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Micro/Nano Lithography

Faster and denser spatial light modulators for maskless lithography

Novel micromirror arrays with higher speed and advanced modulation range will extend lithography systems with precision wavefront control.
5 September 2007, SPIE Newsroom. DOI: 10.1117/2.1200708.0829

Optical maskless lithography may be used to fabicate integrated circuits (IC). One critical barrier for this method lies in developing and implementing beam modulation technology.1 A spatial light modulator (SLM) based on microelectromechanical systems (MEMS) can meet this challenge, enabling systems to achieve sub-50nm critical dimensions (CD) with throughput of at least one 300mm wafer level per hour. These specifications require an SLM with a large number of pixels (several millions) of very small size (5μm or smaller) and capable of very high frame rates (10kHz).

Here we describe the recently developed2–5 MEMS SLM technology that can modulate deep ultraviolet radiation (DUV) for maskless lithography applications. It will also reduce the complexity and cost of such systems by allowing use of simpler optics and providing a digital interface for high data throughput to the SLM.

A conceptual drawing of our MEMS array is shown in Figure 1. Each pixel consists of a silicon micromirror that can be moved like a piston when electrostatically actuated.2 The mirror is supported at its center by an elastic, doubly-clamped spring that provides a mechanical restoring force as well as the electrical contact to the ground lines. The length for springs even as thin as 100nm must span several pixels to provide required compliance for low voltage operation. Each spring is electrically and mechanically connected to a movable plate, with an electrode located underneath it. The plate maximizes the force achieved by the actuator by bringing the electrodes closer and increasing the area. It also shields the spring from the electric fields of neighboring electrodes by completely surrounding them with structures at the ground potential.

In the non-actuated state, all the micromirrors are at the same height and any incident beam will be reflected with minimum loss (fill factor dependent). A change in the relative height between mirrors will modulate the reflected wavefront by changing the optical path and phase of the beams reflected at each single micromirror.4

Figure 1. The microelectromechanical systems-based spatial light modulator. Changing the relative vertical position of the mirrors introduces a phase difference in the light reflected.

Using a polysilicon surface micromachining process with five structural layers and 130nm minimum features, we fabricated arrays of 128×512 electrostatically-actuated piston micromirrors with pixel size of 3μm and 5μm and > 90% optical fill factor.3 To modulate DUV for optical maskless lithography applications, continuously controlled vertical motion of >80nm is achieved at less than 4V. We have observed micromirror response time as fast as 2μs.2

A typical optical profile of two actuated sub-arrays is shown in Figure 2. A wiring layer is used to simultaneously actuate sub-arrays of up to 128×128 mirrors in linear gratings, checkerboards, and other patterns.2 In the arrays shown, micro-mirror size is 3μm. The actuated micromirrors (blue squares) were vertically displaced ∼70nm by applying ∼4V.

Figure 2. Optical profile of two actuated micromirrors arrays (in blue), were vertically displaced by applying a low voltage.

A more detailed view of several micromirrors in an array can be seen in Figure 3. This figure shows an SEM view of mirrors after having been planarized by a chemical mechanical polish.3 As is evident, a very smooth and high-fill-factor optical surface is attainable.

Figure 3. Scanning electron micrograph of the (A) 3μm pitch and (B) 5μm pitch microelectromechanical systems mirrors in arrays. One mirror is snapped down for illustration. The top surface is planarized and coated with evaporated Al for high reflectivity.

In conclusion, we have fabricated SLMs containing many thousands of movable micromirrors measuring just 3μm on each side, with a view toward future advances in maskless lithography systems. These MEMS-based micromirror arrays represent an order-of-magnitude improvement in density compared with similar SLMs.

Current research focuses on integration of these devices with suitable control electronics at this unprecedented density, which represents a critical barrier. Recently we successfully demonstrated electronic circuits and hybrid integration technologies for 5μm-pitch mirror arrays, another order-of-magnitude improvement on state of the art. The technology is suitable for creating arrays of over 10 million pixels and is envisioned as a substitute for expensive photomasks for prototyping and low volume production, reducing cost and providing increased security and flexibility to next-generation lithography systems.