective micro-mirror device where each micro-mirror can have only two stable states altering between plus or minus 12 degrees along its diagonal, although the angle can differ between various types. Each point on the surface of this device alters the phase of the light by introducing an optical path difference, but since these phases are not independent, we can only observer the net effect of a small, tilted mirror.

There are other SLMs operating along the same optical principle, such as the IPMS analog tilt mirror SLM. Since the tilt-mirror devices are overall phase neutral, they rely on the imaging system to express their intended effect. Without it, they just look like a rough reflecting surface. With a properly designed projection system, they turn into high performing pattern generating systems that cannot be distinguished from high-quality binary masks.

The analog tilt-mirror devices are simple to calibrate and simple to use but require an Excimer laser to reach their true potential. There are, however, some properties that a tilted mirror in the image plane brings which are worth to keep in mind. The tilt of an isolated mirror can never hidden by the optical system even if the projection optics is not even close to resolving the micro-mirror. This can sometimes be used to properly set focus, possibly even the most sensitive way to do it without an interferometer or wavefront sensor but probably only useful at lower resolutions.

In order to use a tilt-mirror device of this type for high-end applications, the mirror tilt has to be alternating (as in the figure above). The projection optics will then cancel the phase. Generally speaking, these devices are best understood using Fourier theory.

Here’s an example: A device having flat mirrors has infinite contrast, one just has to tilt the mirrors to the correct angle. The tilted plate (or mirror) has a reflection pattern given by the sinc-function and the zero of that sinc is the correct angle for infinite contrast. What happens when we use partially coherent illumination. Then the SLM is then illuminated with a range of incident angles. Does that mean the contrast is degraded with partially coherent illumination? If we stick with the sinc-picture, we may be led to believe that it is impossible to arrange an optimal tilt angle since we cannot simultaneously adapt the one tilt angle to -range- of incident angles. If we go with the Fourier approach instead, the array illuminated at zero incidence reflects all light along the normal. The effect of the tilt is represented as a set of diffraction modes which are not going to be transmitted through the pupil. For any off-axis plane wave, the reflected light is just a shifted replica of the on-axis pattern. If the first one had infinite contrast (when viewed through the projection optics), so does the replica. The Fourier approach gives the right answer.

MEMS devices are never perfectly flat, which introduces a partially developed speckle in the image and other effects. This sets the limit to image quality when using these devices. Not all manufacturers are forthcoming with a specification regarding these parameters. For example, I have not been able to find it for the Ti DLP. However, some limits to this parameter are given by diffraction efficiency. Since the DLP must overlay many images in order to generate a grayscale, partially developed speckle should not contribute significantly to the degradation of image quality when using these devices.

With time, especially when using short wavelengths, the aluminium surface tends to undergo some annealing and compactification that leads to some curling of the mirrors. For the DLP, this has an insignificant effect on image quality due to the large tilt. For the analogue devices, this is probably the property that sets the usable lifetime of the device. It is visible as a contrast degradation but affects other important imaging properties, like focus sensitivity.

Reflective Phase SLMs

Reflective phase SLMs create a path length difference mostly in two ways. Either by displacing the reflecting surface or by locally changing the refractive index in order to generate a path-length difference. The former would describe spatial light modulators like the RealHolo, the Texas Instruments PLM or Silicon Light Machines PLV. In terms of technical potential, the RealHolo clearly stands out in the MEMS crowd.

Liquid Crystal on Silicon – LCoS

Another popular phase SLM technology is the Liquid Crystal on Silicon, or LCoS. Even though these micro displays require a fairly complex stack or technologies, like the CMOS backplane, metallic pixel layer, a top and bottom alignment layer for the Liquid Crystal, an Indium Tin Oxide layer to generate the electric field over the LC, and finally a cover glass to hold the ITO, they can still be manufactured in volume with good yields and without pixel defects. Additional advantages are that LCoS panels are pretty robust. To the disadvantages we have to count polarization dependence. Also, the wavelength for which the SLM is designed dictates the thickness of the liquid crystal layer which limits the effective resolution of the modulator due to the nature of electric fields and their tendency to diverge from their source. The LCoS is a fancy collection of densely packed transparent capacitors, and when the lateral size of each is smaller than the distance between plates of the capacitor, the electric field spreads, which in this case spreads to a neighboring pixel and causes cross-talk. In the LCoS business this is referred to as fringe-field effects. It causes both unwanted polarization effects and disclinations. Before we jump into MEMS SLMs, we have to mention one major advantage which is that you can buy them today from companies like Hamamatsu or Holoeye Photonics.

MEMS Piston Mirror Arrays

Even if the piston-MEMS devices are not as rare as unicorns, you will not easily spot one in the wild. That said, they do exist and if Texas Instruments eventually releases the PLM, they may in fact become quite common. Piston devices are really flexible from the optical point of view. There are no polarization effects to speak of. No wavelength dependence as long as we illuminate with one wavelength at the time. They are fast and the phase is stable. With LCoS, one has to frequently change the polarity of the driving voltage in order to prevent degradation of the liquid crystal. None of that stuff here. 360 degree phase modulation and no cross talk to talk about. Great. Can I have a bunch? There is one problem, which is equally valid for the LCoS panels, how to you decide which phase to set on each illuminated pixel?

The Computational Challenge

Naturally, this is a “solved” problem, and as many of you know, it is solved—or at least solvable—using the Gerchberg-Saxton algorithm or numerous other phase retrieval algorithms. These algorithms are particularly useful in visual applications, where the brain renders the result more palatable. Perhaps this should have been said first of all, and this is true both for LCoS and piston devices, they do not need any optics. Illuminate with a beam, wide enough to cover most of the device, and the diffracted light can be controlled to generate any pattern. The wavelength divided by the pixel size limits its angular extent but if that is enough, we are done. Otherwise, we may use a Galilean telescope to provide the desired magnification.

For applications where image quality requirements are high, such as lithography, we need an approach that delivers zero phase variation in the projected image and, of course, absolutely no noise. That’s easy, isn’t it? Just combine 4 mirrors into one pixel. That can have any phase, any amplitude and it is a square pixel. But if you care about how fast this device delivers a pattern, this solution slows it down 4 times if the patterning speed is limited by the rate at which we can transfer the data to the modulator. Is it possible to have a piston modulator deliver the same write capacity as a tilt-mirror modulator of the same size and data rate capacity? Yes, it can be done but requires a significant computational effort. So much, in fact, that it may be difficult to apply these SLMs to a maskless / direct-write application. Nevertheless, Nikon is working on, what they call, a digital scanner (DS) with very ambitious targets. The rule – never assume what someone else can’t do – applies.

When the Sky is Not the Limit SLM

The piston modulator applied to lithography is the ultimate modulator for those who want to push sub-wavelength resolution beyond its otherwise practical limits, but it does come at a cost. Here’s the point. A piston modulator can project any phase and amplitude (in relative terms) but for lithography, we actually do not want any amplitude. We want the image to have a constant phase over the entire image because a phase variation is a variation of feature dimensions or placement through focus, and we absolutely don’t want that. This means that we have to spend time in order to find a solution that constrains that particular degree of freedom of this modulator.

To both have the cake and eat it, Micronic invented a tilt-mirror design which included a quarter wavelength step over one half of the mirror. The result was an image amplitude that could reach a (relative) range of -1 to 1 and zero phase variation. All the good stuff that you needed the piston mirror for without the work. The downside was a reduction in reflectivity of the device, which for an application that included an Excimer laser was not an issue. Strictly speaking, there may be 2D topologies that this modulator cannot deal with in a single image. Nevertheless, this was the solution ASML used for their direct-write behemoth, which unfortunately was never released as a product. To my knowledge, this is the largest SLM designed and fabricated to date. 11 million analogue-tilt 8 micron mirrors and 6 kHz image repetition rate. A nice SEM image can be found here.

Want to Know More

If you want to know more, feel free to contact me through the contact page or simply drop and email to contact@senslogic.de. I like talking SLMs. You can also leave a comment right here.